diff --git a/app/api/plugin-proxy/route.ts b/app/api/plugin-proxy/route.ts
index 2715160e7..979eb4c26 100644
--- a/app/api/plugin-proxy/route.ts
+++ b/app/api/plugin-proxy/route.ts
@@ -5,6 +5,7 @@ import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
 import { getStalwartCredentials } from '@/lib/stalwart/credentials';
 import { CREDENTIAL_ID_RE } from '@/lib/plugin-secrets/credentials';
 import { ProxyError, proxyRequest, type CaptureSpec } from '@/lib/plugin-secrets/proxy';
+import { bootGateResponse } from '@/lib/shared-store/boot-gate';
 
 const PLUGIN_ID_RE = /^[a-z0-9][a-z0-9-]*[a-z0-9]$/;
 const FIELD_RE = /^[A-Za-z_][A-Za-z0-9_]{0,63}$/;
@@ -35,6 +36,9 @@ function parseCapture(raw: string | null): CaptureSpec | null | undefined {
  * plugin itself (its iframe has an opaque origin and is refused here).
  */
 export async function POST(request: NextRequest) {
+  // The proxy doesn't run for this route, so it applies the boot gate itself.
+  const notReady = bootGateResponse();
+  if (notReady) return notReady;
   const crossOrigin = rejectCrossOriginRequest(request);
   if (crossOrigin) return crossOrigin;
 
diff --git a/app/api/ready/route.ts b/app/api/ready/route.ts
new file mode 100644
index 000000000..9bb71294a
--- /dev/null
+++ b/app/api/ready/route.ts
@@ -0,0 +1,55 @@
+import { NextResponse } from 'next/server';
+import { bootState, getStore, replicaId } from '@/lib/shared-store';
+
+export const dynamic = 'force-dynamic';
+
+/**
+ * Readiness probe (#1066), separate from the liveness probe /api/health.
+ *
+ * 200 when this instance can serve: always once booted with local files;
+ * with a shared store, once boot finished and the store answered a ping
+ * within a second. 503 otherwise, with the reason in `state`.
+ *
+ * Kubernetes: readinessProbe here, livenessProbe on /api/health. A store
+ * outage then takes pods out of the Service without restarting them.
+ *
+ * The ping result is reused for a second, so probes can't flood the store.
+ */
+
+const CACHE_MS = 1000;
+let last: { at: number; ok: boolean } | null = null;
+
+async function storeAnswers(): Promise<boolean> {
+  if (last && Date.now() - last.at < CACHE_MS) return last.ok;
+  let ok = true;
+  try {
+    await getStore().ping(1000);
+  } catch {
+    ok = false;
+  }
+  last = { at: Date.now(), ok };
+  return ok;
+}
+
+async function readiness(): Promise<{ status: number; body: Record<string, unknown> }> {
+  const store = getStore();
+  const state = bootState();
+  if (state !== 'ready') {
+    return { status: 503, body: { ready: false, store: store.mode, state } };
+  }
+  if (store.mode === 'local') return { status: 200, body: { ready: true, store: 'local' } };
+  if (!(await storeAnswers())) {
+    return { status: 503, body: { ready: false, store: store.mode, state: 'store-down', replica: replicaId() } };
+  }
+  return { status: 200, body: { ready: true, store: store.mode, replica: replicaId() } };
+}
+
+export async function GET() {
+  const { status, body } = await readiness();
+  return NextResponse.json(body, { status, headers: { 'Cache-Control': 'no-store' } });
+}
+
+export async function HEAD() {
+  const { status } = await readiness();
+  return new NextResponse(null, { status, headers: { 'Cache-Control': 'no-store' } });
+}
diff --git a/app/api/webdav/route.ts b/app/api/webdav/route.ts
index c9a0de7b3..3db45873b 100644
--- a/app/api/webdav/route.ts
+++ b/app/api/webdav/route.ts
@@ -6,6 +6,7 @@ import { forwardedClientHeaders, loginClientAddress } from '@/lib/auth/client-ad
 import { fetchJmapServer } from '@/lib/stalwart/server-fetch';
 import { DisallowedUrlError } from '@/lib/security/url-guard';
 import { UpstreamBodyTooLargeError, readBodyCapped, readTextCapped } from '@/lib/security/capped-body';
+import { bootGateResponse } from '@/lib/shared-store/boot-gate';
 
 /** A PROPFIND listing of a folder with many thousand entries still fits. */
 const MAX_PROPFIND_RESPONSE_BYTES = 50 * 1024 * 1024;
@@ -59,6 +60,9 @@ function buildDavTargetUrl(baseUrl: string, username: string, rawPath: string, c
  *   Overwrite: WebDAV Overwrite header for MOVE/COPY
  */
 export async function POST(request: NextRequest) {
+  // The proxy doesn't run for this route, so it applies the boot gate itself.
+  const notReady = bootGateResponse();
+  if (notReady) return notReady;
   // CSRF gate (GHSA-9mvj-98f5-9q6g): this handler acts with the caller's
   // session cookie, which SameSite=Lax still sends from a same-site page.
   const crossOrigin = rejectCrossOriginRequest(request);
diff --git a/app/api/wopi/files/[fileId]/contents/route.ts b/app/api/wopi/files/[fileId]/contents/route.ts
index d60ddf0ed..f70fca6b9 100644
--- a/app/api/wopi/files/[fileId]/contents/route.ts
+++ b/app/api/wopi/files/[fileId]/contents/route.ts
@@ -3,6 +3,7 @@ import { logger } from '@/lib/logger';
 import { downloadFileBlob, getWopiFileNode, updateFileNodeBlob, uploadFileBlob } from '@/lib/wopi/files';
 import { wopiContext } from '@/lib/wopi/request';
 import { readBodyCapped } from '@/lib/security/capped-body';
+import { bootGateResponse } from '@/lib/shared-store/boot-gate';
 
 /**
  * Largest document PutFile takes. proxy.ts doesn't run for this route, so
@@ -39,6 +40,9 @@ export async function GET(
   request: NextRequest,
   { params }: { params: Promise<{ fileId: string }> },
 ) {
+  // The proxy doesn't run for this route, so it applies the boot gate itself.
+  const notReady = bootGateResponse();
+  if (notReady) return notReady;
   // The editor fetches this server-to-server and sends no Sec-Fetch headers.
   // A browser navigating to it, or loading it as a script or style, is not
   // the editor.
@@ -89,6 +93,9 @@ export async function POST(
   request: NextRequest,
   { params }: { params: Promise<{ fileId: string }> },
 ) {
+  // The proxy doesn't run for this route, so it applies the boot gate itself.
+  const notReady = bootGateResponse();
+  if (notReady) return notReady;
   try {
     const { fileId: documentId } = await params;
     const auth = await wopiContext(request, documentId);
diff --git a/instrumentation.node.ts b/instrumentation.node.ts
index 3157b49de..f73112096 100644
--- a/instrumentation.node.ts
+++ b/instrumentation.node.ts
@@ -5,6 +5,9 @@ import { migrateLegacyAdminLayout, migratePolicyUnifiedMailbox } from "./lib/adm
 import { detectSetupState } from "./lib/setup/state";
 import { ensureSetupToken } from "./lib/setup/token";
 import { ensureSessionSecret } from "./lib/auth/session-secret";
+import { bootState, sharedStoreUrl } from "./lib/shared-store";
+import { bootSharedMode } from "./lib/shared-store/boot";
+import { importLocalData } from "./lib/shared-store/import";
 
 const pkg = JSON.parse(
   readFileSync(`${process.cwd()}/package.json`, "utf-8")
@@ -12,23 +15,40 @@ const pkg = JSON.parse(
 const current: string = pkg.version ?? "0.0.0";
 console.info(`Bulwark Webmail v${current}`);
 
+// With REDIS_URL every replica shares one store (#1066). Without it,
+// everything below runs exactly as it always has.
+const shared = sharedStoreUrl() !== null;
+
+async function loadAdmin(): Promise<void> {
+  await configManager.load();
+  await initAdminPassword();
+}
+
+async function bootShared(): Promise<void> {
+  await bootSharedMode({ version: current, importLocal: importLocalData, load: loadAdmin });
+  if (bootState() !== "ready") throw new Error(`shared store boot ended in state ${bootState()}`);
+}
+
 // Initialize admin config and password bootstrap. Migration runs first so
-// existing v1 layouts are split before anything reads admin.json.
+// existing v1 layouts are split before anything reads admin.json (with a
+// shared store: before they are imported).
 migrateLegacyAdminLayout()
   .then(() => migratePolicyUnifiedMailbox())
-  .then(() => configManager.load())
-  .then(() => initAdminPassword())
+  .then(() => (shared ? bootShared() : loadAdmin()))
   .then(async () => {
     console.info("Admin dashboard initialized");
-    try {
-      if (await ensureSessionSecret()) {
-        console.info(
-          "Generated a session secret in the admin state dir. Keep that dir on a " +
-          "persistent volume, or set SESSION_SECRET, so restarts don't sign everyone out.",
-        );
+    // With a shared store, boot already made or loaded the secret.
+    if (!shared) {
+      try {
+        if (await ensureSessionSecret()) {
+          console.info(
+            "Generated a session secret in the admin state dir. Keep that dir on a " +
+            "persistent volume, or set SESSION_SECRET, so restarts don't sign everyone out.",
+          );
+        }
+      } catch (err) {
+        console.warn("Failed to generate a session secret:", err instanceof Error ? err.message : err);
       }
-    } catch (err) {
-      console.warn("Failed to generate a session secret:", err instanceof Error ? err.message : err);
     }
     // If we're in bootstrap state (no JMAP_SERVER_URL env and no
     // setupComplete in config.json), generate/refresh the setup token and
diff --git a/lib/auth/session-secret.ts b/lib/auth/session-secret.ts
index f2c3738a5..cd3b6cee7 100644
--- a/lib/auth/session-secret.ts
+++ b/lib/auth/session-secret.ts
@@ -3,26 +3,28 @@ import { writeFile } from 'node:fs/promises';
 import { configManager } from '@/lib/admin/config-manager';
 import { ensureStateDir, getStatePath } from '@/lib/admin/paths';
 import { readFileEnv } from '@/lib/read-file-env';
+import { getStore, isSharedMode } from '@/lib/shared-store';
 
 /** Written on first start when no secret is configured (see ensureSessionSecret). */
-const GENERATED_SECRET_FILE = '.session-secret';
+export const GENERATED_SECRET_FILE = '.session-secret';
+
+/** Where the generated secret lives in the shared store. */
+export const SHARED_SECRET_KEY = 'session-secret';
 
 /**
- * Resolve the session secret from any of the supported sources, in priority
- * order:
- *   1. SESSION_SECRET env var
- *   2. SESSION_SECRET_FILE-pointed file
- *   3. Admin override in config.json (set by the setup wizard)
- *   4. The secret generated on first start, in the admin state dir
- *
- * Returns an empty string when nothing is configured. Callers must treat
- * empty as "feature disabled" rather than crashing.
- *
- * The configManager fallback exists so the web installer can persist the
- * secret without touching .env files. It only takes effect if the env vars
- * aren't set, so existing deployments aren't affected.
+ * The generated secret as read from the shared store at boot. It never
+ * changes once created, so every module copy (proxy, routes) reads it from
+ * here without waiting on the store.
  */
-export function getSessionSecret(): string {
+const SHARED_CACHE = Symbol.for('bulwark.sessionSecret.shared');
+type SecretGlobals = typeof globalThis & { [SHARED_CACHE]?: string };
+
+function sharedSecret(): string {
+  return (globalThis as SecretGlobals)[SHARED_CACHE] ?? '';
+}
+
+/** A secret the operator set: env, file, or the setup wizard's config. */
+function configuredSessionSecret(): string {
   const fromEnv = process.env.SESSION_SECRET;
   if (fromEnv) return fromEnv;
 
@@ -32,11 +34,29 @@ export function getSessionSecret(): string {
   const fromAdmin = configManager.get<string>('sessionSecret', '');
   if (fromAdmin) return fromAdmin;
 
-  const fromAdminFile = readFileEnv(
-    configManager.get<string>('sessionSecretFile', ''),
-  );
-  if (fromAdminFile) return fromAdminFile;
+  return readFileEnv(configManager.get<string>('sessionSecretFile', '')) || '';
+}
 
+/**
+ * Resolve the session secret from any of the supported sources, in priority
+ * order:
+ *   1. SESSION_SECRET env var
+ *   2. SESSION_SECRET_FILE-pointed file
+ *   3. Admin override in config.json (set by the setup wizard)
+ *   4. The secret generated on first start: in the admin state dir, or with
+ *      a shared store in the store, one for every replica (#1066)
+ *
+ * Returns an empty string when nothing is configured. Callers must treat
+ * empty as "feature disabled" rather than crashing.
+ *
+ * The configManager fallback exists so the web installer can persist the
+ * secret without touching .env files. It only takes effect if the env vars
+ * aren't set, so existing deployments aren't affected.
+ */
+export function getSessionSecret(): string {
+  const configured = configuredSessionSecret();
+  if (configured) return configured;
+  if (isSharedMode()) return sharedSecret();
   return readFileEnv(getStatePath(GENERATED_SECRET_FILE)) || "";
 }
 
@@ -51,9 +71,14 @@ export function hasSessionSecret(): boolean {
  * writable and a volume in docker-compose.yml; without a persistent volume a
  * restart issues a new one and signs everyone out.
  *
+ * With a shared store it lives in the store: the first replica creates it
+ * and every replica caches the stored value, so replicas starting at the
+ * same moment end up with one secret.
+ *
  * Returns true when this call wrote a new secret.
  */
 export async function ensureSessionSecret(): Promise<boolean> {
+  if (isSharedMode()) return ensureSharedSessionSecret();
   if (hasSessionSecret()) return false;
   await ensureStateDir();
   try {
@@ -68,3 +93,33 @@ export async function ensureSessionSecret(): Promise<boolean> {
     throw error;
   }
 }
+
+/**
+ * Shared mode: create the stored secret if there is none (`value`, or a new
+ * random one), then load whatever the store holds into this process. Also
+ * called when a configured secret exists, so the stored one (used once the
+ * configured one is removed) is loaded either way.
+ */
+export async function ensureSharedSessionSecret(value?: string): Promise<boolean> {
+  const store = getStore();
+  const created = await store.kv.set(SHARED_SECRET_KEY, value ?? randomBytes(32).toString('base64'), { ifAbsent: true });
+  const stored = await store.kv.get(SHARED_SECRET_KEY);
+  if (!stored) throw new Error('The shared store lost the session secret right after writing it');
+  (globalThis as SecretGlobals)[SHARED_CACHE] = stored;
+  return created;
+}
+
+/** The generated secret of this install's state dir, if there is one (the import). */
+export function localGeneratedSessionSecret(): string {
+  return readFileEnv(getStatePath(GENERATED_SECRET_FILE)) || '';
+}
+
+/** Whether the operator configured a secret (env, file or config). */
+export function hasConfiguredSessionSecret(): boolean {
+  return configuredSessionSecret().length > 0;
+}
+
+/** Forget the cached shared secret (tests). */
+export function resetSharedSessionSecretForTests(): void {
+  delete (globalThis as SecretGlobals)[SHARED_CACHE];
+}
diff --git a/lib/shared-store/__tests__/boot.test.ts b/lib/shared-store/__tests__/boot.test.ts
new file mode 100644
index 000000000..d6964070d
--- /dev/null
+++ b/lib/shared-store/__tests__/boot.test.ts
@@ -0,0 +1,210 @@
+/**
+ * @vitest-environment node
+ */
+import { mkdtempSync, rmSync, writeFileSync, mkdirSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import path from 'node:path';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('@/lib/admin/config-manager', () => ({
+  configManager: { get: vi.fn((_key: string, fallback: unknown) => fallback) },
+}));
+
+import { bootState, getStore, setBootState, setStoreForTests } from '../index';
+import { resetLocalStoreForTests } from '../local';
+import {
+  INIT_EPOCH,
+  announceChange,
+  markEpochsSeen,
+  onSharedChange,
+  pollChanges,
+  resetChangesForTests,
+} from '../changes';
+import { bootSharedMode } from '../boot';
+import {
+  ensureSharedSessionSecret,
+  getSessionSecret,
+  resetSharedSessionSecretForTests,
+  SHARED_SECRET_KEY,
+} from '@/lib/auth/session-secret';
+import { fakeSharedStore } from './fake-shared';
+
+let stateDir: string;
+
+beforeEach(() => {
+  stateDir = mkdtempSync(path.join(tmpdir(), 'bulwark-boot-'));
+  vi.stubEnv('ADMIN_STATE_DIR', stateDir);
+  vi.stubEnv('SESSION_SECRET', '');
+  vi.stubEnv('SESSION_SECRET_FILE', '');
+  resetLocalStoreForTests();
+  resetChangesForTests();
+  resetSharedSessionSecretForTests();
+});
+
+afterEach(() => {
+  setStoreForTests(null);
+  resetChangesForTests();
+  vi.unstubAllEnvs();
+  rmSync(stateDir, { recursive: true, force: true });
+});
+
+const noImport = async () => ({ imported: {} });
+
+describe('change notices', () => {
+  it('runs this process\'s listeners at once in local mode', async () => {
+    const seen: string[] = [];
+    onSharedChange('config', (origin) => void seen.push(origin));
+    await announceChange('config');
+    expect(seen).toEqual(['local']);
+  });
+
+  it('runs listeners for another replica\'s change on the next poll, once', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    await store.epochs.bump(INIT_EPOCH);
+    await markEpochsSeen();
+    const seen: string[] = [];
+    onSharedChange('policy', (origin) => void seen.push(origin));
+    await store.epochs.bump('policy'); // another replica saved
+    await pollChanges();
+    await pollChanges();
+    expect(seen).toEqual(['remote']);
+  });
+
+  it('does not reload its own change, but does when another landed in between', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    await store.epochs.bump(INIT_EPOCH);
+    await markEpochsSeen();
+    const seen: string[] = [];
+    onSharedChange('config', (origin) => void seen.push(origin));
+    await announceChange('config');
+    await pollChanges();
+    expect(seen).toEqual(['local']);
+    await store.epochs.bump('config'); // other replica
+    await announceChange('config'); // ours right after
+    await pollChanges();
+    expect(seen).toEqual(['local', 'local', 'remote']);
+  });
+
+  it('retries a listener that failed on the next poll', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    await store.epochs.bump(INIT_EPOCH);
+    await markEpochsSeen();
+    let calls = 0;
+    onSharedChange('plugins', () => {
+      calls++;
+      if (calls === 1) throw new Error('store hiccup');
+    });
+    await store.epochs.bump('plugins');
+    await pollChanges();
+    await pollChanges();
+    await pollChanges();
+    expect(calls).toBe(2);
+  });
+
+  it('goes to store-lost when the store forgets the deployment, and back when it returns', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    setBootState('ready');
+    await pollChanges(); // no _init epoch: an emptied store
+    expect(bootState()).toBe('store-lost');
+    const reloaded: string[] = [];
+    onSharedChange('config', (origin) => void reloaded.push(origin));
+    await store.epochs.bump(INIT_EPOCH); // restored
+    await pollChanges();
+    expect(bootState()).toBe('ready');
+    expect(reloaded).toEqual(['remote']);
+  });
+});
+
+describe('session secret with a shared store', () => {
+  it('gives every replica the same generated secret', async () => {
+    setStoreForTests(fakeSharedStore());
+    const created = await Promise.all([ensureSharedSessionSecret(), ensureSharedSessionSecret(), ensureSharedSessionSecret()]);
+    expect(created.filter(Boolean)).toHaveLength(1);
+    const stored = await getStore().kv.get(SHARED_SECRET_KEY);
+    expect(stored).toMatch(/^[A-Za-z0-9+/]{43}=$/);
+    expect(getSessionSecret()).toBe(stored);
+  });
+
+  it('ignores the state dir file in shared mode and lets SESSION_SECRET win', async () => {
+    writeFileSync(path.join(stateDir, '.session-secret'), 'local-file-secret-local-file-secret-xx');
+    setStoreForTests(fakeSharedStore());
+    await ensureSharedSessionSecret();
+    expect(getSessionSecret()).not.toBe('local-file-secret-local-file-secret-xx');
+    vi.stubEnv('SESSION_SECRET', 'from-env-from-env-from-env-from-env-xx');
+    expect(getSessionSecret()).toBe('from-env-from-env-from-env-from-env-xx');
+  });
+});
+
+describe('bootSharedMode', () => {
+  it('initialises an empty store: secret, marker, then loads and becomes ready', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    const order: string[] = [];
+    await bootSharedMode({
+      version: '9.9.9',
+      importLocal: async () => { order.push('import'); return { imported: { settings: 2 } }; },
+      load: async () => { order.push('load'); },
+    });
+    expect(order).toEqual(['import', 'load']);
+    expect(bootState()).toBe('ready');
+    expect((await store.epochs.all())[INIT_EPOCH]).toBe(1);
+    expect(JSON.parse((await store.kv.get('meta:initialized'))!)).toMatchObject({ imported: { settings: 2 } });
+    expect(getSessionSecret()).toBe(await store.kv.get(SHARED_SECRET_KEY));
+    expect((await store.replicas.recent(60_000)).map((r) => r.version)).toEqual(['9.9.9']);
+  });
+
+  it('imports this install\'s generated secret before creating one', async () => {
+    writeFileSync(path.join(stateDir, '.session-secret'), 'old-install-secret-old-install-secret');
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    await bootSharedMode({ version: '1', importLocal: noImport, load: async () => {} });
+    expect(await store.kv.get(SHARED_SECRET_KEY)).toBe('old-install-secret-old-install-secret');
+    expect(getSessionSecret()).toBe('old-install-secret-old-install-secret');
+  });
+
+  it('joins an initialised store without importing again', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    await bootSharedMode({ version: '1', importLocal: noImport, load: async () => {} });
+    const importLocal = vi.fn(noImport);
+    resetSharedSessionSecretForTests();
+    await bootSharedMode({ version: '1', importLocal, load: async () => {} });
+    expect(importLocal).not.toHaveBeenCalled();
+    expect(bootState()).toBe('ready');
+    expect((await store.epochs.all())[INIT_EPOCH]).toBe(1);
+  });
+
+  it('refuses to serve when its own local data was encrypted with another secret', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    await bootSharedMode({ version: '1', importLocal: noImport, load: async () => {} });
+    mkdirSync(stateDir, { recursive: true });
+    writeFileSync(path.join(stateDir, '.session-secret'), 'a-different-install-secret-a-different');
+    const load = vi.fn(async () => {});
+    await bootSharedMode({ version: '1', importLocal: noImport, load });
+    expect(bootState()).toBe('blocked');
+    expect(load).not.toHaveBeenCalled();
+  });
+
+  it('waits for a store that is down at start', async () => {
+    vi.useFakeTimers({ toFake: ['setTimeout', 'setInterval'] });
+    try {
+      const store = fakeSharedStore();
+      store.down = true;
+      setStoreForTests(store);
+      const done = bootSharedMode({ version: '1', importLocal: noImport, load: async () => {} });
+      await vi.advanceTimersByTimeAsync(5000);
+      expect(bootState()).toBe('starting');
+      store.down = false;
+      await vi.advanceTimersByTimeAsync(5000);
+      await done;
+      expect(bootState()).toBe('ready');
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+});
diff --git a/lib/shared-store/__tests__/fake-shared.ts b/lib/shared-store/__tests__/fake-shared.ts
new file mode 100644
index 000000000..13e8fd6c0
--- /dev/null
+++ b/lib/shared-store/__tests__/fake-shared.ts
@@ -0,0 +1,27 @@
+import { createLocalStore } from '../local';
+import type { SharedStore } from '../types';
+
+/**
+ * The local store under the shared store's name: everything that branches
+ * on isSharedMode() takes its shared path, in one process. The contract
+ * suite proves both implementations behave alike, so this stands in for a
+ * Redis in unit tests. `down` makes every call fail as an outage would.
+ */
+export function fakeSharedStore(): SharedStore & { down: boolean } {
+  const local = createLocalStore();
+  const fake = {
+    ...local,
+    mode: 'redis' as const,
+    down: false,
+    async ping() {
+      if (fake.down) {
+        const { StoreUnavailableError } = await import('../types');
+        throw new StoreUnavailableError('down (test)');
+      }
+    },
+    status() {
+      return { mode: 'redis' as const, location: 'redis://fake:6379/0', connected: !fake.down };
+    },
+  };
+  return fake;
+}
diff --git a/lib/shared-store/__tests__/ready-and-gate.test.ts b/lib/shared-store/__tests__/ready-and-gate.test.ts
new file mode 100644
index 000000000..6341107f1
--- /dev/null
+++ b/lib/shared-store/__tests__/ready-and-gate.test.ts
@@ -0,0 +1,92 @@
+/**
+ * @vitest-environment node
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { NextRequest } from 'next/server';
+
+vi.mock('@/lib/admin/config-manager', () => ({
+  configManager: {
+    ensureLoaded: vi.fn(async () => {}),
+    get: vi.fn((_key: string, fallback: unknown) => fallback),
+    getPolicy: vi.fn(() => ({ features: { pluginsEnabled: false } })),
+  },
+}));
+vi.mock('@/lib/setup/state', () => ({ detectSetupState: vi.fn(() => 'configured') }));
+vi.mock('@/lib/admin/csp-frame-origins', () => ({ getEnabledPluginFrameOrigins: vi.fn(async () => []) }));
+vi.mock('next-intl/middleware', async () => {
+  const { NextResponse } = await import('next/server');
+  return { default: () => () => NextResponse.next() };
+});
+
+import { proxy } from '@/proxy';
+import { GET as ready } from '@/app/api/ready/route';
+import { configManager } from '@/lib/admin/config-manager';
+import { setBootState, setStoreForTests } from '../index';
+import { fakeSharedStore } from './fake-shared';
+
+afterEach(() => {
+  setStoreForTests(null);
+  vi.clearAllMocks();
+});
+
+function request(path: string): NextRequest {
+  return new NextRequest('http://localhost:3000' + path, {
+    headers: { 'sec-fetch-dest': 'document', 'sec-fetch-mode': 'navigate', 'sec-fetch-site': 'none' },
+  });
+}
+
+describe('/api/ready', () => {
+  it('is ready with local files, as a single instance always was', async () => {
+    const res = await ready();
+    expect(res.status).toBe(200);
+    expect(await res.json()).toMatchObject({ ready: true, store: 'local' });
+  });
+
+  it('is not ready while a shared-store replica is starting', async () => {
+    setStoreForTests(fakeSharedStore());
+    const res = await ready();
+    expect(res.status).toBe(503);
+    expect(await res.json()).toMatchObject({ ready: false, state: 'starting' });
+  });
+
+  it('follows the store once booted', async () => {
+    const store = fakeSharedStore();
+    setStoreForTests(store);
+    setBootState('ready');
+    expect((await ready()).status).toBe(200);
+    store.down = true;
+    await new Promise((r) => setTimeout(r, 1100)); // past the 1 s ping cache
+    const down = await ready();
+    expect(down.status).toBe(503);
+    expect(await down.json()).toMatchObject({ state: 'store-down' });
+  });
+});
+
+describe('proxy boot gate', () => {
+  it('serves nothing but the probes while a shared-store replica is starting', async () => {
+    setStoreForTests(fakeSharedStore());
+    const page = await proxy(request('/en/mail'));
+    expect(page.status).toBe(503);
+    expect(page.headers.get('retry-after')).toBe('5');
+    const api = await proxy(request('/api/settings'));
+    expect(api.status).toBe(503);
+    expect(await api.json()).toMatchObject({ error: 'not_ready', state: 'starting' });
+    // The probes pass through to their routes, without touching config.
+    for (const probe of ['/api/health', '/api/ready']) {
+      const res = await proxy(request(probe));
+      expect(res.status).toBe(200);
+      expect(res.headers.get('x-middleware-next')).toBe('1');
+    }
+    expect(configManager.ensureLoaded).not.toHaveBeenCalled();
+  });
+
+  it('lets everything through once booted, and always in local mode', async () => {
+    const page = await proxy(request('/en/mail'));
+    expect(page.status).not.toBe(503);
+    setStoreForTests(fakeSharedStore());
+    setBootState('ready');
+    expect((await proxy(request('/en/mail'))).status).not.toBe(503);
+    setBootState('store-lost');
+    expect((await proxy(request('/en/mail'))).status).toBe(503);
+  });
+});
diff --git a/lib/shared-store/boot-gate.ts b/lib/shared-store/boot-gate.ts
new file mode 100644
index 000000000..1c993d17e
--- /dev/null
+++ b/lib/shared-store/boot-gate.ts
@@ -0,0 +1,25 @@
+import { NextResponse } from 'next/server';
+import { bootState } from './index';
+
+const MESSAGES = {
+  starting: 'Bulwark is starting and connecting to its shared store.',
+  'store-lost': 'Bulwark lost its shared store data and is waiting for it to be restored.',
+  blocked: 'This Bulwark instance has local data that conflicts with the shared store. See the server log.',
+} as const;
+
+/**
+ * 503 for every request while this replica can't serve (shared mode only:
+ * local mode is always ready). The proxy calls it for everything but the
+ * health and readiness probes; the three upload routes the proxy skips call
+ * it themselves.
+ */
+export function bootGateResponse(pathname?: string): NextResponse | null {
+  const state = bootState();
+  if (state === 'ready') return null;
+  const message = MESSAGES[state];
+  const headers = { 'Retry-After': '5', 'Cache-Control': 'no-store' };
+  if (!pathname || pathname.startsWith('/api/')) {
+    return NextResponse.json({ error: 'not_ready', state, message }, { status: 503, headers });
+  }
+  return new NextResponse(message, { status: 503, headers: { ...headers, 'Content-Type': 'text/plain; charset=utf-8' } });
+}
diff --git a/lib/shared-store/boot.ts b/lib/shared-store/boot.ts
new file mode 100644
index 000000000..7a434f56b
--- /dev/null
+++ b/lib/shared-store/boot.ts
@@ -0,0 +1,170 @@
+import { logger } from '@/lib/logger';
+import {
+  ensureSharedSessionSecret,
+  hasConfiguredSessionSecret,
+  localGeneratedSessionSecret,
+  SHARED_SECRET_KEY,
+} from '@/lib/auth/session-secret';
+import { INIT_EPOCH, markEpochsSeen, startChangePoller } from './changes';
+import { getStore, replicaId, setBootState } from './index';
+
+/**
+ * Boot with a shared store (#1066). Order:
+ *
+ * 1. Wait until the store answers (the replica reports "starting" meanwhile;
+ *    the proxy answers 503 to everything but the probes).
+ * 2. Under the `boot` lease, so replicas starting together take turns:
+ *    - Store not initialised yet: import this replica's local files (an
+ *      existing single-instance install moving over), create the session
+ *      secret if none was imported, then mark the store initialised. The
+ *      marker is written last, so an import that dies halfway is redone.
+ *    - Already initialised: check that this replica's local data doesn't
+ *      conflict with it (a generated secret of its own would mean data
+ *      encrypted with another key).
+ * 3. Load the secret, take the epochs as the baseline, run `load` (config
+ *    and the rest), start the change poller and the replica heartbeat.
+ */
+
+export interface ImportSummary {
+  /** What was written per store, e.g. { settings: 3, config: 1 }. */
+  imported: Record<string, number>;
+}
+
+export interface SharedBootOptions {
+  version: string;
+  /** Copy this replica's local files into an uninitialised store. */
+  importLocal: () => Promise<ImportSummary>;
+  /** Load everything this process caches from the store (config, …). */
+  load: () => Promise<void>;
+}
+
+const BOOT_LEASE = 'boot';
+const BOOT_LEASE_TTL_MS = 60_000;
+const HEARTBEAT_MS = 15_000;
+const META_KEY = 'meta:initialized';
+
+function sleep(ms: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}
+
+async function waitForStore(): Promise<void> {
+  const store = getStore();
+  let warned = false;
+  for (;;) {
+    try {
+      await store.ping(2000);
+      return;
+    } catch (error) {
+      if (!warned) {
+        warned = true;
+        logger.warn('Waiting for the shared store', {
+          location: store.status().location,
+          error: error instanceof Error ? error.message : 'Unknown error',
+        });
+      }
+      await sleep(2000);
+    }
+  }
+}
+
+/** Hold the boot lease while `task` runs, renewing it; wait for it first. */
+async function withBootLease<T>(task: () => Promise<T>): Promise<T> {
+  const store = getStore();
+  const holder = replicaId();
+  while (!(await store.leases.acquire(BOOT_LEASE, holder, BOOT_LEASE_TTL_MS))) await sleep(1000);
+  const renew = setInterval(() => {
+    void store.leases.acquire(BOOT_LEASE, holder, BOOT_LEASE_TTL_MS).catch(() => {});
+  }, BOOT_LEASE_TTL_MS / 3);
+  renew.unref();
+  try {
+    return await task();
+  } finally {
+    clearInterval(renew);
+    await store.leases.release(BOOT_LEASE, holder).catch(() => {});
+  }
+}
+
+async function initialise(options: SharedBootOptions): Promise<'initialised' | 'joined' | 'blocked'> {
+  const store = getStore();
+  return withBootLease(async () => {
+    const epochs = await store.epochs.all();
+    if (!epochs[INIT_EPOCH]) {
+      // The local generated secret first: settings and plugin secrets
+      // imported with it are encrypted with it.
+      const localSecret = localGeneratedSessionSecret();
+      if (localSecret) await store.kv.set(SHARED_SECRET_KEY, localSecret, { ifAbsent: true });
+      const summary = await options.importLocal();
+      await ensureSharedSessionSecret();
+      await store.kv.set(META_KEY, JSON.stringify({ at: new Date().toISOString(), by: replicaId(), ...summary }));
+      await store.epochs.bump(INIT_EPOCH);
+      const total = Object.values(summary.imported).reduce((a, b) => a + b, 0);
+      logger.info(
+        total > 0 ? 'Shared store initialised from this instance\'s local data' : 'Shared store initialised (no local data to import)',
+        summary.imported,
+      );
+      return 'initialised';
+    }
+
+    // Joining: a generated secret in this replica's state dir that isn't the
+    // store's means its local data belongs to another install. Serving would
+    // be fine, but the admin most likely meant to import that data.
+    const localSecret = localGeneratedSessionSecret();
+    const storedSecret = await store.kv.get(SHARED_SECRET_KEY);
+    if (localSecret && storedSecret && localSecret !== storedSecret && !hasConfiguredSessionSecret()) {
+      logger.error(
+        'This instance has local data (a generated session secret in its state dir) from a single-instance ' +
+        'install, but the shared store was already initialised without it. Its settings and plugin ' +
+        'credentials could not be read. Either remove the local data dirs from this instance, or empty the ' +
+        'shared store and start this instance first so it imports them. See the docs, "Running several instances".',
+      );
+      return 'blocked';
+    }
+    return 'joined';
+  });
+}
+
+function startHeartbeat(version: string): void {
+  const store = getStore();
+  const info = { id: replicaId(), version, startedAt: Date.now() };
+  const beat = () => void store.replicas.beat(info).catch(() => {});
+  beat();
+  setInterval(beat, HEARTBEAT_MS).unref();
+}
+
+export async function bootSharedMode(options: SharedBootOptions): Promise<void> {
+  setBootState('starting');
+  const store = getStore();
+  logger.info('Using a shared store', { location: store.status().location, replica: replicaId() });
+  await waitForStore();
+  // A store that drops mid-boot is waited for again at each step.
+  for (;;) {
+    try {
+      const result = await initialise(options);
+      if (result === 'blocked') {
+        setBootState('blocked');
+        return;
+      }
+      await ensureSharedSessionSecret();
+      if (!hasConfiguredSessionSecret()) {
+        logger.warn(
+          'No SESSION_SECRET set; using the one in the shared store. Set SESSION_SECRET on every replica, ' +
+          'so a lost store does not sign everyone out and make stored settings unreadable.',
+        );
+      }
+      // Baseline before loading, so a change during the load is seen.
+      await markEpochsSeen();
+      await options.load();
+      break;
+    } catch (error) {
+      logger.warn('Shared store boot step failed; retrying', {
+        error: error instanceof Error ? error.message : 'Unknown error',
+      });
+      await sleep(2000);
+      await waitForStore();
+    }
+  }
+  startChangePoller();
+  startHeartbeat(options.version);
+  setBootState('ready');
+  logger.info('Shared store ready', { replica: replicaId() });
+}
diff --git a/lib/shared-store/changes.ts b/lib/shared-store/changes.ts
new file mode 100644
index 000000000..060e8934f
--- /dev/null
+++ b/lib/shared-store/changes.ts
@@ -0,0 +1,182 @@
+import { logger } from '@/lib/logger';
+import { getStore, isSharedMode, setBootState } from './index';
+
+/**
+ * How admin changes reach every replica (#1066).
+ *
+ * Every write to shared state calls announceChange(area): that bumps the
+ * area's epoch in the store and runs this process's listeners at once (they
+ * may live in another module copy: proxy, instrumentation, routes). Every
+ * replica reads all epochs once a second and runs the listeners of each
+ * area whose epoch moved. A replica therefore sees a change within about a
+ * second, and one that couldn't reach the store for a while catches up on
+ * its first successful read.
+ *
+ * The epoch is noted before the listeners reload, so a change that lands
+ * while they read is picked up by the next poll.
+ *
+ * Local mode: no poller; announceChange only runs this process's listeners.
+ */
+
+export type ChangeOrigin = 'local' | 'remote';
+export type ChangeListener = (origin: ChangeOrigin) => void | Promise<void>;
+
+/** Set once the store holds a deployment; its absence later means the store lost its data. */
+export const INIT_EPOCH = '_init';
+
+export const POLL_INTERVAL_MS = 1000;
+
+interface ChangeGlobals {
+  listeners: Map<string, Set<ChangeListener>>;
+  seen: Map<string, number>;
+  poller?: NodeJS.Timeout;
+  polling?: Promise<void>;
+  lastPollOk: number;
+  storeLost: boolean;
+}
+
+const KEY = Symbol.for('bulwark.sharedStore.changes');
+
+function globals(): ChangeGlobals {
+  const g = globalThis as typeof globalThis & { [KEY]?: ChangeGlobals };
+  return (g[KEY] ??= { listeners: new Map(), seen: new Map(), lastPollOk: 0, storeLost: false });
+}
+
+/**
+ * Run `fn` when `area` changes: at once for a write in this process
+ * (origin 'local'), and after another replica's write (origin 'remote',
+ * shared mode only). Returns an unsubscribe function.
+ */
+export function onSharedChange(area: string, fn: ChangeListener): () => void {
+  const listeners = globals().listeners;
+  let set = listeners.get(area);
+  if (!set) listeners.set(area, (set = new Set()));
+  set.add(fn);
+  return () => set!.delete(fn);
+}
+
+async function runListeners(area: string, origin: ChangeOrigin): Promise<boolean> {
+  let ok = true;
+  for (const fn of [...(globals().listeners.get(area) ?? [])]) {
+    try {
+      await fn(origin);
+    } catch (error) {
+      ok = false;
+      logger.warn('Could not apply a change from the shared store', {
+        area,
+        error: error instanceof Error ? error.message : 'Unknown error',
+      });
+    }
+  }
+  return ok;
+}
+
+/**
+ * Tell every replica that `area` changed. Call it after the write went
+ * through. In shared mode a failure to bump the epoch is logged, not thrown:
+ * the write itself succeeded, and other replicas then see it on their next
+ * reload of that area (or after their next restart) instead.
+ */
+export async function announceChange(area: string): Promise<void> {
+  const g = globals();
+  if (isSharedMode()) {
+    try {
+      const epoch = await getStore().epochs.bump(area);
+      // Only skip our own change when nothing else landed in between;
+      // otherwise the poller reloads to pick up the other replica's.
+      if ((g.seen.get(area) ?? 0) === epoch - 1) g.seen.set(area, epoch);
+    } catch (error) {
+      logger.warn('Could not announce a change to the other replicas', {
+        area,
+        error: error instanceof Error ? error.message : 'Unknown error',
+      });
+    }
+  }
+  await runListeners(area, 'local');
+}
+
+/** One poll: read the epochs, run the listeners of every area that moved. */
+export async function pollChanges(): Promise<void> {
+  const g = globals();
+  if (g.polling) return g.polling;
+  g.polling = (async () => {
+    const epochs = await getStore().epochs.all();
+    if (!epochs[INIT_EPOCH]) {
+      if (!g.storeLost) {
+        g.storeLost = true;
+        setBootState('store-lost');
+        logger.error(
+          'The shared store no longer holds this deployment (restarted without persistence, flushed or evicted). ' +
+          'Keeping the last loaded config; nothing is regenerated. Restore the store from a backup, ' +
+          'or restart every replica to start it over.',
+        );
+      }
+      return;
+    }
+    g.lastPollOk = Date.now();
+    if (g.storeLost) {
+      // Restored from a backup: reload everything from it.
+      g.storeLost = false;
+      g.seen.clear();
+      setBootState('ready');
+      logger.warn('The shared store holds this deployment again; reloading from it');
+      for (const area of g.listeners.keys()) if (!(area in epochs)) await runListeners(area, 'remote');
+    }
+    for (const [area, epoch] of Object.entries(epochs)) {
+      if (area === INIT_EPOCH) continue;
+      const before = g.seen.get(area) ?? 0;
+      if (epoch === before) continue;
+      g.seen.set(area, epoch);
+      if (!(await runListeners(area, 'remote'))) {
+        // Try again on the next poll.
+        if (g.seen.get(area) === epoch) g.seen.set(area, before);
+      }
+    }
+  })()
+    .catch(() => {
+      // Store unreachable: readiness reports it; the next poll catches up.
+    })
+    .finally(() => {
+      g.polling = undefined;
+    });
+  return g.polling;
+}
+
+/**
+ * Take the current epochs as the baseline, so the first poll doesn't reload
+ * what boot just loaded. Call before loading the data the listeners reload.
+ */
+export async function markEpochsSeen(): Promise<void> {
+  const epochs = await getStore().epochs.all();
+  const g = globals();
+  for (const [area, epoch] of Object.entries(epochs)) g.seen.set(area, epoch);
+}
+
+export function startChangePoller(intervalMs = POLL_INTERVAL_MS): void {
+  const g = globals();
+  if (g.poller || !isSharedMode()) return;
+  g.poller = setInterval(() => void pollChanges(), intervalMs);
+  g.poller.unref();
+}
+
+export function stopChangePoller(): void {
+  const g = globals();
+  if (g.poller) clearInterval(g.poller);
+  g.poller = undefined;
+}
+
+/** When the epochs were last read successfully (ms), 0 if never. */
+export function lastSuccessfulPoll(): number {
+  return globals().lastPollOk;
+}
+
+export function storeLostItsData(): boolean {
+  return globals().storeLost;
+}
+
+/** Forget listeners and epochs (tests). */
+export function resetChangesForTests(): void {
+  stopChangePoller();
+  const g = globalThis as typeof globalThis & { [KEY]?: ChangeGlobals };
+  delete g[KEY];
+}
diff --git a/lib/shared-store/import.ts b/lib/shared-store/import.ts
new file mode 100644
index 000000000..0256f5204
--- /dev/null
+++ b/lib/shared-store/import.ts
@@ -0,0 +1,13 @@
+import type { ImportSummary } from './boot';
+
+/**
+ * Copy an existing single-instance install's local files into an
+ * uninitialised shared store (#1066). Every store adds its own step here;
+ * each step writes only what the store doesn't hold yet (put with
+ * ifVersion: null), so running it again changes nothing, and it never
+ * changes or deletes a local file.
+ */
+export async function importLocalData(): Promise<ImportSummary> {
+  const imported: Record<string, number> = {};
+  return { imported };
+}
diff --git a/proxy.ts b/proxy.ts
index 28fd6525f..12f1423ed 100644
--- a/proxy.ts
+++ b/proxy.ts
@@ -15,6 +15,7 @@ import { listReachableHosts, noThirdPartyCspSources } from "./lib/admin/reachabl
 import { getWopiEditorOrigins } from "./lib/wopi/discovery";
 import { detectSetupState } from "./lib/setup/state";
 import { isServedOverHttps } from "./lib/security/request-protocol";
+import { bootGateResponse } from "./lib/shared-store/boot-gate";
 
 const intlMiddleware = createIntlMiddleware(routing);
 
@@ -187,6 +188,19 @@ function isSetupPath(pathname: string): boolean {
 }
 
 export async function proxy(request: NextRequest) {
+  // The probes answer without config or store, so a replica that is still
+  // starting (or lost its shared store) can say so (#1066).
+  const probePath = routePathOf(request.nextUrl.pathname);
+  if (probePath === "/api/health" || probePath === "/api/ready") {
+    const probe = NextResponse.next();
+    probe.headers.set("X-Content-Type-Options", "nosniff");
+    probe.headers.set("X-Frame-Options", "DENY");
+    return probe;
+  }
+  // With a shared store, nothing else is served until boot finished.
+  const notReady = bootGateResponse(probePath);
+  if (notReady) return notReady;
+
   // Resolve setup state before deciding what to skip. The first call after
   // boot triggers the config load; subsequent calls are in-memory.
   await configManager.ensureLoaded();