README.md

@@ -9,15 +9,13 @@ PuppetDB and it sees one consistent view spanning all of them.
 
 Running more than one PuppetDB — during a migration between two of them, or
 across regions — means a given node's current data lives in exactly one at any
-moment, and consumers have to know which, or query each in turn. Consider two
-backends being merged during a migration:
+moment, and consumers have to know which, or query each in turn. `pdbmux`
+merges them all so consumers don't have to know (or query twice) which PuppetDB
+a node currently lives in.
 
-- **old** — the PuppetDB nodes are moving off, e.g. `http://puppetdb1.example.com:8080`
-- **new** — the PuppetDB nodes are moving on to, e.g. `http://puppetdb2.example.com:8080`
-
-Nodes move from old to new as they migrate. `pdbmux` merges both so consumers
-don't have to know (or query twice) which PuppetDB a node currently lives in.
-The backend names are arbitrary labels; there is no fixed number of backends.
+All backends are equal — `pdbmux` is never told which one to favour. Backend
+names are arbitrary labels and there is no fixed number of them. The configured
+order is used only as a tie-break, so output is reproducible.
 
 ## Endpoints
 
@@ -32,8 +30,8 @@ not PQL) is forwarded verbatim.
 | `GET /pdb/query/v4/events` | Fan out to all and serve the **union**, deduped by record identity, re-ordered and re-paged. |
 | `GET /pdb/query/v4/event-counts` | Fan out to all and **sum** each subject's counts into one row per subject. |
 | `GET /pdb/query/v4/aggregate-event-counts` | Fan out to all and **sum** the summary object's counts. |
-| `GET /pdb/query/v4/reports/<hash>/{events,logs,metrics}` | Ask every backend; serve the answer from whichever backend actually holds that report. `404` when neither does. |
-| `GET /pdb/query/v4/*` (any other) | Transparently proxied to the **primary** backend, unmerged, streamed verbatim. |
+| `GET /pdb/query/v4/reports/<hash>/{events,logs,metrics}` | Ask every backend; serve the answer from whichever backend actually holds that report. `404` when none does. |
+| `GET /pdb/query/v4/*` (any other) | No merge rule, so backends are tried in configured order and the first success is streamed back verbatim; if all reject it, the first upstream error response is replayed. |
 | `GET /healthz` | Per-backend reachability. `200 {"status":"ok"}` if all reachable, `200 degraded` if some fail, `503 down` if all fail. |
 
 Fan-out is concurrent. If one backend errors or times out, `pdbmux` serves the
@@ -44,25 +42,24 @@ unknown fields survive untouched.
 ## Merge semantics
 
 - **`/nodes`** — dedupe by `certname`; the record with the strictly-newer
-  `report_timestamp` wins. On a tie (or when a node exists in only one backend),
-  the **preferred** backend's record is kept.
+  `report_timestamp` wins. On a tie, the backend listed first in `backends`
+  supplies the record — a tie-break only, so the merged output is deterministic.
 - **`/facts`** — node-level granularity. For a `certname` present in more than
   one backend, `pdbmux` keeps **all** of that node's facts from **one** backend and
-  drops the other's, chosen by the merge strategy:
+  drops the others', chosen by the merge strategy:
   - **`freshness`** (default) — attribute each `certname` to whichever backend
     holds its newer `report_timestamp`. `pdbmux` derives this from a per-certname
     freshness map built by querying `/nodes` from every backend, cached for
-    `freshness_ttl` (default 30s). Ties/fallbacks use `prefer`.
-  - **`static`** — always keep the `prefer` backend's facts for shared nodes.
-    No extra `/nodes` query.
+    `freshness_ttl` (default 30s).
+  - **`static`** — skip the extra `/nodes` query and take each shared node's
+    facts from the first backend in configured order that holds it.
   - A node present in only one backend always appears (falls back to whichever
     backend actually returned facts for it).
 - **`/reports`, `/events`** — **union**, not a per-node winner. Reports are
-  immutable history, so a node that migrated legitimately has reports in the old
-  PuppetDB *and* the new one and both belong in the merged view. Reports dedupe
-  on `hash`; events, which carry no id of their own, dedupe on the verbatim
-  record (a node briefly reporting to both PuppetDBs stores identical records in
-  each).
+  immutable history, so a node's reports can legitimately exist in more than one
+  backend and all of them belong in the merged view. Reports dedupe on `hash`;
+  events, which carry no id of their own, dedupe on the verbatim record (a node
+  reporting to more than one backend stores identical records in each).
 - **Aggregates** — `extract`/`group_by` rows are counts, not records, so each
   backend returns a partial answer that has to be **added**, not deduped. This
   covers `/event-counts`, `/aggregate-event-counts`, and a `/reports` query whose
@@ -86,7 +83,7 @@ Each backend applies `order_by`/`limit`/`offset` to its own slice only, so
 `pdbmux` re-does all three over the union:
 
 - `order_by` is parsed and the merged set re-sorted by those fields (ties keep
-  backend precedence). A record missing an ordered field sorts first.
+  the merged set's existing order). A record missing an ordered field sorts first.
 - Backends are asked for the first `offset + limit` records — never an `offset`
   — and the requested window is then cut from the merged, re-sorted set.
 - `include_total=true` on a union endpoint makes `pdbmux` sum each backend's
@@ -112,14 +109,12 @@ or the paths it searched.
 
 ```yaml
 listen: ":8080"
-backends:
-  - name: old
+backends:                          # order is a tie-break only, not a ranking
+  - name: pdb-a
     url: http://puppetdb1.example.com:8080
-  - name: new
+  - name: pdb-b
     url: https://puppetdb2.example.com
-primary: new          # backend used for non-merged /pdb/query/v4/* pass-through
 merge: freshness      # freshness | static
-prefer: new           # winner on ties / static merge / fallback
 timeout: 10s          # per-upstream request timeout
 freshness_ttl: 30s    # freshness-map cache TTL (freshness merge only)
 ```
@@ -131,14 +126,12 @@ the `/pdb/query/v4/...` path per request.
 |---|---|
 | `PDBMUX_CONFIG` | config file path (not a file key) |
 | `PDBMUX_LISTEN` | `listen` |
-| `PDBMUX_PRIMARY` | `primary` |
 | `PDBMUX_MERGE` | `merge` |
-| `PDBMUX_PREFER` | `prefer` |
 | `PDBMUX_TIMEOUT` | `timeout` (Go duration, e.g. `10s`) |
 | `PDBMUX_FRESHNESS_TTL` | `freshness_ttl` |
 | `PDBMUX_BACKENDS` | whole backend list, as `name=url,name=url` |
 
-Flags: `--config`, `--listen`, `--primary`, `--merge`.
+Flags: `--config`, `--listen`, `--merge`.
 
 `config init` writes to `--config`/`PDBMUX_CONFIG` when set, else to
 `$XDG_CONFIG_HOME/pdbmux/config.yaml`.
@@ -150,7 +143,7 @@ Subcommands: `serve` (default), `config init`, `config show`, `version`. Run
 base URL in place of a PuppetDB one.
 
 ```bash
-PDBMUX_BACKENDS='old=http://puppetdb1.example.com:8080,new=http://puppetdb2.example.com:8080' pdbmux
+PDBMUX_BACKENDS='pdb-a=http://puppetdb1.example.com:8080,pdb-b=http://puppetdb2.example.com:8080' pdbmux
 curl -s --get http://localhost:8080/pdb/query/v4/nodes \
   --data-urlencode 'query=["=","certname","host1.example.com"]'
 ```

aggregate.go

@@ -159,8 +159,9 @@ type sumGroup struct {
 // A row that is not a JSON object passes through untouched, as does the sole row
 // of a key only one backend reported — those keep their upstream bytes. An
 // aggregate column that is absent or non-numeric in a later row is left at the
-// first backend's value rather than being coerced to zero. results must be
-// ordered by precedence; output keeps first-seen order.
+// earlier backend's value rather than being coerced to zero. results come in
+// configured backend order and the output keeps first-seen order, a tie-break
+// only.
 func sumRows(results []backendResult, columns func(map[string]json.RawMessage) ([]string, []string)) []json.RawMessage {
 	type slot struct {
 		raw   json.RawMessage // passthrough row, when group is nil
@@ -237,7 +238,7 @@ func (g *sumGroup) encode() json.RawMessage {
 }
 
 // groupKey builds a row's identity from the named fields' verbatim JSON values.
-// Both backends run the same PuppetDB serialiser, so byte equality is a sound
+// Every backend runs the same PuppetDB serialiser, so byte equality is a sound
 // comparison for object-valued keys such as event-counts' subject. An absent
 // field is distinct from any present value.
 func groupKey(row map[string]json.RawMessage, keys []string) string {

aggregate_test.go

@@ -73,8 +73,8 @@ func TestParseAggregate_NoFunctionIsNotAggregate(t *testing.T) {
 func TestSumRows_SharedKeysAreAdded(t *testing.T) {
 	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(`{"count":3,"status":"changed"}`, `{"count":1,"status":"failed"}`)},
-		{name: "old", records: rows(`{"count":4,"status":"changed"}`, `{"count":2,"status":"failed"}`)},
+		{name: "a", records: rows(`{"count":3,"status":"changed"}`, `{"count":1,"status":"failed"}`)},
+		{name: "b", records: rows(`{"count":4,"status":"changed"}`, `{"count":2,"status":"failed"}`)},
 	}, spec.columns)
 
 	want := []map[string]any{
@@ -89,8 +89,8 @@ func TestSumRows_SharedKeysAreAdded(t *testing.T) {
 func TestSumRows_DisjointKeysAreKept(t *testing.T) {
 	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(`{"count":3,"status":"changed"}`)},
-		{name: "old", records: rows(`{"count":2,"status":"skipped"}`)},
+		{name: "a", records: rows(`{"count":3,"status":"changed"}`)},
+		{name: "b", records: rows(`{"count":2,"status":"skipped"}`)},
 	}, spec.columns)
 
 	want := []map[string]any{
@@ -106,8 +106,8 @@ func TestSumRows_SingleBackendRowKeepsUpstreamBytes(t *testing.T) {
 	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
 	const raw = `{"count":3,"status":"changed","extra":{"kept":true}}`
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(raw)},
-		{name: "old", records: nil},
+		{name: "a", records: rows(raw)},
+		{name: "b", records: nil},
 	}, spec.columns)
 
 	if len(merged) != 1 || string(merged[0]) != raw {
@@ -118,8 +118,8 @@ func TestSumRows_SingleBackendRowKeepsUpstreamBytes(t *testing.T) {
 func TestSumRows_NonNumericAggregateColumnIsNotZeroed(t *testing.T) {
 	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(`{"count":5,"status":"changed"}`)},
-		{name: "old", records: rows(`{"count":null,"status":"changed"}`)},
+		{name: "a", records: rows(`{"count":5,"status":"changed"}`)},
+		{name: "b", records: rows(`{"count":null,"status":"changed"}`)},
 	}, spec.columns)
 
 	want := []map[string]any{{"count": float64(5), "status": "changed"}}
@@ -131,8 +131,8 @@ func TestSumRows_NonNumericAggregateColumnIsNotZeroed(t *testing.T) {
 func TestSumRows_MissingAggregateColumnStartsFromTheNumericRow(t *testing.T) {
 	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(`{"status":"changed"}`)},
-		{name: "old", records: rows(`{"count":6,"status":"changed"}`)},
+		{name: "a", records: rows(`{"status":"changed"}`)},
+		{name: "b", records: rows(`{"count":6,"status":"changed"}`)},
 	}, spec.columns)
 
 	want := []map[string]any{{"count": float64(6), "status": "changed"}}
@@ -144,8 +144,8 @@ func TestSumRows_MissingAggregateColumnStartsFromTheNumericRow(t *testing.T) {
 func TestSumRows_NonObjectRowsPassThrough(t *testing.T) {
 	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(`"surprise"`)},
-		{name: "old", records: rows(`{"count":1,"status":"changed"}`)},
+		{name: "a", records: rows(`"surprise"`)},
+		{name: "b", records: rows(`{"count":1,"status":"changed"}`)},
 	}, spec.columns)
 
 	if len(merged) != 2 || string(merged[0]) != `"surprise"` {
@@ -161,8 +161,8 @@ func TestSumRows_NoKeyColumnsCollapseToOneRow(t *testing.T) {
 		t.Fatal("expected an aggregate spec")
 	}
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(`{"count":10}`)},
-		{name: "old", records: rows(`{"count":32}`)},
+		{name: "a", records: rows(`{"count":10}`)},
+		{name: "b", records: rows(`{"count":32}`)},
 	}, spec.columns)
 
 	want := []map[string]any{{"count": float64(42)}}
@@ -188,11 +188,11 @@ func TestInferredColumns_SplitsCountsFromIdentity(t *testing.T) {
 
 func TestSumRows_EventCountsPerSubject(t *testing.T) {
 	merged := sumRows([]backendResult{
-		{name: "new", records: rows(
+		{name: "a", records: rows(
 			`{"subject_type":"certname","subject":{"title":"h1"},"failures":1,"successes":2,"noops":0,"skips":0}`,
 			`{"subject_type":"certname","subject":{"title":"h2"},"failures":0,"successes":5,"noops":0,"skips":0}`,
 		)},
-		{name: "old", records: rows(
+		{name: "b", records: rows(
 			`{"subject_type":"certname","subject":{"title":"h1"},"failures":3,"successes":4,"noops":1,"skips":0}`,
 		)},
 	}, inferredColumns)

config.go

@@ -28,8 +28,8 @@ const (
 )
 
 var exampleBackends = []Backend{
-	{Name: "primary", URL: "http://puppetdb1.example.com:8080"},
-	{Name: "secondary", URL: "http://puppetdb2.example.com:8080"},
+	{Name: "pdb-a", URL: "http://puppetdb1.example.com:8080"},
+	{Name: "pdb-b", URL: "http://puppetdb2.example.com:8080"},
 }
 
 type Backend struct {
@@ -39,10 +39,8 @@ type Backend struct {
 
 type Config struct {
 	Listen       string        `yaml:"listen"`
-	Backends     []Backend     `yaml:"backends"`
-	Primary      string        `yaml:"primary"`
+	Backends     []Backend     `yaml:"backends"` // all equal; order is only a deterministic tie-break
 	Merge        string        `yaml:"merge"`
-	Prefer       string        `yaml:"prefer"` // wins under static merge, and breaks ties under freshness merge
 	Timeout      time.Duration `yaml:"timeout"`
 	FreshnessTTL time.Duration `yaml:"freshness_ttl"`
 
@@ -69,8 +67,6 @@ func DefaultConfig() Config {
 func ExampleConfig() Config {
 	cfg := DefaultConfig()
 	cfg.Backends = append([]Backend(nil), exampleBackends...)
-	cfg.Primary = exampleBackends[0].Name
-	cfg.Prefer = exampleBackends[0].Name
 	return cfg
 }
 
@@ -143,7 +139,6 @@ func Load(flagPath string) (Config, error) {
 	}
 
 	applyEnv(&cfg, os.Getenv)
-	cfg.normalize()
 	return cfg, nil
 }
 
@@ -151,15 +146,9 @@ func applyEnv(cfg *Config, getenv func(string) string) {
 	if v := getenv(envPrefix + "LISTEN"); v != "" {
 		cfg.Listen = v
 	}
-	if v := getenv(envPrefix + "PRIMARY"); v != "" {
-		cfg.Primary = v
-	}
 	if v := getenv(envPrefix + "MERGE"); v != "" {
 		cfg.Merge = v
 	}
-	if v := getenv(envPrefix + "PREFER"); v != "" {
-		cfg.Prefer = v
-	}
 	if v := getenv(envPrefix + "TIMEOUT"); v != "" {
 		if d, err := time.ParseDuration(v); err == nil {
 			cfg.Timeout = d
@@ -195,18 +184,6 @@ func parseBackends(s string) []Backend {
 	return out
 }
 
-func (c *Config) normalize() {
-	if len(c.Backends) == 0 {
-		return
-	}
-	if c.Primary == "" {
-		c.Primary = c.Backends[0].Name
-	}
-	if c.Prefer == "" {
-		c.Prefer = c.Backends[0].Name
-	}
-}
-
 // configHint names the file a user should edit: the one actually loaded, else
 // the default write target.
 func (c Config) configHint() string {
@@ -231,32 +208,17 @@ func (c Config) Validate() error {
 		}
 		seen[b.Name] = true
 	}
-	if !seen[c.Primary] {
-		return fmt.Errorf("primary %q is not a configured backend", c.Primary)
-	}
 	switch c.Merge {
 	case mergeFreshness, mergeStatic:
 	default:
 		return fmt.Errorf("merge must be %q or %q, got %q", mergeFreshness, mergeStatic, c.Merge)
 	}
-	if !seen[c.Prefer] {
-		return fmt.Errorf("prefer %q is not a configured backend", c.Prefer)
-	}
 	if c.Timeout <= 0 {
 		return fmt.Errorf("timeout must be positive")
 	}
 	return nil
 }
 
-func (c Config) PrimaryBackend() Backend {
-	for _, b := range c.Backends {
-		if b.Name == c.Primary {
-			return b
-		}
-	}
-	return c.Backends[0]
-}
-
 func writeDefaultConfig(path string) error {
 	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return fmt.Errorf("creating config dir: %w", err)
@@ -268,8 +230,8 @@ func writeDefaultConfig(path string) error {
 	header := []byte("# pdbmux configuration\n" +
 		"# A merging proxy presenting one PuppetDB v4 query surface over several\n" +
 		"# PuppetDB backends. The backend URLs below are placeholders — edit them.\n" +
-		"# Env overrides: PDBMUX_LISTEN, PDBMUX_PRIMARY, PDBMUX_MERGE, PDBMUX_PREFER,\n" +
-		"#   PDBMUX_TIMEOUT, PDBMUX_FRESHNESS_TTL, PDBMUX_BACKENDS (name=url,name=url).\n\n")
+		"# Env overrides: PDBMUX_LISTEN, PDBMUX_MERGE, PDBMUX_TIMEOUT,\n" +
+		"#   PDBMUX_FRESHNESS_TTL, PDBMUX_BACKENDS (name=url,name=url).\n\n")
 	if err := os.WriteFile(path, append(header, data...), 0o644); err != nil {
 		return fmt.Errorf("writing config: %w", err)
 	}

config_test.go

@@ -14,11 +14,9 @@ import (
 func testConfigValid() Config {
 	cfg := DefaultConfig()
 	cfg.Backends = []Backend{
-		{Name: "old", URL: "http://localhost:18080"},
-		{Name: "new", URL: "http://localhost:18081"},
+		{Name: "a", URL: "http://localhost:18080"},
+		{Name: "b", URL: "http://localhost:18081"},
 	}
-	cfg.Primary = "new"
-	cfg.Prefer = "new"
 	return cfg
 }
 
@@ -56,7 +54,7 @@ func TestLoad_NoBackendsLoadsButFailsValidation(t *testing.T) {
 	}
 }
 
-func TestLoad_PrimaryDefaultsToFirstBackend(t *testing.T) {
+func TestLoad_BackendsKeepConfiguredOrder(t *testing.T) {
 	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
 	clearEnv(t)
 	t.Setenv(envPrefix+"BACKENDS", "a=http://localhost:18080,b=http://localhost:18081")
@@ -65,8 +63,11 @@ func TestLoad_PrimaryDefaultsToFirstBackend(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if cfg.Primary != "a" || cfg.Prefer != "a" {
-		t.Errorf("primary/prefer should default to first backend, got %q/%q", cfg.Primary, cfg.Prefer)
+	if len(cfg.Backends) != 2 || cfg.Backends[0].Name != "a" || cfg.Backends[1].Name != "b" {
+		t.Errorf("backends should keep the configured order, got %+v", cfg.Backends)
+	}
+	if err := cfg.Validate(); err != nil {
+		t.Errorf("a bare backend list must validate: %v", err)
 	}
 }
 
@@ -79,8 +80,8 @@ func TestLoad_FileAndEnvOverride(t *testing.T) {
 	if err := os.MkdirAll(cfgDir, 0o755); err != nil {
 		t.Fatal(err)
 	}
-	body := "listen: :9999\nmerge: static\nprimary: old\nprefer: old\n" +
-		"backends:\n  - name: old\n    url: http://localhost:18080\n  - name: new\n    url: http://localhost:18081\n"
+	body := "listen: :9999\nmerge: static\n" +
+		"backends:\n  - name: a\n    url: http://localhost:18080\n  - name: b\n    url: http://localhost:18081\n"
 	if err := os.WriteFile(filepath.Join(cfgDir, configFileName), []byte(body), 0o644); err != nil {
 		t.Fatal(err)
 	}
@@ -95,8 +96,8 @@ func TestLoad_FileAndEnvOverride(t *testing.T) {
 	if cfg.Listen != "127.0.0.1:1234" {
 		t.Errorf("env should beat file for listen, got %q", cfg.Listen)
 	}
-	if cfg.Merge != mergeStatic || cfg.Primary != "old" {
-		t.Errorf("file override failed: merge=%s primary=%s", cfg.Merge, cfg.Primary)
+	if cfg.Merge != mergeStatic {
+		t.Errorf("file override failed: merge=%s", cfg.Merge)
 	}
 }
 
@@ -104,8 +105,6 @@ func TestApplyEnv_Backends(t *testing.T) {
 	cfg := testConfigValid()
 	env := map[string]string{
 		envPrefix + "BACKENDS":      "a=http://a:8080,b=http://b:8080",
-		envPrefix + "PRIMARY":       "a",
-		envPrefix + "PREFER":        "a",
 		envPrefix + "TIMEOUT":       "3s",
 		envPrefix + "FRESHNESS_TTL": "45s",
 	}
@@ -127,10 +126,8 @@ func TestValidate(t *testing.T) {
 	}{
 		{"ok", func(*Config) {}, false},
 		{"no backends", func(c *Config) { c.Backends = nil }, true},
-		{"dup name", func(c *Config) { c.Backends = append(c.Backends, Backend{Name: "old", URL: "x"}) }, true},
+		{"dup name", func(c *Config) { c.Backends = append(c.Backends, Backend{Name: "a", URL: "x"}) }, true},
 		{"missing url", func(c *Config) { c.Backends[0].URL = "" }, true},
-		{"primary not a backend", func(c *Config) { c.Primary = "ghost" }, true},
-		{"prefer not a backend", func(c *Config) { c.Prefer = "ghost" }, true},
 		{"bad merge", func(c *Config) { c.Merge = "wrong" }, true},
 		{"zero timeout", func(c *Config) { c.Timeout = 0 }, true},
 	}
@@ -156,13 +153,6 @@ func TestParseBackends(t *testing.T) {
 	}
 }
 
-func TestPrimaryBackend(t *testing.T) {
-	cfg := testConfigValid()
-	if got, want := cfg.PrimaryBackend().URL, cfg.Backends[1].URL; got != want {
-		t.Errorf("primary backend URL = %q, want %q", got, want)
-	}
-}
-
 func TestExampleConfig_IsValidAndNeutral(t *testing.T) {
 	cfg := ExampleConfig()
 	if err := cfg.Validate(); err != nil {
@@ -175,8 +165,8 @@ func TestExampleConfig_IsValidAndNeutral(t *testing.T) {
 	}
 }
 
-const testConfigBody = "listen: \":9999\"\nmerge: static\nprimary: old\nprefer: old\n" +
-	"backends:\n  - name: old\n    url: http://localhost:18080\n  - name: new\n    url: http://localhost:18081\n"
+const testConfigBody = "listen: \":9999\"\nmerge: static\n" +
+	"backends:\n  - name: a\n    url: http://localhost:18080\n  - name: b\n    url: http://localhost:18081\n"
 
 func writeConfigFile(t *testing.T, path string) {
 	t.Helper()
@@ -401,7 +391,7 @@ func captureStdout(t *testing.T, f func()) string {
 
 func clearEnv(t *testing.T) {
 	t.Helper()
-	for _, k := range []string{"CONFIG", "LISTEN", "PRIMARY", "MERGE", "PREFER", "TIMEOUT", "FRESHNESS_TTL", "BACKENDS"} {
+	for _, k := range []string{"CONFIG", "LISTEN", "MERGE", "TIMEOUT", "FRESHNESS_TTL", "BACKENDS"} {
 		t.Setenv(envPrefix+k, "")
 	}
 }

main.go

@@ -23,7 +23,6 @@ func main() {
 		cfg        Config
 		configPath string
 		listen     string
-		primary    string
 		merge      string
 	)
 
@@ -44,9 +43,6 @@ func main() {
 		if cmd.Flags().Changed("listen") {
 			cfg.Listen = listen
 		}
-		if cmd.Flags().Changed("primary") {
-			cfg.Primary = primary
-		}
 		if cmd.Flags().Changed("merge") {
 			cfg.Merge = merge
 		}
@@ -70,7 +66,6 @@ func main() {
 	pf := root.PersistentFlags()
 	pf.StringVar(&configPath, "config", "", "Config file path (overrides PDBMUX_CONFIG and the default search path)")
 	pf.StringVar(&listen, "listen", defaultListen, "HTTP listen address (overrides config and PDBMUX_LISTEN)")
-	pf.StringVar(&primary, "primary", "", "Primary backend name for non-merged pass-through")
 	pf.StringVar(&merge, "merge", mergeFreshness, "Facts merge strategy: freshness or static")
 
 	serveCmd := &cobra.Command{
@@ -132,8 +127,8 @@ func runServer(cfg Config) error {
 		ReadHeaderTimeout: 10 * time.Second,
 	}
 
-	logger.Printf("listening on %s (merge=%s primary=%s backends=%d)",
-		cfg.Listen, cfg.Merge, cfg.Primary, len(cfg.Backends))
+	logger.Printf("listening on %s (merge=%s backends=%d)",
+		cfg.Listen, cfg.Merge, len(cfg.Backends))
 
 	errCh := make(chan error, 1)
 	go func() {
@@ -163,9 +158,7 @@ func printConfig(cfg Config) {
 		fmt.Printf("config file  : none loaded (searched %s)\n", strings.Join(configSearchPaths(), ", "))
 	}
 	fmt.Printf("listen       : %s\n", cfg.Listen)
-	fmt.Printf("primary      : %s\n", cfg.Primary)
 	fmt.Printf("merge        : %s\n", cfg.Merge)
-	fmt.Printf("prefer       : %s\n", cfg.Prefer)
 	fmt.Printf("timeout      : %s\n", durationString(cfg.Timeout))
 	fmt.Printf("freshness_ttl: %s\n", durationString(cfg.FreshnessTTL))
 	fmt.Println("backends:")

merge.go

@@ -49,7 +49,7 @@ func parseTimestamp(s string) time.Time {
 	return time.Time{}
 }
 
-// results must be ordered by precedence: ties keep the earlier backend's record.
+// Ties keep the earlier backend's record — a deterministic tie-break, not a preference.
 func mergeNodes(results []backendResult) []json.RawMessage {
 	type pick struct {
 		raw json.RawMessage
@@ -81,7 +81,7 @@ func mergeNodes(results []backendResult) []json.RawMessage {
 // certname -> name of the backend holding that node's newest report.
 type freshness map[string]string
 
-// results must be ordered by precedence: ties keep the earlier backend.
+// Ties keep the earlier backend — a deterministic tie-break, not a preference.
 func buildFreshness(results []backendResult) freshness {
 	type pick struct {
 		backend string
@@ -104,9 +104,9 @@ func buildFreshness(results []backendResult) freshness {
 	return f
 }
 
-// owner names the winning backend per certname; when it holds no facts for that certname, precedence order wins.
+// owner names the winning backend per certname; a nil owner (static merge), or one holding no facts for that certname, falls back to configured order.
 func mergeFacts(results []backendResult, owner func(certname string) string) []json.RawMessage {
-	present := map[string][]string{} // certname -> backend names, in precedence order
+	present := map[string][]string{} // certname -> backend names, in configured order
 	byKey := map[string][]json.RawMessage{}
 	for _, res := range results {
 		for _, rec := range res.records {
@@ -133,7 +133,10 @@ func mergeFacts(results []backendResult, owner func(certname string) string) []j
 	out := []json.RawMessage{}
 	for _, cn := range order {
 		backends := present[cn]
-		chosen := owner(cn)
+		chosen := ""
+		if owner != nil {
+			chosen = owner(cn)
+		}
 		if !contains(backends, chosen) {
 			chosen = backends[0]
 		}

merge_test.go

@@ -89,10 +89,10 @@ func event(cn, reportHash, resource string) string {
 }
 
 func TestMergeNodes_NewerWins(t *testing.T) {
-	old := recs(t, "old", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-10T00:00:00Z"))
-	nw := recs(t, "new", node("h1", "2026-07-20T00:00:00Z"), node("h3", "2026-07-05T00:00:00Z"))
+	a := recs(t, "a", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-10T00:00:00Z"))
+	b := recs(t, "b", node("h1", "2026-07-20T00:00:00Z"), node("h3", "2026-07-05T00:00:00Z"))
 
-	merged := mergeNodes([]backendResult{old, nw})
+	merged := mergeNodes([]backendResult{a, b})
 	got := map[string]string{}
 	for _, r := range merged {
 		var m recordMeta
@@ -100,13 +100,13 @@ func TestMergeNodes_NewerWins(t *testing.T) {
 		got[m.Certname] = m.ReportTimestamp
 	}
 	if got["h1"] != "2026-07-20T00:00:00Z" {
-		t.Errorf("h1: newer (new) should win, got %s", got["h1"])
+		t.Errorf("h1: newer (b) should win, got %s", got["h1"])
 	}
 	if got["h2"] != "2026-07-10T00:00:00Z" {
-		t.Errorf("h2: only in old, got %s", got["h2"])
+		t.Errorf("h2: only in a, got %s", got["h2"])
 	}
 	if got["h3"] != "2026-07-05T00:00:00Z" {
-		t.Errorf("h3: only in new, got %s", got["h3"])
+		t.Errorf("h3: only in b, got %s", got["h3"])
 	}
 	if len(merged) != 3 {
 		t.Errorf("expected 3 deduped nodes, got %d", len(merged))
@@ -114,20 +114,20 @@ func TestMergeNodes_NewerWins(t *testing.T) {
 }
 
 func TestMergeNodes_OneBackendOnly(t *testing.T) {
-	old := recs(t, "old", node("h1", "2026-07-01T00:00:00Z"))
-	// new returned nothing (e.g. empty result).
-	nw := backendResult{name: "new"}
-	merged := mergeNodes([]backendResult{old, nw})
+	a := recs(t, "a", node("h1", "2026-07-01T00:00:00Z"))
+	// b returned nothing (e.g. empty result).
+	b := backendResult{name: "b"}
+	merged := mergeNodes([]backendResult{a, b})
 	if len(merged) != 1 || certnames(t, merged)[0] != "h1" {
 		t.Fatalf("expected only h1, got %v", certnames(t, merged))
 	}
 }
 
 func TestMergeNodes_TieKeepsEarlierBackend(t *testing.T) {
-	// Equal timestamps: the backend listed first (precedence) wins.
-	prefer := recs(t, "new", node("h1", "2026-07-01T00:00:00Z"))
-	other := recs(t, "old", node("h1", "2026-07-01T00:00:00Z"))
-	merged := mergeNodes([]backendResult{prefer, other})
+	// Equal timestamps: the backend listed first wins, as a tie-break.
+	first := recs(t, "b", node("h1", "2026-07-01T00:00:00Z"))
+	second := recs(t, "a", node("h1", "2026-07-01T00:00:00Z"))
+	merged := mergeNodes([]backendResult{first, second})
 	if len(merged) != 1 {
 		t.Fatalf("expected 1 record, got %d", len(merged))
 	}
@@ -138,8 +138,8 @@ func TestMergeNodes_TieKeepsEarlierBackend(t *testing.T) {
 }
 
 func TestMergeNodes_PreservesUnknownFields(t *testing.T) {
-	old := recs(t, "old", `{"certname":"h1","report_timestamp":"2026-07-01T00:00:00Z","extra":{"deep":42}}`)
-	merged := mergeNodes([]backendResult{old})
+	a := recs(t, "a", `{"certname":"h1","report_timestamp":"2026-07-01T00:00:00Z","extra":{"deep":42}}`)
+	merged := mergeNodes([]backendResult{a})
 	if len(merged) != 1 {
 		t.Fatalf("expected 1 record")
 	}
@@ -150,69 +150,70 @@ func TestMergeNodes_PreservesUnknownFields(t *testing.T) {
 	}
 }
 
-func TestMergeFacts_Static_PreferWins(t *testing.T) {
-	// h1 in both; static prefer=new -> new's facts kept, old's dropped.
-	old := recs(t, "old", fact("h1", "role", "web-old", ""), fact("h2", "role", "db-old", ""))
-	nw := recs(t, "new", fact("h1", "role", "web-new", ""))
+func TestMergeFacts_NilOwnerUsesConfiguredOrder(t *testing.T) {
+	// Static merge passes no owner: h1 is in both, so the first backend in the
+	// slice supplies its facts.
+	a := recs(t, "a", fact("h1", "role", "web-a", ""), fact("h2", "role", "db-a", ""))
+	b := recs(t, "b", fact("h1", "role", "web-b", ""))
 
-	merged := mergeFacts([]backendResult{nw, old}, func(string) string { return "new" })
+	merged := mergeFacts([]backendResult{b, a}, nil)
 	got := factValues(t, merged)
-	assertContains(t, got, "h1:role=web-new")
-	assertNotContains(t, got, "h1:role=web-old")
-	// h2 only in old -> falls back to old.
-	assertContains(t, got, "h2:role=db-old")
+	assertContains(t, got, "h1:role=web-b")
+	assertNotContains(t, got, "h1:role=web-a")
+	// h2 only in a -> still served from a.
+	assertContains(t, got, "h2:role=db-a")
 }
 
 func TestMergeFacts_Freshness_NewerBackendWins(t *testing.T) {
-	// owner map says h1 belongs to old (older backend has the newer report),
-	// h2 belongs to new. Multiple facts per node must all come from the winner.
-	old := recs(t, "old",
-		fact("h1", "role", "web-old", ""), fact("h1", "ip", "10.0.0.1", ""),
-		fact("h2", "role", "db-old", ""))
-	nw := recs(t, "new",
-		fact("h1", "role", "web-new", ""), fact("h1", "ip", "10.9.9.9", ""),
-		fact("h2", "role", "db-new", ""), fact("h2", "ip", "10.0.0.2", ""))
+	// owner map says h1 belongs to a and h2 to b. Multiple facts per node must
+	// all come from the winner.
+	a := recs(t, "a",
+		fact("h1", "role", "web-a", ""), fact("h1", "ip", "10.0.0.1", ""),
+		fact("h2", "role", "db-a", ""))
+	b := recs(t, "b",
+		fact("h1", "role", "web-b", ""), fact("h1", "ip", "10.9.9.9", ""),
+		fact("h2", "role", "db-b", ""), fact("h2", "ip", "10.0.0.2", ""))
 
 	owner := func(cn string) string {
 		if cn == "h1" {
-			return "old"
+			return "a"
 		}
-		return "new"
+		return "b"
 	}
-	merged := mergeFacts([]backendResult{nw, old}, owner)
+	merged := mergeFacts([]backendResult{b, a}, owner)
 	got := factValues(t, merged)
-	// h1 -> all old facts, no new facts.
-	assertContains(t, got, "h1:role=web-old")
+	// h1 -> all a facts, no b facts.
+	assertContains(t, got, "h1:role=web-a")
 	assertContains(t, got, "h1:ip=10.0.0.1")
-	assertNotContains(t, got, "h1:role=web-new")
+	assertNotContains(t, got, "h1:role=web-b")
 	assertNotContains(t, got, "h1:ip=10.9.9.9")
-	// h2 -> all new facts.
-	assertContains(t, got, "h2:role=db-new")
+	// h2 -> all b facts.
+	assertContains(t, got, "h2:role=db-b")
 	assertContains(t, got, "h2:ip=10.0.0.2")
-	assertNotContains(t, got, "h2:role=db-old")
+	assertNotContains(t, got, "h2:role=db-a")
 }
 
-func TestMergeFacts_OwnerMissingFallsBackToPrecedence(t *testing.T) {
-	// owner returns a backend with no facts for h1 -> fall back to first
-	// backend present (precedence order of the slice).
-	prefer := recs(t, "new", fact("h1", "role", "web-new", ""))
-	other := recs(t, "old", fact("h1", "role", "web-old", ""))
-	merged := mergeFacts([]backendResult{prefer, other}, func(string) string { return "ghost" })
+func TestMergeFacts_OwnerMissingFallsBackToConfiguredOrder(t *testing.T) {
+	// owner returns a backend with no facts for h1 -> fall back to the first
+	// backend in the slice that has some.
+	first := recs(t, "b", fact("h1", "role", "web-b", ""))
+	second := recs(t, "a", fact("h1", "role", "web-a", ""))
+	merged := mergeFacts([]backendResult{first, second}, func(string) string { return "ghost" })
 	got := factValues(t, merged)
-	assertContains(t, got, "h1:role=web-new") // new is first in slice
-	assertNotContains(t, got, "h1:role=web-old")
+	assertContains(t, got, "h1:role=web-b") // b is first in slice
+	assertNotContains(t, got, "h1:role=web-a")
 }
 
 func TestBuildFreshness(t *testing.T) {
-	// old has newer report for h1; new has newer for h2.
-	old := recs(t, "old", node("h1", "2026-07-20T00:00:00Z"), node("h2", "2026-07-01T00:00:00Z"))
-	nw := recs(t, "new", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-20T00:00:00Z"))
-	f := buildFreshness([]backendResult{old, nw})
-	if f["h1"] != "old" {
-		t.Errorf("h1 should belong to old, got %q", f["h1"])
+	// a holds the newer report for h1; b holds the newer one for h2.
+	a := recs(t, "a", node("h1", "2026-07-20T00:00:00Z"), node("h2", "2026-07-01T00:00:00Z"))
+	b := recs(t, "b", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-20T00:00:00Z"))
+	f := buildFreshness([]backendResult{a, b})
+	if f["h1"] != "a" {
+		t.Errorf("h1 should belong to a, got %q", f["h1"])
 	}
-	if f["h2"] != "new" {
-		t.Errorf("h2 should belong to new, got %q", f["h2"])
+	if f["h2"] != "b" {
+		t.Errorf("h2 should belong to b, got %q", f["h2"])
 	}
 }
 

reports.go

@@ -9,7 +9,7 @@ import (
 	"strings"
 )
 
-// results must be ordered by precedence; a key func returning ok=false means the record has no identity and is always kept.
+// The first backend in results holding a key supplies the record; a key func returning ok=false means the record has no identity and is always kept.
 func mergeUnion(results []backendResult, key func(record) (string, bool)) []json.RawMessage {
 	seen := make(map[string]bool)
 	out := []json.RawMessage{}
@@ -65,7 +65,7 @@ func parseOrderBy(s string) ([]orderField, error) {
 	return out, nil
 }
 
-// Each backend ordered only its own slice, so the union is re-sorted here; stable, so ties keep backend precedence.
+// Each backend ordered only its own slice, so the union is re-sorted here; stable, so ties keep the merged set's existing order.
 func sortRecords(recs []json.RawMessage, order []orderField) {
 	if len(order) == 0 || len(recs) < 2 {
 		return

reports_test.go

@@ -8,17 +8,17 @@ import (
 )
 
 func TestMergeUnion_KeepsBothBackendsHistory(t *testing.T) {
-	old := recs(t, "old", report("h1", "r1", "2026-07-01T00:00:00Z"))
-	nw := recs(t, "new", report("h1", "r2", "2026-07-02T00:00:00Z"))
-	merged := mergeUnion([]backendResult{nw, old}, reportKey)
+	a := recs(t, "a", report("h1", "r1", "2026-07-01T00:00:00Z"))
+	b := recs(t, "b", report("h1", "r2", "2026-07-02T00:00:00Z"))
+	merged := mergeUnion([]backendResult{b, a}, reportKey)
 	if got := hashesOf(t, merged); !slices.Equal(got, []string{"r2", "r1"}) {
-		t.Errorf("union = %v, want both reports in precedence order", got)
+		t.Errorf("union = %v, want both reports in configured order", got)
 	}
 }
 
 func TestMergeUnion_DedupesSharedHash(t *testing.T) {
 	dup := report("h1", "r1", "2026-07-01T00:00:00Z")
-	merged := mergeUnion([]backendResult{recs(t, "new", dup), recs(t, "old", dup)}, reportKey)
+	merged := mergeUnion([]backendResult{recs(t, "b", dup), recs(t, "a", dup)}, reportKey)
 	if got := hashesOf(t, merged); !slices.Equal(got, []string{"r1"}) {
 		t.Errorf("union = %v, want a single r1", got)
 	}
@@ -27,9 +27,9 @@ func TestMergeUnion_DedupesSharedHash(t *testing.T) {
 func TestMergeUnion_HashlessRowsAreAllKept(t *testing.T) {
 	// extract/group_by queries return synthetic rows with no hash; dropping the
 	// second backend's rows as "duplicates" would silently lose half the data.
-	old := recs(t, "old", `{"status":"changed","count":3}`)
-	nw := recs(t, "new", `{"status":"changed","count":5}`)
-	merged := mergeUnion([]backendResult{old, nw}, reportKey)
+	a := recs(t, "a", `{"status":"changed","count":3}`)
+	b := recs(t, "b", `{"status":"changed","count":5}`)
+	merged := mergeUnion([]backendResult{a, b}, reportKey)
 	if len(merged) != 2 {
 		t.Errorf("expected both aggregate rows, got %d: %v", len(merged), merged)
 	}
@@ -39,7 +39,7 @@ func TestMergeUnion_IdenticalHashlessRowsAreNotCollapsed(t *testing.T) {
 	// Two backends can legitimately produce the same aggregate row; collapsing
 	// them as duplicates undercounts the merged result.
 	same := `{"status":"changed","count":1}`
-	merged := mergeUnion([]backendResult{recs(t, "old", same), recs(t, "new", same)}, reportKey)
+	merged := mergeUnion([]backendResult{recs(t, "a", same), recs(t, "b", same)}, reportKey)
 	if len(merged) != 2 {
 		t.Errorf("expected both backends' aggregate rows, got %d: %v", len(merged), merged)
 	}
@@ -48,7 +48,7 @@ func TestMergeUnion_IdenticalHashlessRowsAreNotCollapsed(t *testing.T) {
 func TestMergeUnion_EventsDedupeOnRawIdentity(t *testing.T) {
 	same := event("h1", "r1", "Package[nginx]")
 	other := event("h1", "r1", "Service[nginx]")
-	merged := mergeUnion([]backendResult{recs(t, "new", same, other), recs(t, "old", same)}, rawKey)
+	merged := mergeUnion([]backendResult{recs(t, "b", same, other), recs(t, "a", same)}, rawKey)
 	if len(merged) != 2 {
 		t.Errorf("expected 2 distinct events, got %d: %v", len(merged), merged)
 	}

server.go

@@ -8,7 +8,6 @@ import (
 	"log"
 	"net/http"
 	"net/url"
-	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -82,7 +81,7 @@ func (s *Server) handleQuery(w http.ResponseWriter, r *http.Request) {
 			s.serveFirstHolder(w, r)
 			return
 		}
-		s.proxyPrimary(w, r)
+		s.proxyUnmerged(w, r)
 	}
 }
 
@@ -125,7 +124,7 @@ func (s *Server) serveUnion(w http.ResponseWriter, r *http.Request, path string,
 		return
 	}
 
-	merged := mergeUnion(s.byPrecedence(alive), key)
+	merged := mergeUnion(alive, key)
 	sortRecords(merged, page.order)
 	if page.wantTotal {
 		if total := sumTotals(alive); total >= 0 {
@@ -158,7 +157,7 @@ func (s *Server) serveSummed(w http.ResponseWriter, r *http.Request, path string
 		return
 	}
 
-	merged := sumRows(s.byPrecedence(alive), columns)
+	merged := sumRows(alive, columns)
 	sortRecords(merged, page.order)
 	if page.wantTotal {
 		w.Header().Set(recordsHeader, strconv.Itoa(len(merged)))
@@ -182,7 +181,7 @@ func (s *Server) serveFirstHolder(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "no backend holds this report", http.StatusNotFound)
 		return
 	}
-	for _, res := range s.byPrecedence(alive) {
+	for _, res := range alive {
 		if len(res.records) > 0 {
 			writeJSON(w, rawRecords(res.records))
 			return
@@ -226,33 +225,15 @@ func rawRecords(recs []record) []json.RawMessage {
 }
 
 func (s *Server) mergeNodesResponse(results []backendResult) []json.RawMessage {
-	return mergeNodes(s.byPrecedence(results))
+	return mergeNodes(results)
 }
 
 func (s *Server) mergeFactsResponse(results []backendResult) []json.RawMessage {
-	ordered := s.byPrecedence(results)
 	if s.cfg.Merge == mergeStatic {
-		prefer := s.cfg.Prefer
-		return mergeFacts(ordered, func(string) string { return prefer })
+		return mergeFacts(results, nil)
 	}
-	fresh := s.freshnessMap(context.Background(), ordered)
-	prefer := s.cfg.Prefer
-	return mergeFacts(ordered, func(cn string) string {
-		if b, ok := fresh[cn]; ok {
-			return b
-		}
-		return prefer
-	})
-}
-
-// Puts Prefer first so it wins ties; the rest keep config order.
-func (s *Server) byPrecedence(results []backendResult) []backendResult {
-	ordered := make([]backendResult, len(results))
-	copy(ordered, results)
-	sort.SliceStable(ordered, func(i, j int) bool {
-		return ordered[i].name == s.cfg.Prefer && ordered[j].name != s.cfg.Prefer
-	})
-	return ordered
+	fresh := s.freshnessMap(context.Background(), results)
+	return mergeFacts(results, func(cn string) string { return fresh[cn] })
 }
 
 // Queries /nodes unfiltered rather than reusing the request's results, because a /facts query's certname set can differ.
@@ -274,7 +255,7 @@ func (s *Server) freshnessMap(ctx context.Context, _ []backendResult) freshness
 		}
 		alive = append(alive, res)
 	}
-	f := buildFreshness(s.byPrecedence(alive))
+	f := buildFreshness(alive)
 
 	s.mu.Lock()
 	s.freshData = f
@@ -329,29 +310,63 @@ func (s *Server) queryBackend(ctx context.Context, b Backend, path string, param
 	return recs, total, err
 }
 
-func (s *Server) proxyPrimary(w http.ResponseWriter, r *http.Request) {
-	b := s.cfg.PrimaryBackend()
+// The record shape is unknown, so a union would be guesswork: the first 2xx wins and the first error response is replayed when none succeeds.
+func (s *Server) proxyUnmerged(w http.ResponseWriter, r *http.Request) {
+	var fallback *bufferedResponse
+	for _, b := range s.cfg.Backends {
+		resp, err := s.passThrough(r, b)
+		if err != nil {
+			s.log.Printf("warning: backend %q pass-through failed for %s: %v", b.Name, r.URL.Path, err)
+			continue
+		}
+		if resp.StatusCode >= 200 && resp.StatusCode < 300 {
+			setContentType(w, resp.Header.Get("Content-Type"))
+			w.WriteHeader(resp.StatusCode)
+			_, _ = io.Copy(w, resp.Body)
+			_ = resp.Body.Close()
+			return
+		}
+		body, _ := io.ReadAll(resp.Body)
+		_ = resp.Body.Close()
+		if fallback == nil {
+			fallback = &bufferedResponse{
+				status:      resp.StatusCode,
+				contentType: resp.Header.Get("Content-Type"),
+				body:        body,
+			}
+		}
+	}
+	if fallback == nil {
+		http.Error(w, "all backends failed", http.StatusBadGateway)
+		return
+	}
+	setContentType(w, fallback.contentType)
+	w.WriteHeader(fallback.status)
+	_, _ = w.Write(fallback.body)
+}
+
+type bufferedResponse struct {
+	status      int
+	contentType string
+	body        []byte
+}
+
+func (s *Server) passThrough(r *http.Request, b Backend) (*http.Response, error) {
 	target := strings.TrimRight(b.URL, "/") + r.URL.Path
 	if r.URL.RawQuery != "" {
 		target += "?" + r.URL.RawQuery
 	}
 	req, err := http.NewRequestWithContext(r.Context(), http.MethodGet, target, nil)
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusBadGateway)
-		return
+		return nil, err
 	}
-	resp, err := s.client.Do(req)
-	if err != nil {
-		s.log.Printf("warning: primary %q pass-through failed for %s: %v", b.Name, r.URL.Path, err)
-		http.Error(w, "primary backend failed", http.StatusBadGateway)
-		return
+	return s.client.Do(req)
+}
+
+func setContentType(w http.ResponseWriter, contentType string) {
+	if contentType != "" {
+		w.Header().Set("Content-Type", contentType)
 	}
-	defer func() { _ = resp.Body.Close() }()
-	if ct := resp.Header.Get("Content-Type"); ct != "" {
-		w.Header().Set("Content-Type", ct)
-	}
-	w.WriteHeader(resp.StatusCode)
-	_, _ = io.Copy(w, resp.Body)
 }
 
 type healthReport struct {

server_test.go

@@ -119,13 +119,11 @@ func truncate(t *testing.T, body, limit string) string {
 	return string(out)
 }
 
-func testConfig(oldURL, newURL, merge string) Config {
+func testConfig(aURL, bURL, merge string) Config {
 	return Config{
 		Listen:       ":0",
-		Backends:     []Backend{{Name: "old", URL: oldURL}, {Name: "new", URL: newURL}},
-		Primary:      "new",
+		Backends:     []Backend{{Name: "a", URL: aURL}, {Name: "b", URL: bURL}},
 		Merge:        merge,
-		Prefer:       "new",
 		Timeout:      2 * time.Second,
 		FreshnessTTL: 30 * time.Second,
 	}
@@ -148,11 +146,11 @@ func doGet(t *testing.T, h http.Handler, path, query string) *httptest.ResponseR
 }
 
 func TestHandler_NodesMerged(t *testing.T) {
-	old := newFakeBackend(t,
+	a := newFakeBackend(t,
 		`[`+node("h1", "2026-07-01T00:00:00Z")+`,`+node("h2", "2026-07-10T00:00:00Z")+`]`, `[]`)
-	nw := newFakeBackend(t,
+	b := newFakeBackend(t,
 		`[`+node("h1", "2026-07-20T00:00:00Z")+`]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), nodesPath, `["=","certname","h1"]`)
 	if rec.Code != http.StatusOK {
@@ -167,76 +165,78 @@ func TestHandler_NodesMerged(t *testing.T) {
 	}
 	for _, m := range got {
 		if m.Certname == "h1" && m.ReportTimestamp != "2026-07-20T00:00:00Z" {
-			t.Errorf("h1 should be new's newer record, got %s", m.ReportTimestamp)
+			t.Errorf("h1 should be the newer record, got %s", m.ReportTimestamp)
 		}
 	}
 }
 
 func TestHandler_QueryPassthrough(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	q := `["=","certname","abc.example.net"]`
 	doGet(t, srv.Handler(), factsPath, q)
-	if old.gotQuery(factsPath) != q {
-		t.Errorf("old backend got query %q, want %q", old.gotQuery(factsPath), q)
+	if a.gotQuery(factsPath) != q {
+		t.Errorf("a backend got query %q, want %q", a.gotQuery(factsPath), q)
 	}
-	if nw.gotQuery(factsPath) != q {
-		t.Errorf("new backend got query %q, want %q", nw.gotQuery(factsPath), q)
+	if b.gotQuery(factsPath) != q {
+		t.Errorf("b backend got query %q, want %q", b.gotQuery(factsPath), q)
 	}
 }
 
 func TestHandler_FactsStaticMerge(t *testing.T) {
-	old := newFakeBackend(t, `[]`,
-		`[`+fact("h1", "role", "web-old", "")+`,`+fact("h2", "role", "db-old", "")+`]`)
-	nw := newFakeBackend(t, `[]`,
-		`[`+fact("h1", "role", "web-new", "")+`]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	// Static merge ignores timestamps: a shared certname resolves to the first
+	// backend in configured order that holds it.
+	a := newFakeBackend(t, `[]`,
+		`[`+fact("h1", "role", "web-a", "")+`,`+fact("h2", "role", "db-a", "")+`]`)
+	b := newFakeBackend(t, `[]`,
+		`[`+fact("h1", "role", "web-b", "")+`,`+fact("h3", "role", "db-b", "")+`]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), factsPath, `["=","name","role"]`)
 	if rec.Code != http.StatusOK {
 		t.Fatalf("status %d", rec.Code)
 	}
 	body := rec.Body.String()
-	if !strings.Contains(body, "web-new") || strings.Contains(body, "web-old") {
-		t.Errorf("static prefer=new should keep web-new, drop web-old: %s", body)
+	if !strings.Contains(body, "web-a") || strings.Contains(body, "web-b") {
+		t.Errorf("h1 should resolve to the first backend holding it: %s", body)
 	}
-	if !strings.Contains(body, "db-old") {
-		t.Errorf("h2 only in old should survive: %s", body)
+	if !strings.Contains(body, "db-a") || !strings.Contains(body, "db-b") {
+		t.Errorf("nodes held by only one backend must all survive: %s", body)
 	}
 }
 
 func TestHandler_FactsFreshnessMerge(t *testing.T) {
-	// Freshness: old holds h1's newer report; new holds h2's newer report.
-	old := newFakeBackend(t,
+	// Freshness: a holds h1's newer report; b holds h2's newer report.
+	a := newFakeBackend(t,
 		`[`+node("h1", "2026-07-20T00:00:00Z")+`,`+node("h2", "2026-07-01T00:00:00Z")+`]`,
-		`[`+fact("h1", "role", "web-old", "")+`,`+fact("h2", "role", "db-old", "")+`]`)
-	nw := newFakeBackend(t,
+		`[`+fact("h1", "role", "web-a", "")+`,`+fact("h2", "role", "db-a", "")+`]`)
+	b := newFakeBackend(t,
 		`[`+node("h1", "2026-07-01T00:00:00Z")+`,`+node("h2", "2026-07-20T00:00:00Z")+`]`,
-		`[`+fact("h1", "role", "web-new", "")+`,`+fact("h2", "role", "db-new", "")+`]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeFreshness))
+		`[`+fact("h1", "role", "web-b", "")+`,`+fact("h2", "role", "db-b", "")+`]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 	rec := doGet(t, srv.Handler(), factsPath, `["=","name","role"]`)
 	if rec.Code != http.StatusOK {
 		t.Fatalf("status %d: %s", rec.Code, rec.Body.String())
 	}
 	body := rec.Body.String()
-	// h1 -> old (newer report there); h2 -> new.
-	if !strings.Contains(body, "web-old") || strings.Contains(body, "web-new") {
-		t.Errorf("h1 should resolve to old: %s", body)
+	// h1 -> a (newer report there); h2 -> b.
+	if !strings.Contains(body, "web-a") || strings.Contains(body, "web-b") {
+		t.Errorf("h1 should resolve to a: %s", body)
 	}
-	if !strings.Contains(body, "db-new") || strings.Contains(body, "db-old") {
-		t.Errorf("h2 should resolve to new: %s", body)
+	if !strings.Contains(body, "db-b") || strings.Contains(body, "db-a") {
+		t.Errorf("h2 should resolve to b: %s", body)
 	}
 }
 
 func TestHandler_OneBackendDown(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.fail = true
-	nw := newFakeBackend(t,
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t,
 		`[`+node("h1", "2026-07-20T00:00:00Z")+`]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), nodesPath, "")
 	if rec.Code != http.StatusOK {
@@ -248,10 +248,10 @@ func TestHandler_OneBackendDown(t *testing.T) {
 }
 
 func TestHandler_BothBackendsDown(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	old.fail, nw.fail = true, true
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	a.fail, b.fail = true, true
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), nodesPath, "")
 	if rec.Code != http.StatusBadGateway {
@@ -259,11 +259,12 @@ func TestHandler_BothBackendsDown(t *testing.T) {
 	}
 }
 
-func TestHandler_PassThroughToPrimary(t *testing.T) {
-	// A non-merged v4 path (e.g. /resources) goes only to the primary (new).
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+func TestHandler_PassThroughFirstAnswer(t *testing.T) {
+	// A path with no merge rule (e.g. /resources) is served by the first backend
+	// that answers; the rest are not asked at all.
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	const path = "/pdb/query/v4/resources"
 	rec := doGet(t, srv.Handler(), path, `["=","certname","h1"]`)
@@ -273,19 +274,51 @@ func TestHandler_PassThroughToPrimary(t *testing.T) {
 	if !strings.Contains(rec.Body.String(), path) {
 		t.Errorf("expected pass-through body, got %s", rec.Body.String())
 	}
-	// Only primary (new) should have been queried.
-	if _, hit := old.params(path); hit {
-		t.Errorf("non-primary backend should not be queried for pass-through")
+	if _, hit := a.params(path); !hit {
+		t.Errorf("first backend should be queried for pass-through")
 	}
-	if _, hit := nw.params(path); !hit {
-		t.Errorf("primary backend should be queried for pass-through")
+	if _, hit := b.params(path); hit {
+		t.Errorf("later backends should not be queried once one answers")
+	}
+}
+
+func TestHandler_PassThroughFallsBackToNextBackend(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	const path = "/pdb/query/v4/resources"
+	rec := doGet(t, srv.Handler(), path, "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected the surviving backend to serve it, got %d: %s", rec.Code, rec.Body.String())
+	}
+	if !strings.Contains(rec.Body.String(), path) {
+		t.Errorf("expected pass-through body, got %s", rec.Body.String())
+	}
+}
+
+func TestHandler_PassThroughReplaysUpstreamError(t *testing.T) {
+	// Every backend rejects it, so PuppetDB's own status reaches the client
+	// rather than a synthetic 502.
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	a.fail, b.fail = true, true
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := doGet(t, srv.Handler(), "/pdb/query/v4/resources", "")
+	if rec.Code != http.StatusInternalServerError {
+		t.Fatalf("expected the upstream 500 replayed, got %d", rec.Code)
+	}
+	if !strings.Contains(rec.Body.String(), "boom") {
+		t.Errorf("expected the upstream body, got %s", rec.Body.String())
 	}
 }
 
 func TestHandler_PostRejected(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 	req := httptest.NewRequest(http.MethodPost, factsPath, nil)
 	rec := httptest.NewRecorder()
 	srv.Handler().ServeHTTP(rec, req)
@@ -295,9 +328,9 @@ func TestHandler_PostRejected(t *testing.T) {
 }
 
 func TestHandler_Health(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), "/healthz", "")
 	if rec.Code != http.StatusOK {
@@ -307,16 +340,16 @@ func TestHandler_Health(t *testing.T) {
 	if err := json.Unmarshal(rec.Body.Bytes(), &hr); err != nil {
 		t.Fatal(err)
 	}
-	if hr.Status != "ok" || hr.Backends["old"] != "ok" || hr.Backends["new"] != "ok" {
+	if hr.Status != "ok" || hr.Backends["a"] != "ok" || hr.Backends["b"] != "ok" {
 		t.Fatalf("unexpected health: %+v", hr)
 	}
 }
 
 func TestHandler_HealthDegradedAndDown(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	old.fail = true
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), "/healthz", "")
 	var hr healthReport
@@ -328,7 +361,7 @@ func TestHandler_HealthDegradedAndDown(t *testing.T) {
 		t.Errorf("degraded should still be 200, got %d", rec.Code)
 	}
 
-	nw.fail = true
+	b.fail = true
 	rec = doGet(t, srv.Handler(), "/healthz", "")
 	_ = json.Unmarshal(rec.Body.Bytes(), &hr)
 	if hr.Status != "down" || rec.Code != http.StatusServiceUnavailable {
@@ -337,21 +370,21 @@ func TestHandler_HealthDegradedAndDown(t *testing.T) {
 }
 
 func TestFreshnessCache_Reused(t *testing.T) {
-	old := newFakeBackend(t,
+	a := newFakeBackend(t,
 		`[`+node("h1", "2026-07-20T00:00:00Z")+`]`,
-		`[`+fact("h1", "role", "web-old", "")+`]`)
-	nw := newFakeBackend(t,
+		`[`+fact("h1", "role", "web-a", "")+`]`)
+	b := newFakeBackend(t,
 		`[`+node("h1", "2026-07-01T00:00:00Z")+`]`,
-		`[`+fact("h1", "role", "web-new", "")+`]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeFreshness))
+		`[`+fact("h1", "role", "web-b", "")+`]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 	// Two facts queries; the freshness /nodes probe should be cached after the
 	// first, so query recording only reflects the last observed nodes query but
-	// results stay consistent (h1 -> old).
+	// results stay consistent (h1 -> a).
 	for i := 0; i < 2; i++ {
 		rec := doGet(t, srv.Handler(), factsPath, `["=","name","role"]`)
-		if !strings.Contains(rec.Body.String(), "web-old") {
-			t.Fatalf("iteration %d: expected h1->old, got %s", i, rec.Body.String())
+		if !strings.Contains(rec.Body.String(), "web-a") {
+			t.Fatalf("iteration %d: expected h1->a, got %s", i, rec.Body.String())
 		}
 	}
 }
@@ -383,15 +416,15 @@ func hashes(t *testing.T, body []byte) []string {
 const receiveDesc = `[{"field":"receive_time","order":"desc"}]`
 
 func TestHandler_ReportsUnioned(t *testing.T) {
-	// h1 migrated: its pre-migration reports are in old, later ones in new.
+	// h1 moved between backends: earlier reports are in a, later ones in b.
 	// Both must show up, unlike /facts where one backend wins the node.
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[` + report("h1", "r2", "2026-07-10T00:00:00Z") + `,` +
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[` + report("h1", "r2", "2026-07-10T00:00:00Z") + `,` +
 		report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[` + report("h1", "r4", "2026-07-30T00:00:00Z") + `,` +
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[` + report("h1", "r4", "2026-07-30T00:00:00Z") + `,` +
 		report("h1", "r3", "2026-07-20T00:00:00Z") + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, url.Values{
 		"query":    {`["=","certname","h1"]`},
@@ -411,11 +444,11 @@ func TestHandler_ReportsDedupedByHash(t *testing.T) {
 	// A node reporting to both PuppetDBs mid-migration stores the same report
 	// hash in each; the merged view must show it once.
 	dup := report("h1", "r1", "2026-07-01T00:00:00Z")
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[` + dup + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[` + dup + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[` + dup + `]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[` + dup + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, nil)
 	if got := hashes(t, rec.Body.Bytes()); !slices.Equal(got, []string{"r1"}) {
@@ -424,15 +457,15 @@ func TestHandler_ReportsDedupedByHash(t *testing.T) {
 }
 
 func TestHandler_ReportsPagedAcrossBackends(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[` + report("h1", "r5", "2026-07-05T00:00:00Z") + `,` +
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[` + report("h1", "r5", "2026-07-05T00:00:00Z") + `,` +
 		report("h1", "r3", "2026-07-03T00:00:00Z") + `,` +
 		report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[` + report("h1", "r6", "2026-07-06T00:00:00Z") + `,` +
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[` + report("h1", "r6", "2026-07-06T00:00:00Z") + `,` +
 		report("h1", "r4", "2026-07-04T00:00:00Z") + `,` +
 		report("h1", "r2", "2026-07-02T00:00:00Z") + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, url.Values{
 		"order_by": {receiveDesc},
@@ -448,7 +481,7 @@ func TestHandler_ReportsPagedAcrossBackends(t *testing.T) {
 	}
 	// Each backend must be asked for the first offset+limit records so the
 	// merged window is fully covered.
-	for name, fb := range map[string]*fakeBackend{"old": old, "new": nw} {
+	for name, fb := range map[string]*fakeBackend{"a": a, "b": b} {
 		p, ok := fb.params(reportsPath)
 		if !ok {
 			t.Fatalf("%s backend was not queried", name)
@@ -463,13 +496,13 @@ func TestHandler_ReportsPagedAcrossBackends(t *testing.T) {
 }
 
 func TestHandler_ReportsIncludeTotalSummed(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[` + report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
-	old.totals[reportsPath] = 40
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[` + report("h1", "r2", "2026-07-02T00:00:00Z") + `]`
-	nw.totals[reportsPath] = 60
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[` + report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
+	a.totals[reportsPath] = 40
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[` + report("h1", "r2", "2026-07-02T00:00:00Z") + `]`
+	b.totals[reportsPath] = 60
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, url.Values{
 		"include_total": {"true"},
@@ -481,13 +514,13 @@ func TestHandler_ReportsIncludeTotalSummed(t *testing.T) {
 }
 
 func TestHandler_ReportsNoTotalWhenNotRequested(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[]`
-	old.totals[reportsPath] = 40
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[]`
-	nw.totals[reportsPath] = 60
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[]`
+	a.totals[reportsPath] = 40
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[]`
+	b.totals[reportsPath] = 60
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, nil)
 	if got := rec.Header().Get(recordsHeader); got != "" {
@@ -496,9 +529,9 @@ func TestHandler_ReportsNoTotalWhenNotRequested(t *testing.T) {
 }
 
 func TestHandler_ReportsBadPagingParam(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	for _, params := range []url.Values{
 		{"limit": {"lots"}},
@@ -515,11 +548,11 @@ func TestHandler_ReportsBadPagingParam(t *testing.T) {
 func TestHandler_EventsUnioned(t *testing.T) {
 	// Puppetboard fetches a report's events as /events?query=["=","report",hash],
 	// and the report may live in either backend.
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[eventsPath] = `[` + event("h1", "r1", "Package[nginx]") + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[eventsPath] = `[` + event("h1", "r2", "Service[nginx]") + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[eventsPath] = `[` + event("h1", "r1", "Package[nginx]") + `]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[eventsPath] = `[` + event("h1", "r2", "Service[nginx]") + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), eventsPath, `["=","certname","h1"]`)
 	if rec.Code != http.StatusOK {
@@ -533,11 +566,11 @@ func TestHandler_EventsUnioned(t *testing.T) {
 
 func TestHandler_EventsDedupedByIdentity(t *testing.T) {
 	dup := event("h1", "r1", "Package[nginx]")
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[eventsPath] = `[` + dup + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[eventsPath] = `[` + dup + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[eventsPath] = `[` + dup + `]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[eventsPath] = `[` + dup + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), eventsPath, "")
 	var got []json.RawMessage
@@ -550,27 +583,27 @@ func TestHandler_EventsDedupedByIdentity(t *testing.T) {
 }
 
 func TestHandler_ReportSubResourceFromHoldingBackend(t *testing.T) {
-	// Only old holds report r1, so its logs must come from old even though new
-	// is the primary — a pass-through would have 404'd.
+	// Only a holds report r1, so its logs come from a; an unmerged pass-through
+	// to whichever backend answered first could have 404'd.
 	const path = reportsPath + "/r1/logs"
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[path] = `[{"level":"notice","message":"from-old"}]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[path] = `[{"level":"notice","message":"from-a"}]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), path, "")
 	if rec.Code != http.StatusOK {
 		t.Fatalf("status %d: %s", rec.Code, rec.Body.String())
 	}
-	if !strings.Contains(rec.Body.String(), "from-old") {
-		t.Errorf("expected old's logs, got %s", rec.Body.String())
+	if !strings.Contains(rec.Body.String(), "from-a") {
+		t.Errorf("expected the holding backend's logs, got %s", rec.Body.String())
 	}
 }
 
 func TestHandler_ReportSubResourceMissingEverywhere(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), reportsPath+"/nope/events", "")
 	if rec.Code != http.StatusNotFound {
@@ -579,11 +612,11 @@ func TestHandler_ReportSubResourceMissingEverywhere(t *testing.T) {
 }
 
 func TestHandler_ReportsOneBackendDown(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.fail = true
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[` + report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[` + report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, url.Values{"order_by": {receiveDesc}})
 	if rec.Code != http.StatusOK {
@@ -621,11 +654,11 @@ func counts(t *testing.T, body []byte, field string) []float64 {
 func TestHandler_EventCountsSummedPerSubject(t *testing.T) {
 	// A node reporting to both PuppetDBs has its run counted in each; the
 	// merged view is the sum, not two rows.
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[eventCountsPath] = `[` + eventCount("h1", 4, 3, 1, 0) + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[eventCountsPath] = `[` + eventCount("h1", 2, 1, 0, 0) + `,` + eventCount("h2", 5, 0, 0, 0) + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[eventCountsPath] = `[` + eventCount("h1", 4, 3, 1, 0) + `]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[eventCountsPath] = `[` + eventCount("h1", 2, 1, 0, 0) + `,` + eventCount("h2", 5, 0, 0, 0) + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), eventCountsPath, url.Values{
 		"query":        {`["=","certname","h1"]`},
@@ -634,7 +667,7 @@ func TestHandler_EventCountsSummedPerSubject(t *testing.T) {
 	if rec.Code != http.StatusOK {
 		t.Fatalf("status %d: %s", rec.Code, rec.Body.String())
 	}
-	// Precedence puts new (prefer) first, so h1 leads.
+	// Rows come out in configured backend order, so h1 leads.
 	if got := counts(t, rec.Body.Bytes(), "successes"); !slices.Equal(got, []float64{6, 5}) {
 		t.Errorf("successes = %v, want [6 5]", got)
 	}
@@ -644,18 +677,18 @@ func TestHandler_EventCountsSummedPerSubject(t *testing.T) {
 }
 
 func TestHandler_EventCountsDisjointSubjectsPassThrough(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[eventCountsPath] = `[` + eventCount("h1", 1, 0, 0, 0) + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[eventCountsPath] = `[` + eventCount("h2", 2, 0, 0, 0) + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[eventCountsPath] = `[` + eventCount("h1", 1, 0, 0, 0) + `]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[eventCountsPath] = `[` + eventCount("h2", 2, 0, 0, 0) + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), eventCountsPath, url.Values{"summarize_by": {"certname"}})
-	if got := counts(t, rec.Body.Bytes(), "successes"); !slices.Equal(got, []float64{2, 1}) {
-		t.Errorf("successes = %v, want [2 1] (both nodes, untouched)", got)
+	if got := counts(t, rec.Body.Bytes(), "successes"); !slices.Equal(got, []float64{1, 2}) {
+		t.Errorf("successes = %v, want [1 2] (both nodes, untouched)", got)
 	}
 	// summarize_by must reach the backends verbatim.
-	for name, fb := range map[string]*fakeBackend{"old": old, "new": nw} {
+	for name, fb := range map[string]*fakeBackend{"a": a, "b": b} {
 		p, _ := fb.params(eventCountsPath)
 		if p.Get("summarize_by") != "certname" {
 			t.Errorf("%s backend got summarize_by=%q, want certname", name, p.Get("summarize_by"))
@@ -666,13 +699,13 @@ func TestHandler_EventCountsDisjointSubjectsPassThrough(t *testing.T) {
 func TestHandler_EventCountsRecordsIsMergedRowCount(t *testing.T) {
 	// Each backend reports one row; they share a subject, so the merged total
 	// is one — not the two the backends' own X-Records add up to.
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[eventCountsPath] = `[` + eventCount("h1", 1, 0, 0, 0) + `]`
-	old.totals[eventCountsPath] = 1
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[eventCountsPath] = `[` + eventCount("h1", 2, 0, 0, 0) + `]`
-	nw.totals[eventCountsPath] = 1
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[eventCountsPath] = `[` + eventCount("h1", 1, 0, 0, 0) + `]`
+	a.totals[eventCountsPath] = 1
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[eventCountsPath] = `[` + eventCount("h1", 2, 0, 0, 0) + `]`
+	b.totals[eventCountsPath] = 1
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), eventCountsPath, url.Values{
 		"summarize_by":  {"certname"},
@@ -684,13 +717,13 @@ func TestHandler_EventCountsRecordsIsMergedRowCount(t *testing.T) {
 }
 
 func TestHandler_AggregateEventCountsSummed(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[aggregateEventCountsPath] =
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[aggregateEventCountsPath] =
 		`[{"successes":2,"failures":1,"noops":0,"skips":3,"total":6,"summarize_by":"certname"}]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[aggregateEventCountsPath] =
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[aggregateEventCountsPath] =
 		`[{"successes":5,"failures":4,"noops":1,"skips":0,"total":10,"summarize_by":"certname"}]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), aggregateEventCountsPath, url.Values{"summarize_by": {"certname"}})
 	if rec.Code != http.StatusOK {
@@ -715,13 +748,13 @@ func TestHandler_AggregateEventCountsSummed(t *testing.T) {
 func TestHandler_AggregateEventCountsNullColumnSurvives(t *testing.T) {
 	// PuppetDB returns null totals for an empty result set; summing must not
 	// crash or blank out the backend that does have numbers.
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[aggregateEventCountsPath] =
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[aggregateEventCountsPath] =
 		`[{"successes":null,"failures":null,"total":null,"summarize_by":"certname"}]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[aggregateEventCountsPath] =
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[aggregateEventCountsPath] =
 		`[{"successes":3,"failures":0,"total":3,"summarize_by":"certname"}]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), aggregateEventCountsPath, url.Values{"summarize_by": {"certname"}})
 	if rec.Code != http.StatusOK {
@@ -737,11 +770,11 @@ const statusCountQuery = `["extract",[["function","count"],"status"],["~","certn
 func TestHandler_ReportsAggregateSummed(t *testing.T) {
 	// Puppetboard's daily-reports chart: each backend counts only its own
 	// reports, so the merged chart needs the per-status sums.
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[{"count":4,"status":"changed"},{"count":2,"status":"failed"}]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[{"count":3,"status":"changed"},{"count":9,"status":"unchanged"}]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[{"count":4,"status":"changed"},{"count":2,"status":"failed"}]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[{"count":3,"status":"changed"},{"count":9,"status":"unchanged"}]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, url.Values{"query": {statusCountQuery}})
 	if rec.Code != http.StatusOK {
@@ -764,13 +797,13 @@ func TestHandler_ReportsAggregateSummed(t *testing.T) {
 }
 
 func TestHandler_ReportsAggregateRecordsIsMergedRowCount(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[{"count":4,"status":"changed"}]`
-	old.totals[reportsPath] = 1
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[{"count":3,"status":"changed"}]`
-	nw.totals[reportsPath] = 1
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[{"count":4,"status":"changed"}]`
+	a.totals[reportsPath] = 1
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[{"count":3,"status":"changed"}]`
+	b.totals[reportsPath] = 1
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, url.Values{
 		"query":         {statusCountQuery},
@@ -784,11 +817,11 @@ func TestHandler_ReportsAggregateRecordsIsMergedRowCount(t *testing.T) {
 func TestHandler_ReportsNonAggregateStillUnioned(t *testing.T) {
 	// An extract with no function is a projection of real reports, so the
 	// union — not a sum — is still the right merge.
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.bodies[reportsPath] = `[` + report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[reportsPath] = `[` + report("h1", "r2", "2026-07-02T00:00:00Z") + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[reportsPath] = `[` + report("h1", "r1", "2026-07-01T00:00:00Z") + `]`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[reportsPath] = `[` + report("h1", "r2", "2026-07-02T00:00:00Z") + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), reportsPath, url.Values{
 		"query":    {`["extract",["hash","certname"],["=","certname","h1"]]`},
@@ -800,11 +833,11 @@ func TestHandler_ReportsNonAggregateStillUnioned(t *testing.T) {
 }
 
 func TestHandler_EventCountsOneBackendDown(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	old.fail = true
-	nw := newFakeBackend(t, `[]`, `[]`)
-	nw.bodies[eventCountsPath] = `[` + eventCount("h1", 2, 0, 0, 0) + `]`
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[eventCountsPath] = `[` + eventCount("h1", 2, 0, 0, 0) + `]`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), eventCountsPath, url.Values{"summarize_by": {"certname"}})
 	if rec.Code != http.StatusOK {
@@ -816,9 +849,9 @@ func TestHandler_EventCountsOneBackendDown(t *testing.T) {
 }
 
 func TestHandler_EventCountsBadPagingParam(t *testing.T) {
-	old := newFakeBackend(t, `[]`, `[]`)
-	nw := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(old.srv.URL, nw.srv.URL, mergeStatic))
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGetParams(t, srv.Handler(), eventCountsPath, url.Values{"limit": {"lots"}})
 	if rec.Code != http.StatusBadRequest {