docker-compose.e2e.yml

@@ -9,6 +9,18 @@ services:
     # No host port needed: only the artifactapi container talks to it, and the
     # tests compare served bytes against the on-disk fixtures.
 
+  # Two constant-body upstreams for the multi-base_url suite: each returns a
+  # distinct, upstream-identifying body for any path, so round-robin
+  # distribution across a two-mirror remote is directly observable.
+  mockupstreama:
+    image: nginx:alpine
+    volumes:
+      - ./e2e-docker/mirror-conf/a.conf:/etc/nginx/conf.d/default.conf:ro,z
+  mockupstreamb:
+    image: nginx:alpine
+    volumes:
+      - ./e2e-docker/mirror-conf/b.conf:/etc/nginx/conf.d/default.conf:ro,z
+
   artifactapi:
     # The host port is set via ARTIFACTAPI_PORT (see scripts/docker-e2e.sh),
     # defaulting to 8000; the e2e run uses 8001 to avoid colliding with a
@@ -16,3 +28,7 @@ services:
     depends_on:
       mockupstream:
         condition: service_started
+      mockupstreama:
+        condition: service_started
+      mockupstreamb:
+        condition: service_started

e2e-docker/README.md

@@ -30,6 +30,12 @@ already-running stack.
   index), rpm (real package + **automatic repodata** generation).
 - **Virtual repositories** — pypi simple-index merge and helm `index.yaml` merge
   across two members.
+- **Mirrorlist** — an rpm remote with a `mirrorlist` of extra upstream mirrors
+  (pool = `base_url` + `mirrorlist`): round-robin distribution across both mirrors
+  (constant-body `mockupstreama` / `mockupstreamb`), failover past a dead primary,
+  no-mirrorlist regression, and a real `dnf` (stock `rockylinux:9` container)
+  `makecache` + `install` through a two-mirror rpm remote whose `base_url` is dead
+  — a dead mirror must not break the client.
 
 ## Fixtures
 

e2e-docker/fixtures/rpm-mirror/repodata/repomd.xml

@@ -0,0 +1,55 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<repomd xmlns="http://linux.duke.edu/metadata/repo" xmlns:rpm="http://linux.duke.edu/metadata/rpm">
+  <revision>1786573032</revision>
+  <data type="primary">
+    <checksum type="sha256">d82f717e4da1afe96b8e7857de9e852f5785c0d75734e50d0f8afdcbc6261b08</checksum>
+    <open-checksum type="sha256">3345bb631380ae6c0620fe2a29cf7dff2ed4e28c5cb06c91e8a1628ba1979bcb</open-checksum>
+    <location href="repodata/d82f717e4da1afe96b8e7857de9e852f5785c0d75734e50d0f8afdcbc6261b08-primary.xml.gz"/>
+    <timestamp>1786573032</timestamp>
+    <size>631</size>
+    <open-size>1192</open-size>
+  </data>
+  <data type="filelists">
+    <checksum type="sha256">daa313cc5eeb7df556e1d4885d7701b10b9f012f436ef239fa46827f966222be</checksum>
+    <open-checksum type="sha256">648bd0ce00fda09abbc6e9c3ff3278518a76f258576ac24cd10c12e41e0e5bd7</open-checksum>
+    <location href="repodata/daa313cc5eeb7df556e1d4885d7701b10b9f012f436ef239fa46827f966222be-filelists.xml.gz"/>
+    <timestamp>1786573032</timestamp>
+    <size>256</size>
+    <open-size>338</open-size>
+  </data>
+  <data type="other">
+    <checksum type="sha256">8510c74a6f288828bbc92abee5d0d8ae9687d3c31a2579ea95e31a4c3a320d85</checksum>
+    <open-checksum type="sha256">c42cfd3843e9c53a60ad84bded44aa46c65faae7b3da99a099a9ca0b018872a9</open-checksum>
+    <location href="repodata/8510c74a6f288828bbc92abee5d0d8ae9687d3c31a2579ea95e31a4c3a320d85-other.xml.gz"/>
+    <timestamp>1786573032</timestamp>
+    <size>296</size>
+    <open-size>399</open-size>
+  </data>
+  <data type="primary_db">
+    <checksum type="sha256">f6bd7755da13d9726381048f467992869104a4c5521338ef740dc35eb85b9b71</checksum>
+    <open-checksum type="sha256">c45c85d12ccb0f8172b7bfae466362c08ac1867559574a9fb9cb2118c04daddb</open-checksum>
+    <location href="repodata/f6bd7755da13d9726381048f467992869104a4c5521338ef740dc35eb85b9b71-primary.sqlite.bz2"/>
+    <timestamp>1786573032</timestamp>
+    <size>1740</size>
+    <open-size>106496</open-size>
+    <database_version>10</database_version>
+  </data>
+  <data type="filelists_db">
+    <checksum type="sha256">be3c6e4c7a13ece48bd5d6a4d6d5e6a2395fe006ef9c5f217f5b87144f465e57</checksum>
+    <open-checksum type="sha256">1ccfa3dff532d782ce3225aae807506a4ce4534291386f1c47455dcc6b70cfd6</open-checksum>
+    <location href="repodata/be3c6e4c7a13ece48bd5d6a4d6d5e6a2395fe006ef9c5f217f5b87144f465e57-filelists.sqlite.bz2"/>
+    <timestamp>1786573032</timestamp>
+    <size>764</size>
+    <open-size>28672</open-size>
+    <database_version>10</database_version>
+  </data>
+  <data type="other_db">
+    <checksum type="sha256">ba593cd8ab5ec1e127888707c1fd882920996f1ce173fd7a589d647918fd7da4</checksum>
+    <open-checksum type="sha256">5d4d38380f0e359bfc0a50033d4faa84c75c5ae8fe2ef11c1c82d64748e9b8e2</open-checksum>
+    <location href="repodata/ba593cd8ab5ec1e127888707c1fd882920996f1ce173fd7a589d647918fd7da4-other.sqlite.bz2"/>
+    <timestamp>1786573032</timestamp>
+    <size>738</size>
+    <open-size>24576</open-size>
+    <database_version>10</database_version>
+  </data>
+</repomd>

e2e-docker/mirror-conf/a.conf

@@ -0,0 +1,9 @@
+# Mock upstream A for the multi-base_url e2e: any path returns a constant,
+# upstream-identifying body so round-robin distribution is observable.
+server {
+    listen 80;
+    location / {
+        default_type text/plain;
+        return 200 "UPSTREAM-A";
+    }
+}

e2e-docker/mirror-conf/b.conf

@@ -0,0 +1,8 @@
+# Mock upstream B for the multi-base_url e2e (see a.conf).
+server {
+    listen 80;
+    location / {
+        default_type text/plain;
+        return 200 "UPSTREAM-B";
+    }
+}

e2e-docker/multibaseurl_test.go

@@ -0,0 +1,163 @@
+//go:build dockere2e
+
+package e2edocker
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+// mockUpstreamA/B are the constant-body upstreams (see docker-compose.e2e.yml)
+// that let the round-robin test observe which mirror served each request.
+func mockUpstreamA() string {
+	if v := os.Getenv("MOCK_UPSTREAM_A_INTERNAL"); v != "" {
+		return strings.TrimRight(v, "/")
+	}
+	return "http://mockupstreama"
+}
+
+func mockUpstreamB() string {
+	if v := os.Getenv("MOCK_UPSTREAM_B_INTERNAL"); v != "" {
+		return strings.TrimRight(v, "/")
+	}
+	return "http://mockupstreamb"
+}
+
+// TestMultiBaseURLRoundRobin configures an rpm remote with base_url = mirror A
+// and mirrorlist = [mirror B] and drives distinct paths through it, asserting
+// both mirrors serve traffic. Each path is a cache miss, so every request reaches
+// upstream and the round-robin cursor alternates mirrors.
+func TestMultiBaseURLRoundRobin(t *testing.T) {
+	name := "e2e-rr"
+	createRepo(t, fmt.Sprintf(`{
+		"name": %q,
+		"package_type": "rpm",
+		"repo_type": "remote",
+		"base_url": %q,
+		"mirrorlist": [%q],
+		"stale_on_error": false
+	}`, name, mockUpstreamA(), mockUpstreamB()))
+	defer deleteRepo(t, name)
+
+	seenA, seenB := false, false
+	const n = 12
+	for i := 0; i < n; i++ {
+		url := api(fmt.Sprintf("/api/v1/remote/%s/rr/%d", name, i))
+		resp, body := doRequest(t, http.MethodGet, url, nil, "")
+		if resp.StatusCode != http.StatusOK {
+			t.Fatalf("request %d: status %d: %s", i, resp.StatusCode, body)
+		}
+		switch strings.TrimSpace(string(body)) {
+		case "UPSTREAM-A":
+			seenA = true
+		case "UPSTREAM-B":
+			seenB = true
+		default:
+			t.Fatalf("request %d: unexpected body %q", i, body)
+		}
+	}
+	if !seenA || !seenB {
+		t.Fatalf("round-robin did not reach both upstreams: A=%v B=%v", seenA, seenB)
+	}
+}
+
+// TestMultiBaseURLFailover points a two-mirror remote at a dead primary and a
+// healthy secondary and asserts every request still succeeds via the secondary.
+func TestMultiBaseURLFailover(t *testing.T) {
+	name := "e2e-failover"
+	createRepo(t, fmt.Sprintf(`{
+		"name": %q,
+		"package_type": "rpm",
+		"repo_type": "remote",
+		"base_url": "http://mockupstream-dead:80",
+		"mirrorlist": [%q],
+		"stale_on_error": false
+	}`, name, mockUpstreamB()))
+	defer deleteRepo(t, name)
+
+	for i := 0; i < 6; i++ {
+		url := api(fmt.Sprintf("/api/v1/remote/%s/fo/%d", name, i))
+		resp, body := doRequest(t, http.MethodGet, url, nil, "")
+		if resp.StatusCode != http.StatusOK {
+			t.Fatalf("request %d: dead primary broke fetch: status %d: %s", i, resp.StatusCode, body)
+		}
+		if got := strings.TrimSpace(string(body)); got != "UPSTREAM-B" {
+			t.Fatalf("request %d: body %q, want UPSTREAM-B (served via failover)", i, got)
+		}
+	}
+}
+
+// TestSingleBaseURLRegression asserts a remote with no mirrorlist works exactly
+// as before the mirrorlist change.
+func TestSingleBaseURLRegression(t *testing.T) {
+	name := "e2e-single"
+	createRepo(t, fmt.Sprintf(`{
+		"name": %q,
+		"package_type": "rpm",
+		"repo_type": "remote",
+		"base_url": %q,
+		"stale_on_error": false
+	}`, name, mockUpstreamA()))
+	defer deleteRepo(t, name)
+
+	resp, body := doRequest(t, http.MethodGet, api("/api/v1/remote/"+name+"/solo/0"), nil, "")
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("single-url fetch: status %d: %s", resp.StatusCode, body)
+	}
+	if got := strings.TrimSpace(string(body)); got != "UPSTREAM-A" {
+		t.Fatalf("single-url body %q, want UPSTREAM-A", got)
+	}
+}
+
+// TestMultiBaseURLDnfFailover drives a real dnf (stock rockylinux container) at
+// a two-mirror rpm remote whose primary is dead: makecache + install must
+// succeed via the live secondary mirror, proving a dead mirror does not break a
+// real package-manager client. Requires the compose network and internal API
+// URL exported by scripts/docker-e2e.sh; skipped when run standalone.
+func TestMultiBaseURLDnfFailover(t *testing.T) {
+	network := os.Getenv("COMPOSE_NETWORK")
+	internal := os.Getenv("ARTIFACTAPI_INTERNAL")
+	if network == "" || internal == "" {
+		t.Skip("COMPOSE_NETWORK/ARTIFACTAPI_INTERNAL not set; run via scripts/docker-e2e.sh")
+	}
+	if _, err := exec.LookPath("docker"); err != nil {
+		t.Skip("docker not available on the test host")
+	}
+
+	name := "e2e-dnf-failover"
+	// Primary base_url is dead; the live mirror serves the real yum repo under
+	// fixtures/rpm-mirror via the shared mock upstream.
+	createRepo(t, fmt.Sprintf(`{
+		"name": %q,
+		"package_type": "rpm",
+		"repo_type": "remote",
+		"base_url": "http://mockupstream-dead:80",
+		"mirrorlist": [%q],
+		"stale_on_error": false
+	}`, name, mockUpstream()))
+	defer deleteRepo(t, name)
+
+	repoURL := strings.TrimRight(internal, "/") + "/api/v1/remote/" + name + "/rpm-mirror"
+	repoConf := fmt.Sprintf("[dnffo]\nname=dnffo\nbaseurl=%s\nenabled=1\ngpgcheck=0\nsslverify=0\nmetadata_expire=0\n", repoURL)
+	script := "set -euo pipefail; " +
+		"printf '%s' \"$REPO\" > /etc/yum.repos.d/dnffo.repo; " +
+		"dnf -y --disablerepo='*' --enablerepo=dnffo makecache; " +
+		"dnf -y --disablerepo='*' --enablerepo=dnffo install e2e-testpkg; " +
+		"rpm -q e2e-testpkg"
+
+	cmd := exec.Command("docker", "run", "--rm",
+		"--network", network,
+		"-e", "REPO="+repoConf,
+		"rockylinux:9", "bash", "-c", script)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("real dnf install through a dead primary mirror failed: %v\n%s", err, out)
+	}
+	if !strings.Contains(string(out), "e2e-testpkg-1.0-1") {
+		t.Fatalf("dnf did not install the expected package via failover; output:\n%s", out)
+	}
+}

internal/api/v2/remotes.go

@@ -78,6 +78,10 @@ func (h *RemotesHandler) create(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "base_url is required for remote repositories", http.StatusBadRequest)
 		return
 	}
+	if err := remote.ValidateMirrorlist(); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
 	if err := remote.ValidatePatterns(); err != nil {
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
@@ -102,6 +106,10 @@ func (h *RemotesHandler) update(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	remote.Name = name
+	if err := remote.ValidateMirrorlist(); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
 	if err := remote.ValidatePatterns(); err != nil {
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return

internal/database/database_test.go

@@ -99,6 +99,39 @@ func TestRemotesCRUD(t *testing.T) {
 	}
 }
 
+func TestRemoteMirrorlistRoundTrip(t *testing.T) {
+	requireDB(t)
+	mirrors := []string{"https://b.example", "https://c.example"}
+	if err := testDB.CreateRemote(ctx(), &models.Remote{
+		Name: "r-mirror", PackageType: models.PackageRPM, RepoType: models.RepoTypeRemote,
+		BaseURL: "https://a.example", Mirrorlist: mirrors, MutableTTL: 3600,
+	}); err != nil {
+		t.Fatalf("create mirrorlist remote: %v", err)
+	}
+	defer testDB.DeleteRemote(ctx(), "r-mirror")
+
+	got, err := testDB.GetRemote(ctx(), "r-mirror")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got.BaseURL != "https://a.example" {
+		t.Fatalf("BaseURL = %q, want https://a.example", got.BaseURL)
+	}
+	if len(got.Mirrorlist) != 2 || got.Mirrorlist[0] != mirrors[0] || got.Mirrorlist[1] != mirrors[1] {
+		t.Fatalf("Mirrorlist round-trip = %v, want %v", got.Mirrorlist, mirrors)
+	}
+
+	// Clearing the mirrorlist on update persists an empty list.
+	got.Mirrorlist = nil
+	if err := testDB.UpdateRemote(ctx(), got); err != nil {
+		t.Fatalf("update clearing mirrorlist: %v", err)
+	}
+	got, _ = testDB.GetRemote(ctx(), "r-mirror")
+	if len(got.Mirrorlist) != 0 {
+		t.Fatalf("mirrorlist after clear = %v, want empty", got.Mirrorlist)
+	}
+}
+
 func TestArtifactsAndBlobs(t *testing.T) {
 	requireDB(t)
 	seedRemote(t, "r-art")

internal/database/postgres.go

@@ -44,6 +44,7 @@ func (db *DB) migrate() error {
 			package_type    TEXT NOT NULL,
 			repo_type       TEXT DEFAULT 'remote',
 			base_url        TEXT NOT NULL DEFAULT '',
+			mirrorlist      TEXT[] DEFAULT '{}',
 			description     TEXT DEFAULT '',
 			username        TEXT DEFAULT '',
 			password        TEXT DEFAULT '',
@@ -124,6 +125,7 @@ func (db *DB) migrate() error {
 		CREATE INDEX IF NOT EXISTS idx_access_log_remote_time ON access_log(remote_name, created_at);
 
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS repo_type TEXT DEFAULT 'remote';
+		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS mirrorlist TEXT[] DEFAULT '{}';
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_dial_timeout INTEGER DEFAULT 0;
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_tls_timeout INTEGER DEFAULT 0;
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_response_header_timeout INTEGER DEFAULT 0;

internal/database/remotes.go

@@ -6,7 +6,7 @@ import (
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
-const remoteCols = `name, package_type, repo_type, base_url, description, username, password,
+const remoteCols = `name, package_type, repo_type, base_url, mirrorlist, description, username, password,
 	immutable_ttl, mutable_ttl, check_mutable,
 	patterns, blocklist, mutable_patterns, immutable_patterns,
 	ban_tags_enabled, ban_tags,
@@ -17,7 +17,7 @@ const remoteCols = `name, package_type, repo_type, base_url, description, userna
 
 func scanRemote(scanner interface{ Scan(...any) error }, r *models.Remote) error {
 	return scanner.Scan(
-		&r.Name, &r.PackageType, &r.RepoType, &r.BaseURL, &r.Description, &r.Username, &r.Password,
+		&r.Name, &r.PackageType, &r.RepoType, &r.BaseURL, &r.Mirrorlist, &r.Description, &r.Username, &r.Password,
 		&r.ImmutableTTL, &r.MutableTTL, &r.CheckMutable,
 		&r.Patterns, &r.Blocklist, &r.MutablePatterns, &r.ImmutablePatterns,
 		&r.BanTagsEnabled, &r.BanTags,
@@ -58,16 +58,16 @@ func (db *DB) ListRemotes(ctx context.Context) ([]models.Remote, error) {
 func (db *DB) CreateRemote(ctx context.Context, r *models.Remote) error {
 	_, err := db.Pool.Exec(ctx, `
 		INSERT INTO remotes (
-			name, package_type, repo_type, base_url, description, username, password,
+			name, package_type, repo_type, base_url, mirrorlist, description, username, password,
 			immutable_ttl, mutable_ttl, check_mutable,
 			patterns, blocklist, mutable_patterns, immutable_patterns,
 			ban_tags_enabled, ban_tags,
 			quarantine_enabled, quarantine_days, stale_on_error,
 			releases_remote, managed_by,
 			upstream_dial_timeout, upstream_tls_timeout, upstream_response_header_timeout
-		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24)
+		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24,$25)
 	`,
-		r.Name, r.PackageType, r.RepoType, r.BaseURL, r.Description, r.Username, r.Password,
+		r.Name, r.PackageType, r.RepoType, r.BaseURL, r.Mirrorlist, r.Description, r.Username, r.Password,
 		r.ImmutableTTL, r.MutableTTL, r.CheckMutable,
 		r.Patterns, r.Blocklist, r.MutablePatterns, r.ImmutablePatterns,
 		r.BanTagsEnabled, r.BanTags,
@@ -81,7 +81,7 @@ func (db *DB) CreateRemote(ctx context.Context, r *models.Remote) error {
 func (db *DB) UpdateRemote(ctx context.Context, r *models.Remote) error {
 	_, err := db.Pool.Exec(ctx, `
 		UPDATE remotes SET
-			package_type=$2, repo_type=$3, base_url=$4, description=$5, username=$6, password=$7,
+			package_type=$2, repo_type=$3, base_url=$4, mirrorlist=$25, description=$5, username=$6, password=$7,
 			immutable_ttl=$8, mutable_ttl=$9, check_mutable=$10,
 			patterns=$11, blocklist=$12, mutable_patterns=$13, immutable_patterns=$14,
 			ban_tags_enabled=$15, ban_tags=$16,
@@ -98,6 +98,7 @@ func (db *DB) UpdateRemote(ctx context.Context, r *models.Remote) error {
 		r.QuarantineEnabled, r.QuarantineDays, r.StaleOnError,
 		r.ReleasesRemote, r.ManagedBy,
 		r.UpstreamDialTimeout, r.UpstreamTLSTimeout, r.UpstreamResponseHeaderTimeout,
+		r.Mirrorlist,
 	)
 	return err
 }

internal/proxy/engine.go

@@ -11,6 +11,8 @@ import (
 	"log/slog"
 	"net/http"
 	"strings"
+	"sync"
+	"sync/atomic"
 	"time"
 
 	"git.unkin.net/unkin/artifactapi/internal/cache"
@@ -35,6 +37,10 @@ type Engine struct {
 	cas       *storage.CAS
 	circuit   *CircuitBreaker
 	accessLog chan database.AccessLogEntry
+	// rrCounters holds a per-remote round-robin cursor (remoteName ->
+	// *atomic.Uint64) used to rotate the starting mirror across upstream base
+	// URLs. Distribution is per-replica and approximate, which is fine.
+	rrCounters sync.Map
 }
 
 func NewEngine(db *database.DB, c *cache.Redis, s *storage.S3) *Engine {
@@ -222,7 +228,30 @@ func (e *Engine) Head(ctx context.Context, remote models.Remote, path string, pr
 	return e.headUpstream(ctx, remote, path, prov)
 }
 
+// headUpstream issues an upstream HEAD, load-balancing across the remote's base
+// URLs and failing over to the next mirror on a network error or 5xx.
 func (e *Engine) headUpstream(ctx context.Context, remote models.Remote, path string, prov provider.Provider) (*HeadResult, error) {
+	order := e.baseURLAttemptOrder(remote)
+	if len(order) == 0 {
+		return nil, &ProxyError{Status: http.StatusBadGateway, Message: "no upstream base_url configured"}
+	}
+	var lastErr error
+	for i, url := range order {
+		result, err := e.headUpstreamOnce(ctx, withBaseURL(remote, url), path, prov)
+		if err == nil {
+			return result, nil
+		}
+		lastErr = err
+		if i < len(order)-1 && shouldFailover(err) {
+			slog.Warn("upstream HEAD failed, failing over", "remote", remote.Name, "base_url", url, "error", err)
+			continue
+		}
+		return nil, err
+	}
+	return nil, lastErr
+}
+
+func (e *Engine) headUpstreamOnce(ctx context.Context, remote models.Remote, path string, prov provider.Provider) (*HeadResult, error) {
 	url := prov.UpstreamURL(remote, path)
 
 	authHeaders, err := prov.AuthHeaders(ctx, remote)
@@ -277,7 +306,31 @@ func (e *Engine) headUpstream(ctx context.Context, remote models.Remote, path st
 	return &HeadResult{ContentType: contentType, Size: resp.ContentLength, Source: "remote"}, nil
 }
 
+// fetchFromUpstream fetches an artifact from upstream, load-balancing across the
+// remote's base URLs and failing over to the next mirror on a network error or
+// 5xx before returning an error.
 func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, path string, prov provider.Provider, class Classification, ttl time.Duration, clientHeaders http.Header) (*FetchResult, error) {
+	order := e.baseURLAttemptOrder(remote)
+	if len(order) == 0 {
+		return nil, &ProxyError{Status: http.StatusBadGateway, Message: "no upstream base_url configured"}
+	}
+	var lastErr error
+	for i, url := range order {
+		result, err := e.fetchFromUpstreamOnce(ctx, withBaseURL(remote, url), path, prov, class, ttl, clientHeaders)
+		if err == nil {
+			return result, nil
+		}
+		lastErr = err
+		if i < len(order)-1 && shouldFailover(err) {
+			slog.Warn("upstream fetch failed, failing over", "remote", remote.Name, "base_url", url, "error", err)
+			continue
+		}
+		return nil, err
+	}
+	return nil, lastErr
+}
+
+func (e *Engine) fetchFromUpstreamOnce(ctx context.Context, remote models.Remote, path string, prov provider.Provider, class Classification, ttl time.Duration, clientHeaders http.Header) (*FetchResult, error) {
 	url := prov.UpstreamURL(remote, path)
 
 	authHeaders, err := prov.AuthHeaders(ctx, remote)
@@ -454,7 +507,31 @@ func (e *Engine) serveFromStore(ctx context.Context, remote models.Remote, path
 	}, nil
 }
 
+// checkUpstream issues a conditional upstream HEAD (If-None-Match), load
+// balancing across the remote's base URLs and failing over to the next mirror on
+// a network error or 5xx.
 func (e *Engine) checkUpstream(ctx context.Context, remote models.Remote, path, etag string, prov provider.Provider) (bool, error) {
+	order := e.baseURLAttemptOrder(remote)
+	if len(order) == 0 {
+		return false, &ProxyError{Status: http.StatusBadGateway, Message: "no upstream base_url configured"}
+	}
+	var lastErr error
+	for i, url := range order {
+		notModified, err := e.checkUpstreamOnce(ctx, withBaseURL(remote, url), path, etag, prov)
+		if err == nil {
+			return notModified, nil
+		}
+		lastErr = err
+		if i < len(order)-1 && shouldFailover(err) {
+			slog.Warn("upstream revalidation failed, failing over", "remote", remote.Name, "base_url", url, "error", err)
+			continue
+		}
+		return false, err
+	}
+	return false, lastErr
+}
+
+func (e *Engine) checkUpstreamOnce(ctx context.Context, remote models.Remote, path, etag string, prov provider.Provider) (bool, error) {
 	url := prov.UpstreamURL(remote, path)
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodHead, url, nil)
@@ -649,3 +726,45 @@ func isNetworkError(err error) bool {
 	var ue *UpstreamError
 	return errors.As(err, &ue)
 }
+
+// baseURLAttemptOrder returns the ordered upstream base URLs to try for a single
+// request, drawn from the remote's pool ([base_url] + mirrorlist). A multi-mirror
+// remote starts at the next round-robin position and advances linearly for
+// failover; a remote with no mirrorlist yields exactly [base_url], preserving the
+// original single-attempt behavior.
+func (e *Engine) baseURLAttemptOrder(remote models.Remote) []string {
+	urls := remote.UpstreamPool()
+	if len(urls) <= 1 {
+		return urls
+	}
+	v, _ := e.rrCounters.LoadOrStore(remote.Name, new(atomic.Uint64))
+	start := int(v.(*atomic.Uint64).Add(1) - 1)
+	ordered := make([]string, len(urls))
+	for i := range urls {
+		ordered[i] = urls[(start+i)%len(urls)]
+	}
+	return ordered
+}
+
+// withBaseURL narrows a remote's active BaseURL to a single selected mirror so
+// providers (UpstreamURL/AuthHeaders/RewriteResponse) operate on exactly that
+// upstream for this attempt.
+func withBaseURL(remote models.Remote, url string) models.Remote {
+	remote.BaseURL = url
+	remote.Mirrorlist = nil
+	return remote
+}
+
+// shouldFailover reports whether an upstream attempt error is worth retrying
+// against the next mirror: network errors/timeouts and upstream 5xx responses.
+// Definitive statuses (404/403/401/...) are returned to the caller unchanged.
+func shouldFailover(err error) bool {
+	if isNetworkError(err) {
+		return true
+	}
+	var pe *ProxyError
+	if errors.As(err, &pe) {
+		return pe.Status >= 500
+	}
+	return false
+}

internal/proxy/multibaseurl_test.go

@@ -0,0 +1,188 @@
+package proxy
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+// TestFetchMultiBaseURLRoundRobin drives distinct artifact paths through a
+// remote configured with two upstreams and asserts both receive traffic.
+func TestFetchMultiBaseURLRoundRobin(t *testing.T) {
+	requireStack(t)
+	ctx := context.Background()
+
+	var hitsA, hitsB atomic.Int64
+	upA := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hitsA.Add(1)
+		w.Write([]byte("A"))
+	}))
+	defer upA.Close()
+	upB := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hitsB.Add(1)
+		w.Write([]byte("B"))
+	}))
+	defer upB.Close()
+
+	r := seed(t, models.Remote{
+		Name:         "eng-rr",
+		PackageType:  models.PackageGeneric,
+		RepoType:     models.RepoTypeRemote,
+		BaseURL:      upA.URL,
+		Mirrorlist:   []string{upB.URL},
+		StaleOnError: true,
+	})
+	p := prov(t, models.PackageGeneric)
+
+	const n = 10
+	for i := 0; i < n; i++ {
+		res, err := testEngine.Fetch(ctx, r, fmt.Sprintf("rr-%d.bin", i), p)
+		if err != nil {
+			t.Fatalf("fetch %d: %v", i, err)
+		}
+		res.Reader.Close()
+	}
+
+	if hitsA.Load() == 0 || hitsB.Load() == 0 {
+		t.Fatalf("round-robin did not spread across both upstreams: A=%d B=%d", hitsA.Load(), hitsB.Load())
+	}
+	if total := hitsA.Load() + hitsB.Load(); total != n {
+		t.Fatalf("expected %d upstream hits total, got %d (A=%d B=%d)", n, total, hitsA.Load(), hitsB.Load())
+	}
+}
+
+// TestFetchMultiBaseURLFailover asserts that a dead/erroring primary mirror
+// transparently fails over to a healthy secondary, for both a 5xx primary and a
+// network-unreachable primary.
+func TestFetchMultiBaseURLFailover(t *testing.T) {
+	requireStack(t)
+	ctx := context.Background()
+
+	var hitsB atomic.Int64
+	upB := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hitsB.Add(1)
+		w.Write([]byte("served-by-B"))
+	}))
+	defer upB.Close()
+	up500 := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer up500.Close()
+
+	p := prov(t, models.PackageGeneric)
+
+	// Primary returns 5xx: every request must still succeed via the secondary.
+	r5xx := seed(t, models.Remote{
+		Name:        "eng-failover-5xx",
+		PackageType: models.PackageGeneric,
+		RepoType:    models.RepoTypeRemote,
+		BaseURL:     up500.URL,
+		Mirrorlist:  []string{upB.URL},
+	})
+	for i := 0; i < 6; i++ {
+		res, err := testEngine.Fetch(ctx, r5xx, fmt.Sprintf("fo5-%d.bin", i), p)
+		if err != nil {
+			t.Fatalf("5xx failover fetch %d: %v", i, err)
+		}
+		if got := readAll(t, res); got != "served-by-B" {
+			t.Fatalf("5xx failover fetch %d body=%q, want served-by-B", i, got)
+		}
+	}
+
+	// Primary is network-unreachable: failover must still reach the secondary.
+	rNet := seed(t, models.Remote{
+		Name:        "eng-failover-net",
+		PackageType: models.PackageGeneric,
+		RepoType:    models.RepoTypeRemote,
+		BaseURL:     "http://127.0.0.1:1",
+		Mirrorlist:  []string{upB.URL},
+	})
+	res, err := testEngine.Fetch(ctx, rNet, "fonet.bin", p)
+	if err != nil {
+		t.Fatalf("network failover fetch: %v", err)
+	}
+	if got := readAll(t, res); got != "served-by-B" {
+		t.Fatalf("network failover body=%q, want served-by-B", got)
+	}
+	if hitsB.Load() == 0 {
+		t.Fatal("secondary upstream never served during failover")
+	}
+}
+
+// TestFetchDefinitiveStatusNoFailover asserts a definitive 404 from the first
+// mirror is returned as-is (not failed over): a missing artifact is not a mirror
+// outage. The remote is fresh so its round-robin cursor starts at index 0.
+func TestFetchDefinitiveStatusNoFailover(t *testing.T) {
+	requireStack(t)
+	ctx := context.Background()
+
+	var hitsB atomic.Int64
+	up404 := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.NotFound(w, r)
+	}))
+	defer up404.Close()
+	upB := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hitsB.Add(1)
+		w.Write([]byte("B"))
+	}))
+	defer upB.Close()
+
+	r := seed(t, models.Remote{
+		Name:        "eng-no-failover-404",
+		PackageType: models.PackageGeneric,
+		RepoType:    models.RepoTypeRemote,
+		BaseURL:     up404.URL,
+		Mirrorlist:  []string{upB.URL},
+	})
+	_, err := testEngine.Fetch(ctx, r, "missing.bin", prov(t, models.PackageGeneric))
+	var pe *ProxyError
+	if err == nil || !asProxyError(err, &pe) || pe.Status != http.StatusNotFound {
+		t.Fatalf("expected 404 ProxyError without failover, got %v", err)
+	}
+	if hitsB.Load() != 0 {
+		t.Fatalf("404 from primary must not fail over, but secondary was hit %d times", hitsB.Load())
+	}
+}
+
+// TestFetchSingleBaseURLUnchanged asserts a single-URL remote behaves exactly as
+// before: one healthy URL succeeds, and one dead URL errors with no failover.
+func TestFetchSingleBaseURLUnchanged(t *testing.T) {
+	requireStack(t)
+	ctx := context.Background()
+
+	upB := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte("solo"))
+	}))
+	defer upB.Close()
+
+	p := prov(t, models.PackageGeneric)
+
+	rOK := seed(t, models.Remote{
+		Name:        "eng-solo",
+		PackageType: models.PackageGeneric,
+		RepoType:    models.RepoTypeRemote,
+		BaseURL:     upB.URL,
+	})
+	res, err := testEngine.Fetch(ctx, rOK, "solo.bin", p)
+	if err != nil {
+		t.Fatalf("single-url fetch: %v", err)
+	}
+	if got := readAll(t, res); got != "solo" {
+		t.Fatalf("single-url body=%q, want solo", got)
+	}
+
+	rDead := seed(t, models.Remote{
+		Name:        "eng-solo-dead",
+		PackageType: models.PackageGeneric,
+		RepoType:    models.RepoTypeRemote,
+		BaseURL:     "http://127.0.0.1:1",
+	})
+	if _, err := testEngine.Fetch(ctx, rDead, "x.bin", p); err == nil {
+		t.Fatal("single dead upstream should error, not succeed")
+	}
+}

pkg/models/remote.go

@@ -2,6 +2,7 @@ package models
 
 import (
 	"fmt"
+	"net/url"
 	"regexp"
 	"time"
 )
@@ -39,9 +40,13 @@ type Remote struct {
 	PackageType PackageType `json:"package_type"`
 	RepoType    RepoType    `json:"repo_type"`
 	BaseURL     string      `json:"base_url"`
-	Description string      `json:"description,omitempty"`
-	Username    string      `json:"-"`
-	Password    string      `json:"-"`
+	// Mirrorlist holds additional upstream mirror base URLs. The effective
+	// upstream pool is [base_url] + mirrorlist, load-balanced round-robin with
+	// failover by the proxy engine. Only valid on remote rpm/deb/apk repos.
+	Mirrorlist  []string `json:"mirrorlist,omitempty"`
+	Description string   `json:"description,omitempty"`
+	Username    string   `json:"-"`
+	Password    string   `json:"-"`
 
 	ImmutableTTL int  `json:"immutable_ttl"`
 	MutableTTL   int  `json:"mutable_ttl"`
@@ -72,6 +77,51 @@ type Remote struct {
 	UpdatedAt time.Time `json:"updated_at"`
 }
 
+// mirrorlistPackageTypes are the package types for which a mirrorlist is
+// allowed: OS package repos (rpm, deb, apk/alpine) that fetch many small files
+// and benefit most from mirror load-balancing and failover.
+var mirrorlistPackageTypes = map[PackageType]bool{
+	PackageRPM:    true,
+	PackageDeb:    true,
+	PackageAlpine: true,
+}
+
+// UpstreamPool returns the ordered upstream base URLs for this remote: the
+// primary base_url first, followed by any mirrorlist entries. The proxy engine
+// load-balances round-robin across the pool and fails over between them.
+func (r Remote) UpstreamPool() []string {
+	pool := make([]string, 0, 1+len(r.Mirrorlist))
+	if r.BaseURL != "" {
+		pool = append(pool, r.BaseURL)
+	}
+	pool = append(pool, r.Mirrorlist...)
+	return pool
+}
+
+// ValidateMirrorlist enforces that a mirrorlist is only configured on remote
+// rpm/deb/apk repositories and that every entry is a parseable http/https URL.
+func (r *Remote) ValidateMirrorlist() error {
+	if len(r.Mirrorlist) == 0 {
+		return nil
+	}
+	if r.RepoType != RepoTypeRemote {
+		return fmt.Errorf("mirrorlist is only allowed on remote repositories")
+	}
+	if !mirrorlistPackageTypes[r.PackageType] {
+		return fmt.Errorf("mirrorlist is only allowed for rpm, deb and alpine package types, not %q", r.PackageType)
+	}
+	for _, u := range r.Mirrorlist {
+		parsed, err := url.ParseRequestURI(u)
+		if err != nil {
+			return fmt.Errorf("invalid mirrorlist url %q: %w", u, err)
+		}
+		if parsed.Scheme != "http" && parsed.Scheme != "https" {
+			return fmt.Errorf("mirrorlist url %q must be http or https", u)
+		}
+	}
+	return nil
+}
+
 // ValidatePatterns ensures every configured regex compiles. Storing an
 // invalid pattern would otherwise be silently dropped at match time, which
 // for the blocklist is a fail-open: a mistyped deny rule becomes a no-op.

pkg/models/remote_test.go

@@ -1,6 +1,10 @@
 package models
 
-import "testing"
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
 
 func TestRemote_ValidatePatterns(t *testing.T) {
 	valid := &Remote{
@@ -17,3 +21,81 @@ func TestRemote_ValidatePatterns(t *testing.T) {
 		t.Fatal("expected error for invalid blocklist regex, got nil")
 	}
 }
+
+func TestRemoteMirrorlistJSON(t *testing.T) {
+	// base_url stays a plain string; mirrorlist round-trips as an array.
+	var r Remote
+	body := `{"name":"x","package_type":"rpm","repo_type":"remote","base_url":"https://a.example","mirrorlist":["https://b.example","https://c.example"]}`
+	if err := json.Unmarshal([]byte(body), &r); err != nil {
+		t.Fatal(err)
+	}
+	if r.BaseURL != "https://a.example" {
+		t.Errorf("BaseURL = %q, want https://a.example", r.BaseURL)
+	}
+	if len(r.Mirrorlist) != 2 || r.Mirrorlist[0] != "https://b.example" || r.Mirrorlist[1] != "https://c.example" {
+		t.Errorf("Mirrorlist = %v, want two entries", r.Mirrorlist)
+	}
+
+	out, err := json.Marshal(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(out), `"base_url":"https://a.example"`) {
+		t.Errorf("marshal lost base_url: %s", out)
+	}
+	if !strings.Contains(string(out), `"mirrorlist":["https://b.example","https://c.example"]`) {
+		t.Errorf("marshal lost mirrorlist: %s", out)
+	}
+}
+
+func TestRemoteMirrorlistOmitempty(t *testing.T) {
+	out, err := json.Marshal(Remote{Name: "x", PackageType: PackageRPM, RepoType: RepoTypeRemote, BaseURL: "https://a.example"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(string(out), "mirrorlist") {
+		t.Errorf("empty mirrorlist should be omitted: %s", out)
+	}
+}
+
+func TestUpstreamPool(t *testing.T) {
+	// base_url first, then mirrorlist.
+	r := Remote{BaseURL: "https://a.example", Mirrorlist: []string{"https://b.example", "https://c.example"}}
+	pool := r.UpstreamPool()
+	want := []string{"https://a.example", "https://b.example", "https://c.example"}
+	if strings.Join(pool, ",") != strings.Join(want, ",") {
+		t.Errorf("UpstreamPool = %v, want %v", pool, want)
+	}
+
+	// No mirrorlist ⇒ pool is just [base_url].
+	solo := Remote{BaseURL: "https://a.example"}
+	if got := solo.UpstreamPool(); len(got) != 1 || got[0] != "https://a.example" {
+		t.Errorf("solo UpstreamPool = %v, want [base_url]", got)
+	}
+}
+
+func TestValidateMirrorlist(t *testing.T) {
+	cases := []struct {
+		name    string
+		remote  Remote
+		wantErr bool
+	}{
+		{"empty is ok on anything", Remote{RepoType: RepoTypeRemote, PackageType: PackageGeneric}, false},
+		{"rpm remote ok", Remote{RepoType: RepoTypeRemote, PackageType: PackageRPM, Mirrorlist: []string{"https://m.example"}}, false},
+		{"deb remote ok", Remote{RepoType: RepoTypeRemote, PackageType: PackageDeb, Mirrorlist: []string{"http://m.example"}}, false},
+		{"alpine remote ok", Remote{RepoType: RepoTypeRemote, PackageType: PackageAlpine, Mirrorlist: []string{"https://m.example"}}, false},
+		{"generic remote rejected", Remote{RepoType: RepoTypeRemote, PackageType: PackageGeneric, Mirrorlist: []string{"https://m.example"}}, true},
+		{"docker remote rejected", Remote{RepoType: RepoTypeRemote, PackageType: PackageDocker, Mirrorlist: []string{"https://m.example"}}, true},
+		{"local rpm rejected", Remote{RepoType: RepoTypeLocal, PackageType: PackageRPM, Mirrorlist: []string{"https://m.example"}}, true},
+		{"bad scheme rejected", Remote{RepoType: RepoTypeRemote, PackageType: PackageRPM, Mirrorlist: []string{"ftp://m.example"}}, true},
+		{"unparseable rejected", Remote{RepoType: RepoTypeRemote, PackageType: PackageRPM, Mirrorlist: []string{"://nope"}}, true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := tc.remote.ValidateMirrorlist()
+			if (err != nil) != tc.wantErr {
+				t.Errorf("ValidateMirrorlist() err = %v, wantErr = %v", err, tc.wantErr)
+			}
+		})
+	}
+}

scripts/docker-e2e.sh

@@ -17,8 +17,8 @@ cleanup() {
 }
 trap cleanup EXIT
 
-echo "==> building and starting stack (postgres, redis, minio, mockupstream, artifactapi)"
-"${COMPOSE[@]}" up -d --build postgres redis minio mockupstream artifactapi
+echo "==> building and starting stack (postgres, redis, minio, mockupstream(s), artifactapi)"
+"${COMPOSE[@]}" up -d --build postgres redis minio mockupstream mockupstreama mockupstreamb artifactapi
 
 echo "==> waiting for artifactapi health at ${API_URL}"
 for i in $(seq 1 60); do
@@ -34,7 +34,17 @@ for i in $(seq 1 60); do
   sleep 1
 done
 
-echo "==> running dockerised e2e suite"
+# Resolve the compose network the artifactapi container is attached to, so the
+# real-package-manager test can launch a stock distro container on the same
+# network and reach artifactapi by service name.
+API_CID="$("${COMPOSE[@]}" ps -q artifactapi)"
+COMPOSE_NETWORK="$(docker inspect -f '{{range $k,$_ := .NetworkSettings.Networks}}{{$k}}{{end}}' "${API_CID}" 2>/dev/null || true)"
+
+echo "==> running dockerised e2e suite (compose network: ${COMPOSE_NETWORK:-unknown})"
 ARTIFACTAPI_URL="${API_URL}" \
 MOCK_UPSTREAM_INTERNAL="${MOCK_UPSTREAM_INTERNAL:-http://mockupstream}" \
+MOCK_UPSTREAM_A_INTERNAL="${MOCK_UPSTREAM_A_INTERNAL:-http://mockupstreama}" \
+MOCK_UPSTREAM_B_INTERNAL="${MOCK_UPSTREAM_B_INTERNAL:-http://mockupstreamb}" \
+ARTIFACTAPI_INTERNAL="${ARTIFACTAPI_INTERNAL:-http://artifactapi:8000}" \
+COMPOSE_NETWORK="${COMPOSE_NETWORK}" \
   go test -tags=dockere2e -count=1 -timeout=10m -v ./e2e-docker/...