Deduplicate custom background tasks

CHANGES.md

@@ -285,8 +285,18 @@ To be released.
         `FederationOptions.taskQueueResolution` option is set to `"strict"`.
         `Federation.startQueue()` now accepts `queue: "task"` to run
         a task-only worker.
-
-    [[#206], [#797], [#803] by ChanHaeng Lee]
+     -  Tasks can request at-most-once enqueue with a `deduplicationKey`
+        (new `TaskEnqueueOptions.deduplicationKey`).  A queue declaring the new
+        `MessageQueue.nativeDeduplication` capability owns the check and
+        receives the key through the new
+        `MessageQueueEnqueueOptions.deduplicationKey`; otherwise Fedify
+        performs a best-effort key–value guard through the optional
+        `KvStore.cas` primitive, under a new `taskDeduplication` key prefix.
+        The marker TTL and the no-`cas` fallback are tunable with the new
+        `FederationOptions.taskDeduplicationTtl` and
+        `FederationOptions.taskDeduplicationFallback` options.
+
+    [[#206], [#797], [#798], [#803] by ChanHaeng Lee]
 
 [Standard Schema]: https://standardschema.dev/
 [#206]: https://github.com/fedify-dev/fedify/issues/206
@@ -320,6 +330,7 @@ To be released.
 [#782]: https://github.com/fedify-dev/fedify/issues/782
 [#787]: https://github.com/fedify-dev/fedify/pull/787
 [#797]: https://github.com/fedify-dev/fedify/issues/797
+[#798]: https://github.com/fedify-dev/fedify/issues/798
 [#800]: https://github.com/fedify-dev/fedify/pull/800
 [#803]: https://github.com/fedify-dev/fedify/pull/803
 

docs/manual/tasks.md

@@ -1,3 +1,5 @@
+<!-- deno-fmt-ignore-file -->
+
 Background tasks
 ================
 
@@ -139,6 +141,10 @@ Both methods accept options:
 :   Tasks with the same ordering key are processed sequentially (one at
     a time), like the same option on the message queue layer.
 
+`deduplicationKey`
+:   Requests at-most-once enqueue for tasks that share the key; see
+    [Deduplication](#deduplication) below.
+
 ~~~~ typescript
 await ctx.enqueueTask(sendDigest, payload, {
   delay: { minutes: 30 },
@@ -257,12 +263,96 @@ delivered it.
 > queue and set `taskQueueResolution: "strict"`.
 
 
+Deduplication
+-------------
+
+A task often needs *at-most-once-per-key* enqueue: a digest mailer must not
+send twice when a request is retried, and a cleanup job should coalesce
+duplicate triggers.  Passing a `deduplicationKey` requests this—a second
+enqueue with the same key is dropped while the first is still within the
+deduplication window:
+
+~~~~ typescript
+await ctx.enqueueTask(sendDigest, payload, {
+  deduplicationKey: `digest:${payload.userId}`,  // [!code highlight]
+});
+~~~~
+
+How the key is resolved depends on the queue and the key–value store:
+
+1.  **Native backend.**  When the task's queue declares
+    `~MessageQueue.nativeDeduplication`, Fedify forwards the key in the
+    message queue's `~MessageQueueEnqueueOptions.deduplicationKey` and the
+    backend owns the check.  Fedify does not touch the key–value store.
+
+2.  **Key–value fallback.**  Otherwise, if the configured `~KvStore` exposes
+    the optional compare-and-swap (`~KvStore.cas`) primitive, Fedify records
+    the key under a dedicated `taskDeduplication` prefix with a TTL and skips
+    the enqueue while a marker is present.  The TTL defaults to one hour and is
+    configurable with `~FederationOptions.taskDeduplicationTtl`:
+
+    ~~~~ typescript
+    const federation = createFederation<void>({
+      // ...
+      taskDeduplicationTtl: { minutes: 10 },  // [!code highlight]
+    });
+    ~~~~
+
+3.  **No conditional write.**  When neither applies—no native deduplication and
+    a key–value store without `~KvStore.cas`—the behavior is governed by
+    `~FederationOptions.taskDeduplicationFallback`.  `"open"` (the default)
+    lets the enqueue proceed without deduplication after a debug-level log;
+    `"closed"` throws a `TypeError` before enqueuing:
+
+    ~~~~ typescript
+    const federation = createFederation<void>({
+      // ...
+      taskDeduplicationFallback: "closed",  // [!code highlight]
+    });
+    ~~~~
+
+Among the first-party adapters, the in-memory, Deno KV, SQLite, and MySQL
+key–value stores implement `~KvStore.cas`; PostgreSQL, Redis, and
+Cloudflare Workers KV do not yet, so those deployments take the
+`taskDeduplicationFallback` branch until per-adapter follow-ups add it.
+
+For `~Context.enqueueTaskMany()`, a single `deduplicationKey` applies to the
+whole batch: the batch enqueues as a unit or is skipped as a unit, never
+partially.  Per-item deduplication means calling `~Context.enqueueTask()` in
+a loop, each with its own key.  Deduplicating a multi-item batch requires the
+queue to implement `~MessageQueue.enqueueMany()` so the batch enqueues
+atomically—whether the check is native or the key–value fallback.  Fanning the
+key out across separate `~MessageQueue.enqueue()` calls cannot enqueue a whole
+batch as one unit: a native per-message key cannot cover it, and a key–value
+marker could not be rolled back cleanly if only some of the fanned-out enqueues
+failed.  So when deduplication is actually applied—a native queue, or a
+key–value store with `~KvStore.cas`—Fedify rejects a multi-item batch with a
+`deduplicationKey` on a queue without `~MessageQueue.enqueueMany()` instead of
+risking duplicates.  Under the `"open"` fallback (no native deduplication and no
+`cas`), no marker is taken, so the batch simply fans out without deduplication.
+
+This applies through `~ParallelMessageQueue` as well: wrapping a queue that
+lacks `~MessageQueue.enqueueMany()` does not make batch enqueue atomic, so a
+deduplicated multi-item batch on such a wrapper is likewise rejected rather than
+collapsed onto one message.
+
+> [!WARNING]
+> The key–value fallback is **best-effort, not transactional**.  The marker
+> write and the enqueue are separate operations.  Fedify rolls the marker back
+> when an enqueue fails, so a transient failure does not suppress the retry, but
+> a crash before that rollback, the `"open"` fallback under concurrency, a
+> non-atomic third-party `~KvStore.cas`, or reuse of a key within its TTL window
+> can still admit a duplicate or suppress a task.  Cleanup is otherwise by TTL
+> expiry, not active deletion on handler success.  Deployments needing strict
+> guarantees use a queue with `nativeDeduplication: true`, where the backend
+> owns an atomic check.
+
+
 Limitations
 -----------
 
-The current API intentionally ships without deduplication, task-specific
-OpenTelemetry spans and metrics, cron-style periodic scheduling, result
-backends, and per-task priority.  Some of these are planned as follow-ups;
-see the [tracking issue].
+The current API intentionally ships without task-specific OpenTelemetry spans
+and metrics, cron-style periodic scheduling, result backends, and per-task
+priority.  Some of these are planned as follow-ups; see the [tracking issue].
 
 [tracking issue]: https://github.com/fedify-dev/fedify/issues/206

packages/fedify/src/federation/context.ts

@@ -459,17 +459,21 @@ export interface Context<TContextData> {
 
   /**
    * Enqueues multiple payloads for a custom background task at once.
-   * Uses the queue's bulk enqueue operation when available, falling back
-   * to parallel single enqueues.
+   * Uses the queue's bulk enqueue operation when available.  Without
+   * deduplication, it may fall back to parallel single enqueues when the
+   * queue does not implement bulk enqueue.
    * @template TData The type of the task payload, inferred from the task's
    *                 schema.
    * @param task The handle returned by {@link TaskRegistry.defineTask}.
    * @param payloads The task payloads.  Each is validated against the
    *                 task's schema before being enqueued.
    * @param options Options for enqueuing the tasks.
    * @throws {TypeError} If the task is not defined on this federation,
-   *                     if no message queue is configured for tasks, or if
-   *                     a payload fails schema validation.
+   *                     if no message queue is configured for tasks, if
+   *                     a payload fails schema validation, or if a
+   *                     deduplicated multi-item batch cannot be enqueued
+   *                     atomically because the queue does not implement
+   *                     bulk enqueue.
    * @since 2.x.x
    */
   enqueueTaskMany<TData>(

packages/fedify/src/federation/federation.ts

@@ -1103,6 +1103,32 @@ export interface FederationOptions<TContextData> {
    */
   taskQueueResolution?: "fallback" | "strict";
 
+  /**
+   * The time-to-live for a {@link TaskEnqueueOptions.deduplicationKey} marker
+   * stored in the key–value deduplication fallback.  A second enqueue with the
+   * same key within this window is skipped; once it expires, the key may
+   * enqueue again.  Ignored when the task's queue declares
+   * {@link MessageQueue.nativeDeduplication} (the backend owns the window).
+   * @default `{ hours: 1 }`
+   * @since 2.x.x
+   */
+  taskDeduplicationTtl?: Temporal.DurationLike;
+
+  /**
+   * The behavior when a {@link TaskEnqueueOptions.deduplicationKey} is supplied
+   * but the task's queue does not declare
+   * {@link MessageQueue.nativeDeduplication} *and* the configured
+   * {@link KvStore} exposes no `cas` (compare-and-swap) primitive:
+   *
+   * - `"open"` (the default): proceeds without deduplication after logging at
+   *   debug level.
+   * - `"closed"`: rejects with a `TypeError` before enqueuing.
+   *
+   * @default `"open"`
+   * @since 2.x.x
+   */
+  taskDeduplicationFallback?: "open" | "closed";
+
   /**
    * Activity transformers that are applied to outgoing activities.  It is
    * useful for adjusting outgoing activities to satisfy some ActivityPub

packages/fedify/src/federation/middleware.test.ts

@@ -10565,20 +10565,23 @@ const withTimeout = <T>(
   return Promise.race([promise, timeout]).finally(() => clearTimeout(timer));
 };
 
-const taskFederationOptions = {
+// A factory, not a shared constant: each task test gets its own
+// MemoryKvStore so deduplication markers never leak across tests and the
+// suite stays order-independent as more cases are added.
+const mockOptions = () => ({
   kv: new MemoryKvStore(),
   documentLoaderFactory: () => mockDocumentLoader,
   contextLoaderFactory: () => mockDocumentLoader,
   manuallyStartQueue: true,
-};
+});
 
 test("ContextImpl.enqueueTask()", async (t) => {
   await t.step(
     "builds the task message envelope and round-trips a vocab payload",
     async () => {
       const queue = new MockQueue({ supportsEnqueueMany: true });
       const federation = createFederation<void>({
-        ...taskFederationOptions,
+        ...mockOptions(),
         queue: { task: queue },
       });
       const task = federation.defineTask("greet", {
@@ -10619,7 +10622,7 @@ test("ContextImpl.enqueueTaskMany()", async (t) => {
     async () => {
       const queue = new MockQueue({ supportsEnqueueMany: true });
       const federation = createFederation<void>({
-        ...taskFederationOptions,
+        ...mockOptions(),
         queue: { task: queue },
       });
       const task = federation.defineTask("bulk", {
@@ -10652,7 +10655,7 @@ test("ContextImpl.enqueueTaskMany()", async (t) => {
     async () => {
       const queue = new MockQueue({ supportsEnqueueMany: true });
       const federation = createFederation<void>({
-        ...taskFederationOptions,
+        ...mockOptions(),
         queue: { task: queue },
       });
       const task = federation.defineTask("bulk-single", {
@@ -10674,7 +10677,7 @@ test("ContextImpl.enqueueTaskMany()", async (t) => {
     async () => {
       const queue = new RendezvousQueue(2);
       const federation = createFederation<void>({
-        ...taskFederationOptions,
+        ...mockOptions(),
         queue: { task: queue },
       });
       const task = federation.defineTask("bulk-fallback", {
@@ -10716,7 +10719,7 @@ test("ContextImpl.enqueueTaskMany()", async (t) => {
     async () => {
       const queue = new MockQueue();
       const federation = createFederation<void>({
-        ...taskFederationOptions,
+        ...mockOptions(),
         queue: { task: queue },
       });
       const task = federation.defineTask("bulk-typed", {