feat: cache merged /facts and /nodes in memory, stale on backend failure

## Why
A busy Puppetboard re-fans-out the same /facts query every few seconds, and a
502 is worse than 30-second-old facts when every PuppetDB is unreachable.

## How
- Add a `Cache` interface (get reports fresh/stale/miss, put, stats) keyed on
  `<path>?<params>` with keys and repeated values sorted, plus a no-op default
  so uncached paths behave exactly as before.
- Route serveMerged/serveUnion/serveSummed through `serveCached`, so the
  reports cache drops in at `cacheFor` without touching a handler.
- Back /facts and /nodes with a byte-bounded LRU: `facts_ttl` (default 30s,
  clamped to a 30s cap) and `facts_cache_bytes` (default 64 MiB); expired
  entries are kept and served only when every backend fails.
- Single-flight identical keys so N concurrent requests cause one fan-out.
- Surface `cache` state and `serving_stale` in /healthz and the cache settings
  in `config show`.

README.md

@@ -32,7 +32,7 @@ not PQL) is forwarded verbatim.
 | `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 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. |
+| `GET /healthz` | Per-backend reachability plus cache state. `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
 surviving backends' results and logs a warning; a merged endpoint only returns `502` when
@@ -93,6 +93,39 @@ Each backend applies `order_by`/`limit`/`offset` to its own slice only, so
 - A malformed `limit`, `offset` or `order_by` gets a `400` rather than being
   forwarded.
 
+## Caching
+
+`pdbmux` caches merged `/nodes` and `/facts` responses **in memory** so a busy
+Puppetboard does not re-fan-out the same query every few seconds. Everything else
+runs uncached; the cache is an interface, and `/reports` gets its own (S3-backed)
+backend later without further handler changes.
+
+- **Key** — `<path>?<params>`, where the params are the ones that actually
+  determine the response, URL-encoded with keys sorted ascending and a repeated
+  param's values sorted ascending. Param order in the request is therefore
+  irrelevant: one canonical key per distinct request. A request with no params
+  keys on the bare path.
+- **TTL** — `facts_ttl`, default `30s`, **hard cap `30s`**. A larger configured
+  value is **clamped** down to the cap, not rejected, so a stray env var cannot
+  crash-loop a container; `pdbmux config show` prints
+  `facts_ttl : 30s (clamped from 600s, cap 30s)` when that happens. `facts_ttl: 0`
+  disables the cache entirely and the merged endpoints behave exactly as before.
+- **Stale on failure only** — an expired entry is kept, not dropped. When the TTL
+  has passed `pdbmux` always re-queries the backends; the expired copy is served
+  **only** if every backend fails, which turns a `502` into slightly-old data. A
+  healthy backend is never shadowed by a stale entry.
+- **Bounded** — `facts_cache_bytes` (default 64 MiB) is a byte budget, evicted
+  least-recently-used; reads count as use, so a stale entry that is still being
+  asked for survives. A single response larger than the whole budget is not
+  cached at all.
+- **Single-flight** — concurrent requests for the same key collapse into one
+  upstream fan-out; the rest wait for it and share the result.
+- **Visibility** — `/healthz` carries a `cache` object: `backend`
+  (`memory`/`none`), `ttl`, `entries`, `stale_entries`, `bytes`, `serving_stale`,
+  `stale_served` and `last_stale_served`. `serving_stale` is `true` from the
+  moment a stale fallback is served until the next response comes from a live
+  fan-out or a fresh entry.
+
 ## Config
 
 Precedence (lowest → highest): **defaults < config file < env vars (`PDBMUX_*`) < flags**.
@@ -114,9 +147,11 @@ backends:                          # order is a tie-break only, not a ranking
     url: http://puppetdb1.example.com:8080
   - name: pdb-b
     url: https://puppetdb2.example.com
-merge: freshness      # freshness | static
+merge: freshness             # freshness | static
-timeout: 10s          # per-upstream request timeout
+timeout: 10s                 # per-upstream request timeout
-freshness_ttl: 30s    # freshness-map cache TTL (freshness merge only)
+freshness_ttl: 30s           # freshness-map cache TTL (freshness merge only)
+facts_ttl: 30s               # /facts + /nodes response cache TTL; 0 disables, capped at 30s
+facts_cache_bytes: 67108864  # byte budget for that cache (64 MiB), LRU-evicted
 ```
 
 `backends[*].url` is a **base** URL (`scheme://host[:port]`); `pdbmux` appends
@@ -129,6 +164,8 @@ the `/pdb/query/v4/...` path per request.
 | `PDBMUX_MERGE` | `merge` |
 | `PDBMUX_TIMEOUT` | `timeout` (Go duration, e.g. `10s`) |
 | `PDBMUX_FRESHNESS_TTL` | `freshness_ttl` |
+| `PDBMUX_FACTS_TTL` | `facts_ttl` (clamped to 30s) |
+| `PDBMUX_FACTS_CACHE_BYTES` | `facts_cache_bytes` (plain integer bytes) |
 | `PDBMUX_BACKENDS` | whole backend list, as `name=url,name=url` |
 
 Flags: `--config`, `--listen`, `--merge`.
@@ -162,5 +199,6 @@ A static (`CGO_ENABLED=0`) binary on a distroless base. Configure it with
 `PDBMUX_*` env vars (at minimum `PDBMUX_BACKENDS`), or mount a config file — a
 configmap at `/etc/pdbmux/config.yaml` is picked up with no env var at all, and
 any other mount path works via `PDBMUX_CONFIG`. Env vars still override file
-values, so the two mix. Stateless, so run as many replicas as you like; use
+values, so the two mix. Run as many replicas as you like — the only state is the
-`/healthz` for liveness/readiness probes.
+in-memory cache, which is per-replica and bounded by `facts_cache_bytes`, so size
+the memory limit above it. Use `/healthz` for liveness/readiness probes.

cache.go

@@ -0,0 +1,248 @@
+package main
+
+import (
+	"container/list"
+	"context"
+	"net/url"
+	"sort"
+	"sync"
+	"time"
+)
+
+// CacheStatus distinguishes the three outcomes of a lookup: nothing stored, a
+// stored entry within its TTL, and a stored entry past it.
+type CacheStatus int
+
+const (
+	CacheMiss CacheStatus = iota
+	CacheFresh
+	CacheStale
+)
+
+func (s CacheStatus) String() string {
+	switch s {
+	case CacheFresh:
+		return "fresh"
+	case CacheStale:
+		return "stale"
+	default:
+		return "miss"
+	}
+}
+
+// CacheEntry is a stored response body and the time it was stored.
+type CacheEntry struct {
+	Body     []byte
+	StoredAt time.Time
+}
+
+// CacheStats is the cache state reported by /healthz.
+type CacheStats struct {
+	Backend      string `json:"backend"`
+	Entries      int    `json:"entries"`
+	StaleEntries int    `json:"stale_entries"`
+	Bytes        int64  `json:"bytes"`
+}
+
+// Cache stores merged responses keyed by cacheKey. Get reports freshness rather
+// than hiding expired entries so a caller can fall back to a stale body when the
+// upstream fetch fails. The context and error exist for out-of-process backends
+// (the reports cache lands on S3); an in-process backend ignores both.
+//
+// A Body handed back by Get aliases the cache's copy and must not be mutated.
+type Cache interface {
+	Get(ctx context.Context, key string) (CacheEntry, CacheStatus, error)
+	Put(ctx context.Context, key string, body []byte) error
+	Stats() CacheStats
+}
+
+// cacheKey is the request path when there are no params, else "<path>?<params>"
+// where params is url.Values.Encode() over a copy whose repeated values have
+// been sorted. Encode() already sorts keys ascending, so both the order params
+// arrive in and the order of a repeated param's values are irrelevant to the
+// key: one canonical string per distinct request.
+func cacheKey(path string, params url.Values) string {
+	if len(params) == 0 {
+		return path
+	}
+	norm := make(url.Values, len(params))
+	for k, vs := range params {
+		sorted := append([]string(nil), vs...)
+		sort.Strings(sorted)
+		norm[k] = sorted
+	}
+	encoded := norm.Encode()
+	if encoded == "" {
+		return path
+	}
+	return path + "?" + encoded
+}
+
+// noopCache is the default for every path with no cache configured, so wiring a
+// handler through the cache leaves its behaviour unchanged.
+type noopCache struct{}
+
+func (noopCache) Get(context.Context, string) (CacheEntry, CacheStatus, error) {
+	return CacheEntry{}, CacheMiss, nil
+}
+
+func (noopCache) Put(context.Context, string, []byte) error { return nil }
+
+func (noopCache) Stats() CacheStats { return CacheStats{Backend: "none"} }
+
+type memoryEntry struct {
+	key      string
+	body     []byte
+	storedAt time.Time
+}
+
+// memoryCache is a byte-bounded LRU. Expired entries are kept, not dropped, so
+// they remain available as a stale fallback; only the byte budget evicts.
+type memoryCache struct {
+	ttl      time.Duration
+	maxBytes int64
+	now      func() time.Time
+
+	mu    sync.Mutex
+	ll    *list.List // front = most recently used
+	items map[string]*list.Element
+	bytes int64
+}
+
+func newMemoryCache(ttl time.Duration, maxBytes int64) *memoryCache {
+	return &memoryCache{
+		ttl:      ttl,
+		maxBytes: maxBytes,
+		now:      time.Now,
+		ll:       list.New(),
+		items:    make(map[string]*list.Element),
+	}
+}
+
+func (c *memoryCache) Get(_ context.Context, key string) (CacheEntry, CacheStatus, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	el, ok := c.items[key]
+	if !ok {
+		return CacheEntry{}, CacheMiss, nil
+	}
+	c.ll.MoveToFront(el)
+	e := el.Value.(*memoryEntry)
+	status := CacheFresh
+	if c.now().Sub(e.storedAt) >= c.ttl {
+		status = CacheStale
+	}
+	return CacheEntry{Body: e.body, StoredAt: e.storedAt}, status, nil
+}
+
+func (c *memoryCache) Put(_ context.Context, key string, body []byte) error {
+	// A response larger than the whole budget would evict everything else.
+	if int64(len(body)) > c.maxBytes {
+		return nil
+	}
+	stored := append([]byte(nil), body...)
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if el, ok := c.items[key]; ok {
+		e := el.Value.(*memoryEntry)
+		c.bytes += int64(len(stored)) - int64(len(e.body))
+		e.body, e.storedAt = stored, c.now()
+		c.ll.MoveToFront(el)
+	} else {
+		c.items[key] = c.ll.PushFront(&memoryEntry{key: key, body: stored, storedAt: c.now()})
+		c.bytes += int64(len(stored))
+	}
+	for c.bytes > c.maxBytes {
+		back := c.ll.Back()
+		if back == nil {
+			break
+		}
+		e := c.ll.Remove(back).(*memoryEntry)
+		delete(c.items, e.key)
+		c.bytes -= int64(len(e.body))
+	}
+	return nil
+}
+
+func (c *memoryCache) Stats() CacheStats {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	st := CacheStats{Backend: "memory", Entries: len(c.items), Bytes: c.bytes}
+	now := c.now()
+	for el := c.ll.Front(); el != nil; el = el.Next() {
+		if now.Sub(el.Value.(*memoryEntry).storedAt) >= c.ttl {
+			st.StaleEntries++
+		}
+	}
+	return st
+}
+
+// flightGroup collapses concurrent identical builds so N simultaneous requests
+// for one key cause one upstream fan-out.
+type flightGroup struct {
+	mu    sync.Mutex
+	calls map[string]*flightCall
+}
+
+type flightCall struct {
+	wg   sync.WaitGroup
+	resp cachedResponse
+	err  error
+}
+
+// Do returns fn's result and whether this caller shared another's in-flight run.
+func (g *flightGroup) Do(key string, fn func() (cachedResponse, error)) (cachedResponse, error, bool) {
+	g.mu.Lock()
+	if g.calls == nil {
+		g.calls = make(map[string]*flightCall)
+	}
+	if c, ok := g.calls[key]; ok {
+		g.mu.Unlock()
+		c.wg.Wait()
+		return c.resp, c.err, true
+	}
+	c := &flightCall{}
+	c.wg.Add(1)
+	g.calls[key] = c
+	g.mu.Unlock()
+
+	defer func() {
+		c.wg.Done()
+		g.mu.Lock()
+		delete(g.calls, key)
+		g.mu.Unlock()
+	}()
+
+	c.resp, c.err = fn()
+	return c.resp, c.err, false
+}
+
+// staleTracker records stale fallbacks for /healthz. serving flips back to false
+// as soon as a response is served from a live fan-out or a fresh entry.
+type staleTracker struct {
+	mu      sync.Mutex
+	serving bool
+	served  uint64
+	last    time.Time
+}
+
+func (t *staleTracker) markStale(now time.Time) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.serving = true
+	t.served++
+	t.last = now
+}
+
+func (t *staleTracker) markFresh() {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.serving = false
+}
+
+func (t *staleTracker) snapshot() (serving bool, served uint64, last time.Time) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.serving, t.served, t.last
+}

cache_test.go

@@ -0,0 +1,595 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// fakeClock drives the cache's TTL without sleeping.
+type fakeClock struct {
+	mu sync.Mutex
+	t  time.Time
+}
+
+func newFakeClock() *fakeClock { return &fakeClock{t: time.Unix(1_800_000_000, 0)} }
+
+func (c *fakeClock) now() time.Time {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.t
+}
+
+func (c *fakeClock) advance(d time.Duration) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.t = c.t.Add(d)
+}
+
+// countingBackend is a PuppetDB stand-in that counts requests per path and can
+// be made to fail or block, so cache hits and single-flight are observable.
+type countingBackend struct {
+	srv *httptest.Server
+
+	mu     sync.Mutex
+	hits   map[string]int
+	bodies map[string]string
+	fail   bool
+	block  chan struct{}
+}
+
+func newCountingBackend(t *testing.T, bodies map[string]string) *countingBackend {
+	t.Helper()
+	cb := &countingBackend{hits: map[string]int{}, bodies: bodies}
+	cb.srv = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cb.mu.Lock()
+		cb.hits[r.URL.Path]++
+		fail, block, body := cb.fail, cb.block, cb.bodies[r.URL.Path]
+		cb.mu.Unlock()
+
+		if block != nil {
+			<-block
+		}
+		if fail {
+			http.Error(w, "boom", http.StatusInternalServerError)
+			return
+		}
+		if body == "" {
+			body = "[]"
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = io.WriteString(w, body)
+	}))
+	t.Cleanup(cb.srv.Close)
+	return cb
+}
+
+func (cb *countingBackend) hitCount(path string) int {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	return cb.hits[path]
+}
+
+func (cb *countingBackend) setFail(v bool) {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	cb.fail = v
+}
+
+func (cb *countingBackend) setBody(path, body string) {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	cb.bodies[path] = body
+}
+
+func (cb *countingBackend) setBlock(ch chan struct{}) {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	cb.block = ch
+}
+
+// newCachedServer builds a server whose facts cache runs on a fake clock.
+func newCachedServer(t *testing.T, cfg Config) (*Server, *fakeClock) {
+	t.Helper()
+	srv := newTestServer(cfg)
+	mc, ok := srv.factsCache.(*memoryCache)
+	if !ok {
+		t.Fatalf("expected a memory cache, got %T", srv.factsCache)
+	}
+	clk := newFakeClock()
+	mc.now = clk.now
+	return srv, clk
+}
+
+func cacheTestConfig(aURL, bURL string) Config {
+	cfg := testConfig(aURL, bURL, mergeStatic)
+	cfg.FactsTTL = 30 * time.Second
+	cfg.CacheBytes = defaultCacheSize
+	return cfg
+}
+
+func TestCacheKey_CanonicalOrdering(t *testing.T) {
+	a := cacheKey(factsPath, url.Values{
+		"query":  {`["=","name","role"]`},
+		"limit":  {"10"},
+		"expand": {"z", "a"},
+	})
+	b := cacheKey(factsPath, url.Values{
+		"expand": {"a", "z"},
+		"limit":  {"10"},
+		"query":  {`["=","name","role"]`},
+	})
+	if a != b {
+		t.Errorf("param order must not change the key:\n %s\n %s", a, b)
+	}
+	if !strings.HasPrefix(a, factsPath+"?") {
+		t.Errorf("key must start with the path, got %q", a)
+	}
+	if got := cacheKey(nodesPath, nil); got != nodesPath {
+		t.Errorf("no params should give the bare path, got %q", got)
+	}
+	if cacheKey(factsPath, url.Values{"query": {"x"}}) == cacheKey(factsPath, url.Values{"query": {"y"}}) {
+		t.Error("different queries must not share a key")
+	}
+	if cacheKey(factsPath, url.Values{"query": {"x"}}) == cacheKey(nodesPath, url.Values{"query": {"x"}}) {
+		t.Error("different paths must not share a key")
+	}
+}
+
+func TestMemoryCache_FreshThenStale(t *testing.T) {
+	c := newMemoryCache(30*time.Second, 1<<20)
+	clk := newFakeClock()
+	c.now = clk.now
+
+	if _, status, _ := c.Get(context.Background(), "k"); status != CacheMiss {
+		t.Fatalf("empty cache should miss, got %s", status)
+	}
+	if err := c.Put(context.Background(), "k", []byte(`{"body":[]}`)); err != nil {
+		t.Fatal(err)
+	}
+
+	ent, status, err := c.Get(context.Background(), "k")
+	if err != nil || status != CacheFresh || string(ent.Body) != `{"body":[]}` {
+		t.Fatalf("want a fresh hit, got %s %q (%v)", status, ent.Body, err)
+	}
+
+	clk.advance(29 * time.Second)
+	if _, status, _ := c.Get(context.Background(), "k"); status != CacheFresh {
+		t.Fatalf("inside the TTL should still be fresh, got %s", status)
+	}
+
+	clk.advance(2 * time.Second)
+	ent, status, _ = c.Get(context.Background(), "k")
+	if status != CacheStale {
+		t.Fatalf("past the TTL should be stale, got %s", status)
+	}
+	if string(ent.Body) != `{"body":[]}` {
+		t.Errorf("a stale entry must still carry its body, got %q", ent.Body)
+	}
+	if st := c.Stats(); st.Entries != 1 || st.StaleEntries != 1 {
+		t.Errorf("stats should report 1 entry, 1 stale, got %+v", st)
+	}
+}
+
+func TestMemoryCache_EvictsLeastRecentlyUsed(t *testing.T) {
+	body := []byte("0123456789") // 10 bytes
+	c := newMemoryCache(time.Minute, 25)
+	ctx := context.Background()
+
+	for _, k := range []string{"a", "b"} {
+		if err := c.Put(ctx, k, body); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// Reading "a" makes "b" the eviction candidate.
+	if _, status, _ := c.Get(ctx, "a"); status != CacheFresh {
+		t.Fatalf("a should be cached, got %s", status)
+	}
+	if err := c.Put(ctx, "c", body); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, status, _ := c.Get(ctx, "b"); status != CacheMiss {
+		t.Errorf("b was least recently used and should have been evicted, got %s", status)
+	}
+	for _, k := range []string{"a", "c"} {
+		if _, status, _ := c.Get(ctx, k); status != CacheFresh {
+			t.Errorf("%s should have survived eviction, got %s", k, status)
+		}
+	}
+	if st := c.Stats(); st.Entries != 2 || st.Bytes != 20 {
+		t.Errorf("stats after eviction = %+v, want 2 entries / 20 bytes", st)
+	}
+}
+
+func TestMemoryCache_SkipsOversizedEntry(t *testing.T) {
+	c := newMemoryCache(time.Minute, 8)
+	if err := c.Put(context.Background(), "big", []byte("123456789")); err != nil {
+		t.Fatal(err)
+	}
+	if _, status, _ := c.Get(context.Background(), "big"); status != CacheMiss {
+		t.Error("an entry larger than the whole budget must not be stored")
+	}
+	if st := c.Stats(); st.Bytes != 0 {
+		t.Errorf("bytes = %d, want 0", st.Bytes)
+	}
+}
+
+func TestNoopCache_AlwaysMisses(t *testing.T) {
+	var c Cache = noopCache{}
+	if err := c.Put(context.Background(), "k", []byte("x")); err != nil {
+		t.Fatal(err)
+	}
+	if _, status, _ := c.Get(context.Background(), "k"); status != CacheMiss {
+		t.Errorf("noop cache must always miss, got %s", status)
+	}
+	if st := c.Stats(); st.Backend != "none" {
+		t.Errorf("backend = %q, want none", st.Backend)
+	}
+}
+
+func TestHandler_FactsCacheFreshHit(t *testing.T) {
+	body := `[` + fact("h1", "role", "web", "") + `]`
+	a := newCountingBackend(t, map[string]string{factsPath: body})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	first := doGet(t, srv.Handler(), factsPath, `["=","name","role"]`)
+	second := doGet(t, srv.Handler(), factsPath, `["=","name","role"]`)
+
+	if first.Code != http.StatusOK || second.Code != http.StatusOK {
+		t.Fatalf("statuses %d/%d", first.Code, second.Code)
+	}
+	if first.Body.String() != second.Body.String() {
+		t.Errorf("cache hit changed the body:\n %s\n %s", first.Body.String(), second.Body.String())
+	}
+	if got := a.hitCount(factsPath); got != 1 {
+		t.Errorf("backend a saw %d requests, want 1 (second served from cache)", got)
+	}
+	if got := b.hitCount(factsPath); got != 1 {
+		t.Errorf("backend b saw %d requests, want 1", got)
+	}
+
+	// A different query is a different key and must go upstream.
+	doGet(t, srv.Handler(), factsPath, `["=","name","osfamily"]`)
+	if got := a.hitCount(factsPath); got != 2 {
+		t.Errorf("a different query should refetch: %d requests, want 2", got)
+	}
+}
+
+func TestHandler_FactsCacheExpires(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "web", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	doGet(t, srv.Handler(), factsPath, "")
+	clk.advance(31 * time.Second)
+	doGet(t, srv.Handler(), factsPath, "")
+
+	if got := a.hitCount(factsPath); got != 2 {
+		t.Errorf("an expired entry should refetch: %d requests, want 2", got)
+	}
+}
+
+func TestHandler_ServesStaleOnlyWhenBackendsFail(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "old", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	warm := doGet(t, srv.Handler(), factsPath, "")
+	if warm.Code != http.StatusOK {
+		t.Fatalf("warm-up status %d", warm.Code)
+	}
+	if !strings.Contains(warm.Body.String(), `"old"`) {
+		t.Fatalf("warm-up body = %s", warm.Body.String())
+	}
+
+	// Backends healthy but the entry expired: fresh data wins, never the stale copy.
+	clk.advance(31 * time.Second)
+	a.setBody(factsPath, `[`+fact("h1", "role", "new", "")+`]`)
+	refetch := doGet(t, srv.Handler(), factsPath, "")
+	if !strings.Contains(refetch.Body.String(), `"new"`) {
+		t.Errorf("a healthy backend must not be shadowed by the stale entry: %s", refetch.Body.String())
+	}
+	if serving, _, _ := srv.stale.snapshot(); serving {
+		t.Error("serving_stale must stay false while backends are healthy")
+	}
+
+	// Every backend down and the entry expired: the stale copy is served.
+	clk.advance(31 * time.Second)
+	a.setFail(true)
+	b.setFail(true)
+	stale := doGet(t, srv.Handler(), factsPath, "")
+	if stale.Code != http.StatusOK {
+		t.Fatalf("stale fallback status %d: %s", stale.Code, stale.Body.String())
+	}
+	if !strings.Contains(stale.Body.String(), `"new"`) {
+		t.Errorf("stale body = %s, want the last cached copy", stale.Body.String())
+	}
+
+	serving, served, last := srv.stale.snapshot()
+	if !serving || served != 1 || last.IsZero() {
+		t.Errorf("stale tracker = %v/%d/%v, want serving=true served=1", serving, served, last)
+	}
+}
+
+func TestHandler_NoCacheEntryMeansBackendFailureIs502(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	a.setFail(true)
+	b.setFail(true)
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	rec := doGet(t, srv.Handler(), factsPath, "")
+	if rec.Code != http.StatusBadGateway {
+		t.Fatalf("status %d, want 502", rec.Code)
+	}
+	if !strings.Contains(rec.Body.String(), "all backends failed") {
+		t.Errorf("body = %q", rec.Body.String())
+	}
+}
+
+func TestHandler_SingleFlightCollapsesConcurrentRequests(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "web", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+	h := srv.Handler()
+
+	release := make(chan struct{})
+	a.setBlock(release)
+	b.setBlock(release)
+
+	const callers = 16
+	var wg sync.WaitGroup
+	codes := make([]int, callers)
+	for i := range callers {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			codes[i] = doGet(t, h, factsPath, "").Code
+		}(i)
+	}
+
+	// The leader is parked inside the blocked backend, so every caller that
+	// reaches the handler before the release joins its flight.
+	waitFor(t, func() bool { return a.hitCount(factsPath) >= 1 })
+	time.Sleep(250 * time.Millisecond)
+	close(release)
+	wg.Wait()
+
+	for i, code := range codes {
+		if code != http.StatusOK {
+			t.Fatalf("caller %d got %d", i, code)
+		}
+	}
+	if got := a.hitCount(factsPath); got != 1 {
+		t.Errorf("backend a saw %d requests, want 1 for %d concurrent callers", got, callers)
+	}
+	if got := b.hitCount(factsPath); got != 1 {
+		t.Errorf("backend b saw %d requests, want 1 for %d concurrent callers", got, callers)
+	}
+}
+
+func TestFlightGroup_LeaderRunsOnce(t *testing.T) {
+	var g flightGroup
+	var calls, shared atomic.Int64
+	entered := make(chan struct{})
+	release := make(chan struct{})
+
+	run := func() (cachedResponse, error) {
+		if calls.Add(1) == 1 {
+			close(entered)
+		}
+		<-release
+		return cachedResponse{Body: json.RawMessage(`[]`), Records: -1}, nil
+	}
+
+	var wg sync.WaitGroup
+	for range 16 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, _, s := g.Do("k", run); s {
+				shared.Add(1)
+			}
+		}()
+	}
+	<-entered
+	time.Sleep(250 * time.Millisecond)
+	close(release)
+	wg.Wait()
+
+	if calls.Load() != 1 {
+		t.Errorf("fn ran %d times, want 1", calls.Load())
+	}
+	if shared.Load() != 15 {
+		t.Errorf("%d callers shared the flight, want 15", shared.Load())
+	}
+	// The key is released once the flight finishes.
+	if _, _, s := g.Do("k", func() (cachedResponse, error) { return cachedResponse{Records: -1}, nil }); s {
+		t.Error("a later call must start its own flight")
+	}
+}
+
+func TestHandler_HealthzReportsCacheState(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "web", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	hr := health(t, srv)
+	if hr.Cache.Backend != "memory" || hr.Cache.TTL != "30s" {
+		t.Fatalf("cache health = %+v, want memory/30s", hr.Cache)
+	}
+	if hr.Cache.Entries != 0 || hr.Cache.ServingStale {
+		t.Fatalf("a cold cache should be empty and not stale: %+v", hr.Cache)
+	}
+
+	doGet(t, srv.Handler(), factsPath, "")
+	hr = health(t, srv)
+	if hr.Cache.Entries != 1 || hr.Cache.StaleEntries != 0 || hr.Cache.Bytes == 0 {
+		t.Fatalf("after one request: %+v", hr.Cache)
+	}
+
+	clk.advance(31 * time.Second)
+	if hr = health(t, srv); hr.Cache.StaleEntries != 1 {
+		t.Fatalf("expired entry should count as stale: %+v", hr.Cache)
+	}
+	if hr.Cache.ServingStale {
+		t.Error("an expired entry alone is not serving_stale")
+	}
+
+	a.setFail(true)
+	b.setFail(true)
+	doGet(t, srv.Handler(), factsPath, "")
+	hr = health(t, srv)
+	if !hr.Cache.ServingStale || hr.Cache.StaleServed != 1 || hr.Cache.LastStale == "" {
+		t.Errorf("staleness not surfaced in /healthz: %+v", hr.Cache)
+	}
+	if hr.Status != "down" {
+		t.Errorf("status = %q, want down", hr.Status)
+	}
+}
+
+func TestHandler_HealthzReportsDisabledCache(t *testing.T) {
+	a := newCountingBackend(t, nil)
+	b := newCountingBackend(t, nil)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic)) // FactsTTL zero
+
+	hr := health(t, srv)
+	if hr.Cache.Backend != "none" || hr.Cache.TTL != "0" {
+		t.Errorf("disabled cache health = %+v, want none/0", hr.Cache)
+	}
+}
+
+func TestHandler_UncachedPathsStillRefetch(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{reportsPath: `[{"hash":"h"}]`})
+	b := newCountingBackend(t, map[string]string{reportsPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	doGet(t, srv.Handler(), reportsPath, "")
+	doGet(t, srv.Handler(), reportsPath, "")
+	if got := a.hitCount(reportsPath); got != 2 {
+		t.Errorf("/reports is uncached: %d requests, want 2", got)
+	}
+}
+
+func TestConfig_FactsTTLClampedAtCap(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("XDG_CONFIG_HOME", dir)
+	clearEnv(t)
+
+	path := filepath.Join(dir, appName, configFileName)
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte("facts_ttl: 5m\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.FactsTTL != maxFactsTTL {
+		t.Errorf("file facts_ttl = %s, want it clamped to %s", cfg.FactsTTL, maxFactsTTL)
+	}
+
+	t.Setenv(envPrefix+"FACTS_TTL", "10m")
+	cfg, err = Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.FactsTTL != maxFactsTTL {
+		t.Errorf("env facts_ttl = %s, want it clamped to %s", cfg.FactsTTL, maxFactsTTL)
+	}
+	if cfg.factsTTLClamped != 10*time.Minute {
+		t.Errorf("pre-clamp value = %s, want 10m0s", cfg.factsTTLClamped)
+	}
+
+	out := captureStdout(t, func() { printConfig(cfg) })
+	if !strings.Contains(out, "facts_ttl    : 30s (clamped from 600s, cap 30s)") {
+		t.Errorf("config show must report the clamp, got:\n%s", out)
+	}
+
+	t.Setenv(envPrefix+"FACTS_TTL", "5s")
+	cfg, err = Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.FactsTTL != 5*time.Second || cfg.factsTTLClamped != 0 {
+		t.Errorf("a value under the cap must pass through: %s / %s", cfg.FactsTTL, cfg.factsTTLClamped)
+	}
+
+	t.Setenv(envPrefix+"FACTS_TTL", "0s")
+	cfg, err = Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.cacheEnabled() {
+		t.Error("facts_ttl 0 must disable the cache")
+	}
+	out = captureStdout(t, func() { printConfig(cfg) })
+	if !strings.Contains(out, "(cache disabled)") {
+		t.Errorf("config show must say the cache is off, got:\n%s", out)
+	}
+}
+
+func TestNewServer_ClampsFactsTTL(t *testing.T) {
+	cfg := testConfig("http://a.invalid", "http://b.invalid", mergeStatic)
+	cfg.FactsTTL = time.Hour
+	cfg.CacheBytes = 1 << 20
+	srv := newTestServer(cfg)
+
+	mc, ok := srv.factsCache.(*memoryCache)
+	if !ok {
+		t.Fatalf("expected a memory cache, got %T", srv.factsCache)
+	}
+	if mc.ttl != maxFactsTTL {
+		t.Errorf("cache ttl = %s, want %s", mc.ttl, maxFactsTTL)
+	}
+}
+
+func TestConfig_ValidateRejectsNegativeCacheSettings(t *testing.T) {
+	base := testConfig("http://a.invalid", "http://b.invalid", mergeStatic)
+	for name, mutate := range map[string]func(*Config){
+		"negative facts_ttl":         func(c *Config) { c.FactsTTL = -time.Second },
+		"negative facts_cache_bytes": func(c *Config) { c.CacheBytes = -1 },
+	} {
+		cfg := base
+		mutate(&cfg)
+		if err := cfg.Validate(); err == nil {
+			t.Errorf("%s should not validate", name)
+		}
+	}
+}
+
+func health(t *testing.T, srv *Server) healthReport {
+	t.Helper()
+	rec := doGet(t, srv.Handler(), "/healthz", "")
+	var hr healthReport
+	if err := json.Unmarshal(rec.Body.Bytes(), &hr); err != nil {
+		t.Fatalf("decode healthz: %v (%s)", err, rec.Body.String())
+	}
+	return hr
+}
+
+func waitFor(t *testing.T, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for !cond() {
+		if time.Now().After(deadline) {
+			t.Fatal("condition not met within 5s")
+		}
+		time.Sleep(time.Millisecond)
+	}
+}

config.go

@@ -25,6 +25,13 @@ const (
 
 	defaultTimeout      = 10 * time.Second
 	defaultFreshnessTTL = 30 * time.Second
+
+	// maxFactsTTL is a hard cap, not a default: a larger configured value is
+	// clamped down to it rather than rejected, so a stray env var cannot make a
+	// container crash-loop.
+	maxFactsTTL      = 30 * time.Second
+	defaultFactsTTL  = 30 * time.Second
+	defaultCacheSize = int64(64 << 20)
 )
 
 var exampleBackends = []Backend{
@@ -43,8 +50,11 @@ type Config struct {
 	Merge        string        `yaml:"merge"`
 	Timeout      time.Duration `yaml:"timeout"`
 	FreshnessTTL time.Duration `yaml:"freshness_ttl"`
+	FactsTTL     time.Duration `yaml:"facts_ttl"`         // 0 disables the /facts+/nodes cache
+	CacheBytes   int64         `yaml:"facts_cache_bytes"` // byte budget for that cache
 
-	sourcePath string // file this config was read from, empty if none was found
+	sourcePath      string        // file this config was read from, empty if none was found
+	factsTTLClamped time.Duration // pre-clamp facts_ttl, zero when nothing was clamped
 }
 
 // SourcePath returns the config file Load read, or "" when none was loaded.
@@ -61,6 +71,8 @@ func DefaultConfig() Config {
 		Merge:        mergeFreshness,
 		Timeout:      defaultTimeout,
 		FreshnessTTL: defaultFreshnessTTL,
+		FactsTTL:     defaultFactsTTL,
+		CacheBytes:   defaultCacheSize,
 	}
 }
 
@@ -139,9 +151,19 @@ func Load(flagPath string) (Config, error) {
 	}
 
 	applyEnv(&cfg, os.Getenv)
+	cfg.clampFactsTTL()
 	return cfg, nil
 }
 
+// clampFactsTTL pins facts_ttl to maxFactsTTL, remembering the configured value
+// so `config show` can say the cap was applied.
+func (c *Config) clampFactsTTL() {
+	if c.FactsTTL > maxFactsTTL {
+		c.factsTTLClamped = c.FactsTTL
+		c.FactsTTL = maxFactsTTL
+	}
+}
+
 func applyEnv(cfg *Config, getenv func(string) string) {
 	if v := getenv(envPrefix + "LISTEN"); v != "" {
 		cfg.Listen = v
@@ -159,6 +181,16 @@ func applyEnv(cfg *Config, getenv func(string) string) {
 			cfg.FreshnessTTL = d
 		}
 	}
+	if v := getenv(envPrefix + "FACTS_TTL"); v != "" {
+		if d, err := time.ParseDuration(v); err == nil {
+			cfg.FactsTTL = d
+		}
+	}
+	if v := getenv(envPrefix + "FACTS_CACHE_BYTES"); v != "" {
+		if n, err := strconv.ParseInt(v, 10, 64); err == nil {
+			cfg.CacheBytes = n
+		}
+	}
 	if v := getenv(envPrefix + "BACKENDS"); v != "" {
 		if bs := parseBackends(v); len(bs) > 0 {
 			cfg.Backends = bs
@@ -216,9 +248,19 @@ func (c Config) Validate() error {
 	if c.Timeout <= 0 {
 		return fmt.Errorf("timeout must be positive")
 	}
+	if c.FactsTTL < 0 {
+		return fmt.Errorf("facts_ttl must not be negative (0 disables the cache)")
+	}
+	if c.CacheBytes < 0 {
+		return fmt.Errorf("facts_cache_bytes must not be negative")
+	}
 	return nil
 }
 
+// cacheEnabled reports whether a facts/nodes cache should be built: both a TTL
+// and a byte budget are required.
+func (c Config) cacheEnabled() bool { return c.FactsTTL > 0 && c.CacheBytes > 0 }
+
 func writeDefaultConfig(path string) error {
 	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return fmt.Errorf("creating config dir: %w", err)
@@ -231,7 +273,10 @@ func writeDefaultConfig(path string) error {
 		"# 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_MERGE, PDBMUX_TIMEOUT,\n" +
-		"#   PDBMUX_FRESHNESS_TTL, PDBMUX_BACKENDS (name=url,name=url).\n\n")
+		"#   PDBMUX_FRESHNESS_TTL, PDBMUX_FACTS_TTL, PDBMUX_FACTS_CACHE_BYTES,\n" +
+		"#   PDBMUX_BACKENDS (name=url,name=url).\n" +
+		"# facts_ttl caches merged /facts and /nodes in memory; it is capped at 30s\n" +
+		"# (a larger value is clamped) and 0 disables the cache.\n\n")
 	if err := os.WriteFile(path, append(header, data...), 0o644); err != nil {
 		return fmt.Errorf("writing config: %w", err)
 	}

config_test.go

@@ -391,7 +391,7 @@ func captureStdout(t *testing.T, f func()) string {
 
 func clearEnv(t *testing.T) {
 	t.Helper()
-	for _, k := range []string{"CONFIG", "LISTEN", "MERGE", "TIMEOUT", "FRESHNESS_TTL", "BACKENDS"} {
+	for _, k := range []string{"CONFIG", "LISTEN", "MERGE", "TIMEOUT", "FRESHNESS_TTL", "FACTS_TTL", "FACTS_CACHE_BYTES", "BACKENDS"} {
 		t.Setenv(envPrefix+k, "")
 	}
 }

main.go

@@ -151,6 +151,18 @@ func runServer(cfg Config) error {
 	}
 }
 
+func factsTTLString(cfg Config) string {
+	s := durationString(cfg.FactsTTL)
+	switch {
+	case cfg.factsTTLClamped > 0:
+		return fmt.Sprintf("%s (clamped from %s, cap %s)",
+			s, durationString(cfg.factsTTLClamped), durationString(maxFactsTTL))
+	case !cfg.cacheEnabled():
+		return s + " (cache disabled)"
+	}
+	return s
+}
+
 func printConfig(cfg Config) {
 	if p := cfg.SourcePath(); p != "" {
 		fmt.Printf("config file  : %s (loaded)\n", p)
@@ -161,6 +173,8 @@ func printConfig(cfg Config) {
 	fmt.Printf("merge        : %s\n", cfg.Merge)
 	fmt.Printf("timeout      : %s\n", durationString(cfg.Timeout))
 	fmt.Printf("freshness_ttl: %s\n", durationString(cfg.FreshnessTTL))
+	fmt.Printf("facts_ttl    : %s\n", factsTTLString(cfg))
+	fmt.Printf("facts_cache  : %d bytes\n", cfg.CacheBytes)
 	fmt.Println("backends:")
 	for _, b := range cfg.Backends {
 		fmt.Printf("  - %-8s %s\n", b.Name, b.URL)

server.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -34,11 +35,18 @@ type backendResult struct {
 	err     error
 }
 
+var errAllBackendsFailed = errors.New("all backends failed")
+
 type Server struct {
 	cfg    Config
 	client *http.Client
 	log    *log.Logger
 
+	// factsCache is nil when caching is disabled; cacheFor hands out a noop then.
+	factsCache Cache
+	flights    flightGroup
+	stale      staleTracker
+
 	// freshness cache (freshness merge only).
 	mu        sync.Mutex
 	freshData freshness
@@ -46,11 +54,29 @@ type Server struct {
 }
 
 func NewServer(cfg Config, logger *log.Logger) *Server {
-	return &Server{
+	cfg.clampFactsTTL()
+	s := &Server{
 		cfg:    cfg,
 		client: &http.Client{Timeout: cfg.Timeout},
 		log:    logger,
 	}
+	if cfg.cacheEnabled() {
+		s.factsCache = newMemoryCache(cfg.FactsTTL, cfg.CacheBytes)
+	}
+	return s
+}
+
+// cacheFor picks the cache backing a path. /facts and /nodes share the in-memory
+// cache; every other path is uncached until the reports cache lands, and a new
+// backend is a case here rather than a change to any handler.
+func (s *Server) cacheFor(path string) (Cache, bool) {
+	switch path {
+	case factsPath, nodesPath:
+		if s.factsCache != nil {
+			return s.factsCache, true
+		}
+	}
+	return noopCache{}, false
 }
 
 func (s *Server) Handler() http.Handler {
@@ -103,11 +129,14 @@ func isReportSubResource(path string) bool {
 }
 
 func (s *Server) serveMerged(w http.ResponseWriter, r *http.Request, path string, merge func([]backendResult) []json.RawMessage) {
-	alive, ok := s.aliveResults(w, r, path, queryParams(r.URL.Query().Get("query")))
+	params := queryParams(r.URL.Query().Get("query"))
-	if !ok {
+	s.serveCached(w, r, path, params, func() (cachedResponse, error) {
-		return
+		alive, err := s.aliveResults(r.Context(), path, params)
-	}
+		if err != nil {
-	writeJSON(w, merge(alive))
+			return cachedResponse{}, err
+		}
+		return cachedResponse{Body: encodeRecords(merge(alive)), Records: -1}, nil
+	})
 }
 
 // Reports and events are immutable history, so both backends' records belong in the merged view.
@@ -119,19 +148,23 @@ func (s *Server) serveUnion(w http.ResponseWriter, r *http.Request, path string,
 		return
 	}
 
-	alive, ok := s.aliveResults(w, r, path, page.upstreamParams(in))
+	// Keyed on the request's own params, not the upstream ones: upstreamParams
-	if !ok {
+	// folds offset into limit, so different windows would collide.
-		return
+	s.serveCached(w, r, path, in, func() (cachedResponse, error) {
-	}
+		alive, err := s.aliveResults(r.Context(), path, page.upstreamParams(in))
-
+		if err != nil {
-	merged := mergeUnion(alive, key)
+			return cachedResponse{}, err
-	sortRecords(merged, page.order)
-	if page.wantTotal {
-		if total := sumTotals(alive); total >= 0 {
-			w.Header().Set(recordsHeader, strconv.Itoa(total))
 		}
-	}
+		merged := mergeUnion(alive, key)
-	writeJSON(w, page.apply(merged))
+		sortRecords(merged, page.order)
+		resp := cachedResponse{Body: encodeRecords(page.apply(merged)), Records: -1}
+		if page.wantTotal {
+			if total := sumTotals(alive); total >= 0 {
+				resp.Records = total
+			}
+		}
+		return resp, nil
+	})
 }
 
 // An `extract` query with a `function` column returns synthetic aggregate rows that carry no identity, so they are summed rather than unioned.
@@ -152,17 +185,19 @@ func (s *Server) serveSummed(w http.ResponseWriter, r *http.Request, path string
 		return
 	}
 
-	alive, ok := s.aliveResults(w, r, path, page.upstreamParams(in))
+	s.serveCached(w, r, path, in, func() (cachedResponse, error) {
-	if !ok {
+		alive, err := s.aliveResults(r.Context(), path, page.upstreamParams(in))
-		return
+		if err != nil {
-	}
+			return cachedResponse{}, err
-
+		}
-	merged := sumRows(alive, columns)
+		merged := sumRows(alive, columns)
-	sortRecords(merged, page.order)
+		sortRecords(merged, page.order)
-	if page.wantTotal {
+		resp := cachedResponse{Body: encodeRecords(page.apply(merged)), Records: -1}
-		w.Header().Set(recordsHeader, strconv.Itoa(len(merged)))
+		if page.wantTotal {
-	}
+			resp.Records = len(merged)
-	writeJSON(w, page.apply(merged))
+		}
+		return resp, nil
+	})
 }
 
 // A backend without the report answers 404, indistinguishable from a failure, so every backend is consulted before serving empty.
@@ -190,9 +225,9 @@ func (s *Server) serveFirstHolder(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, nil)
 }
 
-// Writes a 502 and returns ok=false only when every backend failed.
+// Returns errAllBackendsFailed only when every backend failed.
-func (s *Server) aliveResults(w http.ResponseWriter, r *http.Request, path string, params url.Values) ([]backendResult, bool) {
+func (s *Server) aliveResults(ctx context.Context, path string, params url.Values) ([]backendResult, error) {
-	results := s.fanOut(r.Context(), path, params)
+	results := s.fanOut(ctx, path, params)
 
 	var alive []backendResult
 	for _, res := range results {
@@ -203,10 +238,114 @@ func (s *Server) aliveResults(w http.ResponseWriter, r *http.Request, path strin
 		alive = append(alive, res)
 	}
 	if len(alive) == 0 {
-		http.Error(w, "all backends failed", http.StatusBadGateway)
+		return nil, errAllBackendsFailed
-		return nil, false
 	}
-	return alive, true
+	return alive, nil
+}
+
+// cachedResponse is the stored form of a merged response: the JSON body plus the
+// X-Records value it carried, so a cache hit reproduces both.
+type cachedResponse struct {
+	Body    json.RawMessage `json:"body"`
+	Records int             `json:"records"` // -1 when the response sets no X-Records
+}
+
+// serveCached answers from the cache when the entry is fresh, otherwise runs
+// build — single-flighted, so N concurrent identical requests cause one upstream
+// fan-out — and stores the result. A build failure falls back to a stale entry
+// when one exists; that is the only path on which stale data is served. Paths
+// with no cache configured run build directly, unchanged.
+func (s *Server) serveCached(w http.ResponseWriter, r *http.Request, path string, params url.Values, build func() (cachedResponse, error)) {
+	cache, enabled := s.cacheFor(path)
+	if !enabled {
+		resp, err := build()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusBadGateway)
+			return
+		}
+		writeCached(w, resp)
+		return
+	}
+
+	key := cacheKey(path, params)
+	var stale *CacheEntry
+	ent, status, err := cache.Get(r.Context(), key)
+	switch {
+	case err != nil:
+		s.log.Printf("warning: cache lookup for %s failed: %v", key, err)
+	case status == CacheFresh:
+		s.stale.markFresh()
+		s.writeStored(w, ent.Body)
+		return
+	case status == CacheStale:
+		stale = &ent
+	}
+
+	resp, err, _ := s.flights.Do(key, func() (cachedResponse, error) {
+		built, buildErr := build()
+		if buildErr != nil {
+			return cachedResponse{}, buildErr
+		}
+		body, marshalErr := json.Marshal(built)
+		if marshalErr != nil {
+			s.log.Printf("warning: encoding cache entry for %s failed: %v", key, marshalErr)
+			return built, nil
+		}
+		// The leader's request may be cancelled while followers still wait.
+		if putErr := cache.Put(context.WithoutCancel(r.Context()), key, body); putErr != nil {
+			s.log.Printf("warning: cache store for %s failed: %v", key, putErr)
+		}
+		return built, nil
+	})
+	if err != nil {
+		if stale != nil {
+			s.stale.markStale(time.Now())
+			s.log.Printf("warning: serving stale %s from cache (stored %s): %v",
+				path, stale.StoredAt.UTC().Format(time.RFC3339), err)
+			s.writeStored(w, stale.Body)
+			return
+		}
+		http.Error(w, err.Error(), http.StatusBadGateway)
+		return
+	}
+	s.stale.markFresh()
+	writeCached(w, resp)
+}
+
+func (s *Server) writeStored(w http.ResponseWriter, body []byte) {
+	var resp cachedResponse
+	if err := json.Unmarshal(body, &resp); err != nil {
+		s.log.Printf("warning: unreadable cache entry: %v", err)
+		http.Error(w, "unreadable cache entry", http.StatusBadGateway)
+		return
+	}
+	writeCached(w, resp)
+}
+
+func writeCached(w http.ResponseWriter, resp cachedResponse) {
+	w.Header().Set("Content-Type", "application/json")
+	if resp.Records >= 0 {
+		w.Header().Set(recordsHeader, strconv.Itoa(resp.Records))
+	}
+	// resp.Body is shared with the cache and with every caller of a single
+	// flight, so it is written, never appended to.
+	body := []byte(resp.Body)
+	if len(body) == 0 {
+		body = []byte("[]")
+	}
+	_, _ = w.Write(body)
+	_, _ = w.Write([]byte("\n"))
+}
+
+func encodeRecords(recs []json.RawMessage) json.RawMessage {
+	if recs == nil {
+		recs = []json.RawMessage{}
+	}
+	b, err := json.Marshal(recs)
+	if err != nil {
+		return json.RawMessage("[]")
+	}
+	return b
 }
 
 func queryParams(query string) url.Values {
@@ -372,13 +511,48 @@ func setContentType(w http.ResponseWriter, contentType string) {
 type healthReport struct {
 	Status   string            `json:"status"`
 	Backends map[string]string `json:"backends"` // name -> "ok" | error text
+	Cache    cacheHealth       `json:"cache"`
+}
+
+type cacheHealth struct {
+	Backend      string `json:"backend"` // "memory" | "none"
+	TTL          string `json:"ttl"`
+	Entries      int    `json:"entries"`
+	StaleEntries int    `json:"stale_entries"` // cached entries past their TTL
+	Bytes        int64  `json:"bytes"`
+	ServingStale bool   `json:"serving_stale"` // last cached response came from a stale entry
+	StaleServed  uint64 `json:"stale_served"`
+	LastStale    string `json:"last_stale_served,omitempty"`
+}
+
+func (s *Server) cacheHealth() cacheHealth {
+	stats := CacheStats{Backend: "none"}
+	ttl := time.Duration(0)
+	if s.factsCache != nil {
+		stats = s.factsCache.Stats()
+		ttl = s.cfg.FactsTTL
+	}
+	serving, served, last := s.stale.snapshot()
+	h := cacheHealth{
+		Backend:      stats.Backend,
+		TTL:          durationString(ttl),
+		Entries:      stats.Entries,
+		StaleEntries: stats.StaleEntries,
+		Bytes:        stats.Bytes,
+		ServingStale: serving,
+		StaleServed:  served,
+	}
+	if !last.IsZero() {
+		h.LastStale = last.UTC().Format(time.RFC3339)
+	}
+	return h
 }
 
 func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 	probe := `["=","certname","pdbmux-healthz-probe"]`
 	results := s.fanOut(r.Context(), nodesPath, queryParams(probe))
 
-	report := healthReport{Backends: map[string]string{}}
+	report := healthReport{Backends: map[string]string{}, Cache: s.cacheHealth()}
 	healthy := 0
 	for _, res := range results {
 		if res.err != nil {