JetStream and the first consumer

messages          12,930
consumers         0
subjects          ["events.>"]
retention         limits
storage           file
num_replicas      1
max_age           0            <- no limit; NFR-REL-08 asks for >= 24 h
max_bytes         -1           <- unbounded
discard           old
duplicate_window  120 s        <- NATS's default, inherited

@@ -1,5 +1,8 @@
 import {
   connect,
+  DiscardPolicy,
+  RetentionPolicy,
+  StorageType,
   type JetStreamClient,
   type NatsConnection,
 } from "nats";
@@ -15,16 +18,82 @@ import type { Publisher, PublishedMessage } from "./publisher";
 
 export const DEFAULT_NATS_URL = "nats://localhost:4222";
 
-/** One stream over `events.>`, file-backed. This is the MINIMUM a publisher
- * needs in order to be provable — publishing into a broker with no stream is
- * fire-and-forget, and the chapter's claim would be false at the last hop.
+/** One stream over `events.>`, file-backed.
  *
- * The real design of the subject space — FR-WHK-02's full event-type list,
- * per-environment sharding, retention, replicas — belongs to chapter 3.4 along
- * with every consumer. */
+ * Chapter 3.3 created this with a name, its subjects and file storage, and left
+ * everything else at whatever NATS defaults to — which on a development broker
+ * meant no age limit, no size limit, and a two-minute duplicate window nobody
+ * had chosen. Chapter 3.4 makes every setting a decision (research R2).
+ *
+ * Two of them can never be changed again, and both happen to be right:
+ * `retention` and `storage` are immutable on an existing stream (measured, R1).
+ * Had 3.3 taken memory storage as a convenience, applying this configuration
+ * would have meant deleting the stream and every event in it. */
 const STREAM = "EVENTS";
 const SUBJECTS = ["events.>"];
 
+const SECOND_NS = 1_000_000_000;
+
+/** NFR-REL-08 asks the queue to retain events for at least 24 hours so that a
+ * consumer outage is absorbed. Seven days is chosen over the floor for a reason
+ * the floor does not cover: an outage that starts on a Friday evening is not
+ * noticed until Monday. The floor protects a process crash; this protects a
+ * weekend. */
+const MAX_AGE_NS = 7 * 24 * 60 * 60 * SECOND_NS;
+
+/** An unbounded stream is a full disk with extra steps. The bound turns that
+ * into a number an operator can watch — and with `discard: old`, hitting it
+ * loses the OLDEST events rather than refusing new publishes. Refusing
+ * publishes would take the write path down with the event spine, which is the
+ * inversion chapter 3.3's outbox exists to prevent. */
+const MAX_BYTES = 1024 * 1024 * 1024;
+
+/** ADR-02 specifies R3 replication. The compose stack is a single node, so this
+ * is environment-derived rather than hardcoded to either value: a chapter that
+ * wrote `3` would not run locally, and one that wrote `1` would ship a
+ * single-replica event spine to production. */
+function replicaCount(): number {
+  const configured = Number(process.env.RELAY_NATS_REPLICAS ?? "");
+  if (Number.isInteger(configured) && configured > 0) return configured;
+  return process.env.NODE_ENV === "production" ? 3 : 1;
+}
+
+/** Apply the stream's configuration, whether or not it exists yet.
+ *
+ * Idempotent on purpose: two api instances starting together both run this, and
+ * the second must be a no-op rather than an error. On an existing stream the
+ * MUTABLE settings are merged onto whatever is there, and the immutable ones are
+ * carried through untouched — attempting to change `retention` or `storage` is
+ * an error the broker refuses rather than a difference it reconciles (R1).
+ *
+ * `duplicate_window` is deliberately left where 3.3 found it. Raising it looks
+ * like the fix for a republished event and is not: the outbox can republish
+ * hours after an outage, no window is a safe guess about the longest one, and a
+ * window measured in hours would hold that dedupe index in the broker's memory
+ * for hours. The guarantee belongs where the work happens — at the consumer
+ * (research R3, SAD risk R5). */
+export async function ensureStream(nc: NatsConnection): Promise<void> {
+  const jsm = await nc.jetstreamManager();
+  const mutable = {
+    subjects: [...SUBJECTS],
+    max_age: MAX_AGE_NS,
+    max_bytes: MAX_BYTES,
+    discard: DiscardPolicy.Old,
+    num_replicas: replicaCount(),
+  };
+  const existing = await jsm.streams.info(STREAM).catch(() => null);
+  if (existing === null) {
+    await jsm.streams.add({
+      name: STREAM,
+      retention: RetentionPolicy.Limits,
+      storage: StorageType.File,
+      ...mutable,
+    });
+    return;
+  }
+  await jsm.streams.update(STREAM, { ...existing.config, ...mutable });
+}
+
 export function createJetStreamPublisher({
   url = process.env.RELAY_NATS_URL ?? DEFAULT_NATS_URL,
 }: { url?: string } = {}): Publisher {
@@ -38,16 +107,7 @@ export function createJetStreamPublisher({
   async function client(): Promise<JetStreamClient> {
     if (js && connection && !connection.isClosed()) return js;
     const nc = await connect({ servers: url });
-    const jsm = await nc.jetstreamManager();
-    // Created if absent, left alone if present: two api instances starting
-    // together must not fight over it.
-    const existing = await jsm.streams
-      .info(STREAM)
-      .then(() => true)
-      .catch(() => false);
-    if (!existing) {
-      await jsm.streams.add({ name: STREAM, subjects: [...SUBJECTS] });
-    }
+    await ensureStream(nc);
     connection = nc;
     js = nc.jetstream();
     return js;

@@ -82,6 +82,41 @@ export const internalBackfillResponseSchema = z.strictObject({
   ),
 });
 
+// ---------------------------------------------------------------------------
+// Event subjects (chapter 3.4, ADR-02).
+//
+// The grammar is `events.{domain}.{action}.{env}` — ADR-02's, verbatim. It lived
+// inside the api's outbox module in 3.3 because nothing else needed it. A
+// consumer needs it now, and the package whose whole job is the shapes both
+// sides share is where a shape shared by both sides belongs (1.3's premise).
+//
+// Built here and nowhere else. A consumer that filters on a subject it
+// assembled itself is a consumer that silently receives nothing the day the
+// grammar changes — no error, no warning, just an empty stream position.
+// ---------------------------------------------------------------------------
+
+/** Every subject the platform publishes on, and the wildcard that reads them
+ * all. One entry today; FR-WHK-02 names seven more, each arriving with the
+ * feature that can produce it. */
+export const EVENT_SUBJECT_PREFIX = "events";
+export const ALL_EVENTS_SUBJECT = `${EVENT_SUBJECT_PREFIX}.>`;
+
+/** `message.created` → `msg.created`: the domain abbreviation ADR-02's example
+ * uses (`events.msg.created.{env}`). Kept as a mapping rather than a string
+ * operation so that a type whose subject form is NOT its dotted name has an
+ * obvious place to be added. */
+const DOMAIN_ABBREVIATION: Record<string, string> = {
+  message: "msg",
+};
+
+export function subjectFor(type: string, environmentId: string): string {
+  if (!type) throw new Error("an event type is required");
+  if (!environmentId) throw new Error("an environment id is required");
+  const [domain, ...rest] = type.split(".");
+  const abbreviated = DOMAIN_ABBREVIATION[domain!] ?? domain!;
+  return [EVENT_SUBJECT_PREFIX, abbreviated, ...rest, environmentId].join(".");
+}
+
 /** api → gateway: the channels this user may hear (FR-RTM-01). */
 export const internalMembershipsResponseSchema = z.strictObject({
   channel_ids: z.array(z.string().min(1)),

@@ -1,3 +1,6 @@
+import { subjectFor } from "@relay/protocol";
+import { z } from "zod";
+
 // The event envelope (chapter 3.3). Built in ONE place, complete, inside the
 // transaction that caused it — so the relay is a mover of bytes and never an
 // author of them (ADR-04, research R7).
@@ -36,13 +39,10 @@ export interface PendingEvent {
   payload: OutboxEvent;
 }
 
-/** `events.msg.created.{environment_id}` — the shape SAD §6.1's own comment
- * gives. The full subject taxonomy for FR-WHK-02's other seven types, and any
- * per-environment sharding, belongs to chapter 3.4. */
-export function subjectFor(type: string, environmentId: string): string {
-  const leaf = type.replace(/^message\./, "msg.");
-  return `events.${leaf}.${environmentId}`;
-}
+// The subject grammar moved to @relay/protocol in chapter 3.4, because a
+// consumer needs it too and both sides must agree on it. Imported for use
+// below and re-exported so 3.3's callers keep working.
+export { subjectFor };
 
 export function messageCreatedEvent({
   eventId,
@@ -69,3 +69,26 @@ export function messageCreatedEvent({
     },
   };
 }
+
+/** The envelope as a CONSUMER receives it (chapter 3.4).
+ *
+ * The producing side builds this object and knows it is well formed; the
+ * consuming side reads bytes off a broker and knows nothing. Chapter 2.5 made
+ * the same argument about the internal HTTP hop — an internal caller has no
+ * more right to assume a payload's shape than an external one does — and a
+ * message that has been sitting in a stream for six days has had even longer to
+ * stop matching what the code expects. */
+export const outboxEventSchema = z.strictObject({
+  id: z.string().uuid(),
+  type: z.literal("message.created"),
+  environment_id: z.string().min(1),
+  occurred_at: z.iso.datetime(),
+  data: z.strictObject({
+    id: z.string().min(1),
+    channel_id: z.string().min(1),
+    seq: z.number().int().positive(),
+    user: z.string().nullable(),
+    text: z.string().nullable(),
+    created_at: z.iso.datetime(),
+  }),
+});

$ node scripts/consumer-walk.mjs --kill-before-ack
durable consumer           walk-f74fc0ad
environment                7d7d764e-4d01-474e-854d-7edeccaad53b
published                  7678edc8-65cd-42c8-ac50-bdaeda79d069
delivered                  7678edc8-65cd-42c8-ac50-bdaeda79d069 attempt=1 redelivered=false
claim                      handled
times handled              1
MARKER kill-me-now
Killed

$ node scripts/consumer-walk.mjs --resume=walk-f74fc0ad
durable consumer           walk-f74fc0ad
waiting                    nothing yet — the broker redelivers after ack_wait (30s)
delivered                  7678edc8-65cd-42c8-ac50-bdaeda79d069 attempt=2 redelivered=true
claim                      duplicate
times handled              1
acknowledged               7678edc8-65cd-42c8-ac50-bdaeda79d069

@@ -325,3 +325,37 @@ export const outbox = pgTable(
       .where(sql`${t.publishedAt} IS NULL`),
   ],
 );
+
+// The consumer's deduplication ledger (chapter 3.4).
+//
+// DECISION (chapter 3.4): no source document defines a table for this. SAD risk
+// R5 requires the BEHAVIOUR — "consumer template with dedup built in", so that
+// "a future consumer forgets to dedupe → double webhooks / double metering"
+// cannot happen — and leaves the shape open. This is therefore a chapter
+// derivation, recorded here the way 2.1 recorded `members`, 3.2 recorded
+// `api_keys` and 3.3 recorded the outbox's index.
+//
+// The PRIMARY KEY is the deduplication. Not a SELECT-then-INSERT: the insert
+// itself is the check, so two instances fetching the same message concurrently
+// cannot both decide they were first. 2.3 learned that on idempotency keys and
+// 3.1 learned it again on signup.
+//
+// Keyed per CONSUMER, not globally. The dispatcher and the ingester must each
+// receive every event; one ledger shared between them would let whichever
+// arrived first silence the other.
+//
+// No environment_id, for the reason the outbox has none: this is the platform's
+// own bookkeeping rather than tenant data (constitution I, 3.3's data model).
+// No event body either — recording that an event was handled needs none of a
+// tenant's message text (NFR-SEC-06).
+export const consumedEvents = pgTable(
+  "consumed_events",
+  {
+    consumer: text("consumer").notNull(),
+    eventId: uuid("event_id").notNull(),
+    handledAt: timestamp("handled_at", { withTimezone: true })
+      .notNull()
+      .defaultNow(),
+  },
+  (t) => [primaryKey({ columns: [t.consumer, t.eventId] })],
+);

-- Chapter 3.4 — the consumer deduplication ledger (SAD risk R5).
--
-- REVIEW DISPOSITION: drizzle-kit generated this from schema.ts and it was read
-- line by line before being applied (the ADR-16 workflow). Nothing was
-- rewritten. Two things were checked rather than assumed:
--
--   * the PRIMARY KEY is composite, (consumer, event_id) — that constraint IS
--     the deduplication, so a wrong key here would silently turn "handled once"
--     into "handled once per consumer per restart";
--   * there is no foreign key to anything. The ledger records that a consumer
--     handled an event id, and the events themselves live in the broker, not in
--     a table this could reference.
--
-- Deliberately absent: an environment_id (this is platform bookkeeping, like
-- the outbox), an event body (recording that something was handled needs none
-- of a tenant's message text, NFR-SEC-06), and any pruning. Rows stop earning
-- their keep once an event is older than the stream's 7-day retention, because
-- a message that can no longer be redelivered can no longer be a duplicate.
 
CREATE TABLE "consumed_events" (
	"consumer" text NOT NULL,
	"event_id" uuid NOT NULL,
	"handled_at" timestamp with time zone DEFAULT now() NOT NULL,
	CONSTRAINT "consumed_events_consumer_event_id_pk" PRIMARY KEY("consumer","event_id")
);

@@ -7,6 +7,7 @@ import {
   apiKeys,
   applications,
   channels,
+  consumedEvents,
   environments,
   humans,
   members,
@@ -301,6 +302,77 @@ export async function outboxDepth(db: Db): Promise<number> {
   return result.rows[0]?.pending ?? 0;
 }
 
+// ---------------------------------------------------------------------------
+// The consumer's deduplication ledger (chapter 3.4, SAD risk R5). Admin surface
+// for the same reason the outbox drain is: it runs on behalf of the platform
+// rather than of a tenant, and one consumer reads every environment's events.
+// ---------------------------------------------------------------------------
+
+/** What happened when a consumer tried to take an event. */
+export type ClaimResult = "handled" | "duplicate";
+
+/** Claim an event for a consumer and run its effect — **in one transaction**.
+ *
+ * This is the shape chapter 3.3 used for the outbox row and the message it
+ * describes, pointed the other way: the ledger row and the effect share a fate.
+ * A handler that throws rolls the claim back with it, so the redelivery finds no
+ * claim and runs again. Claiming outside the transaction would mean a failed
+ * handler leaves a claim behind, and the redelivery would be waved through as a
+ * duplicate — an event silently never handled, which is worse than one handled
+ * twice.
+ *
+ * The INSERT is the check. `ON CONFLICT DO NOTHING` with a `RETURNING` tells us
+ * whether this call won the row; a SELECT-then-INSERT would let two instances
+ * fetching the same message both believe they were first (2.3's lesson on
+ * idempotency keys, 3.1's on signup).
+ *
+ * **The limit of this, stated because chapter 3.5 will meet it**: the effect has
+ * to be transactional for the fate to be shared, which means it has to be in
+ * Postgres. A handler whose effect is an HTTP call to a customer cannot be
+ * rolled back, and no ledger makes it so. That consumer must choose which way to
+ * be wrong, and choosing is its chapter's work.
+ */
+export async function claimEvent(
+  db: Db,
+  consumer: string,
+  eventId: string,
+  effect: () => Promise<void>,
+): Promise<ClaimResult> {
+  return db.transaction(async (tx) => {
+    const claimed = await tx
+      .insert(consumedEvents)
+      .values({ consumer, eventId })
+      .onConflictDoNothing({
+        target: [consumedEvents.consumer, consumedEvents.eventId],
+      })
+      .returning({ eventId: consumedEvents.eventId });
+
+    if (claimed.length === 0) return "duplicate";
+    await effect();
+    return "handled";
+  });
+}
+
+/** How many times a consumer has handled a given event. Zero or one, always —
+ * which is the assertion the redelivery test makes, and the reason this exists
+ * rather than the test reaching into the table itself. */
+export async function timesHandled(
+  db: Db,
+  consumer: string,
+  eventId: string,
+): Promise<number> {
+  const rows = await db
+    .select({ eventId: consumedEvents.eventId })
+    .from(consumedEvents)
+    .where(
+      and(
+        eq(consumedEvents.consumer, consumer),
+        eq(consumedEvents.eventId, eventId),
+      ),
+    );
+  return rows.length;
+}
+
 /** What a signup produced — or found. `created` answers "was an organisation
  * created on this call?", NOT "was the identity new": a known human who owned
  * nothing gets `created: true`, because one really was created for them. */

import type { OutboxEvent } from "../outbox/event";
 
// What a handler is, and — more importantly — what it is not (chapter 3.4).
//
// SAD risk R5: "a future consumer forgets to dedupe → double webhooks / double
// metering", mitigated by a "consumer template with dedup built in". The way to
// make forgetting impossible is to leave a handler nothing to forget. It cannot
// acknowledge, cannot negatively acknowledge, cannot retry, cannot deduplicate,
// and cannot see the raw message. It can return, or it can throw.
 
export interface EventContext {
  /** The broker's delivery count for this message: 1 on the first attempt.
   * A handler may LOG it. It must not use it to decide correctness — a handler
   * that behaves differently on attempt three is a handler whose behaviour
   * depends on a timeout somewhere else. */
  attempt: number;
}
 
/** Returns → handled. Throws → not handled, try again. */
export type EventHandler = (
  event: OutboxEvent,
  context: EventContext,
) => Promise<void>;

import {
  AckPolicy,
  connect,
  DeliverPolicy,
  type JsMsg,
  type NatsConnection,
} from "nats";
import { ALL_EVENTS_SUBJECT } from "@relay/protocol";
import type { Logger } from "@relay/service-kit";
 
import { createDb, createPool, type Db } from "../db/client";
import { claimEvent, type ClaimResult } from "../db/repository";
import { outboxEventSchema, type OutboxEvent } from "../outbox/event";
import { DEFAULT_NATS_URL, ensureStream } from "../outbox/jetstream.publisher";
import type { EventHandler } from "./handler";
 
// The consumer runtime (chapter 3.4). Fetch, decide, acknowledge — and
// deduplicate, so that a handler cannot forget to.
//
// SAD risk R5 is the whole reason this file exists rather than a page of
// instructions: "a future consumer forgets to dedupe → double webhooks / double
// metering", mitigated by a "consumer template with dedup built in". Built in
// means a handler is given no way to skip it.
 
const STREAM = "EVENTS";
 
/** Batch size per fetch. Small enough that a slow handler does not hold an
 * acknowledgement deadline over a hundred messages; large enough that a backlog
 * of twelve thousand drains in sensible steps rather than one round trip each. */
const BATCH = 25;
 
/** How long the broker waits for an acknowledgement before redelivering. Long
 * enough for a real handler, short enough that a killed instance's work comes
 * back promptly — which is also what makes the redelivery test tolerable to run
 * (research R7). */
const ACK_WAIT_NS = 30 * 1_000_000_000;
 
/** Bounded, because forever is not a retry policy. After this many attempts the
 * broker stops delivering and the message leaves the consumer's view entirely —
 * measured, not assumed (research R4). Nothing catches it. A dead-letter store
 * is FR-WHK-04's, in chapter 3.5. */
const MAX_DELIVER = 5;
 
/** Back-pressure: the broker stops handing out work when this much is
 * outstanding, so a stalled consumer cannot accumulate an unbounded pile of
 * unacknowledged messages. */
const MAX_ACK_PENDING = 100;
 
/** What the runtime does with a message. Extracted from the loop so the decision
 * table can be read in one place — and tested without a broker. */
export type Outcome = "acknowledge" | "retry" | "terminate";
 
/** The decision, given a parsed payload and a way to claim it.
 *
 * `claim` receives the effect to run and reports whether this call won the
 * ledger row. It is passed in rather than called directly so this function has
 * no database of its own to reason about.
 */
export async function decideOutcome({
  parsed,
  attempt = 1,
  claim,
  handler,
}: {
  parsed: OutboxEvent | null;
  attempt?: number;
  claim: (effect: () => Promise<void>) => Promise<ClaimResult>;
  handler?: EventHandler;
}): Promise<Outcome> {
  // A payload that will never parse must not consume five delivery attempts
  // before being dropped anyway. The same bytes fail the same way every time.
  if (parsed === null) return "terminate";
 
  try {
    const result = await claim(async () => {
      await handler?.(parsed, { attempt });
    });
    // "duplicate" means somebody already handled this — including a previous
    // delivery to this same consumer that crashed after committing. The message
    // is acknowledged because it genuinely has been handled, just not now.
    return result === "duplicate" ? "acknowledge" : "acknowledge";
  } catch {
    // The handler threw, so the claim rolled back with it (see `claimEvent`).
    // Not acknowledging is how the runtime asks for a redelivery; the handler
    // never sees an acknowledgement to withhold.
    return "retry";
  }
}
 
export interface ConsumerRuntime {
  /** Runs until `stop()`. Never rejects: a broker that is down is an expected
   * state, not a crash. */
  start(): void;
  stop(): Promise<void>;
  /** One fetch-and-decide pass, for tests and for the walk script — the same
   * code path `start` runs, so nothing is proven about a loop only tests use. */
  pollOnce(): Promise<{ handled: number; duplicates: number; retried: number }>;
}
 
export function createConsumerRuntime({
  durable,
  handler,
  logger,
  db = createDb(createPool()),
  url = process.env.RELAY_NATS_URL ?? DEFAULT_NATS_URL,
  batch = BATCH,
  filterSubject = ALL_EVENTS_SUBJECT,
  fromNewOnly = false,
}: {
  durable: string;
  handler: EventHandler;
  logger: Logger;
  db?: Db;
  url?: string;
  batch?: number;
  /** Which subjects this consumer wants. The default is everything, which is
   * what the recorder needs; a narrower filter is how chapter 3.5's dispatcher
   * will subscribe to the event types a customer asked for, and how a test
   * scopes itself to one environment's subject rather than replaying the whole
   * stream (contracts §consumer). */
  filterSubject?: string;
  /** Start at the head rather than at the beginning. A durable consumer's
   * default is to deliver everything the stream still holds — which is correct
   * for a consumer that must not miss anything, and impractical for a test that
   * would otherwise replay twelve thousand events from earlier chapters before
   * reaching its own. */
  fromNewOnly?: boolean;
}): ConsumerRuntime {
  let connection: NatsConnection | null = null;
  let running = false;
  let loop: Promise<void> = Promise.resolve();
 
  /** Lazy, like the publisher's: the api must start and serve writes with the
   * broker unreachable. The durable consumer is created if absent and left
   * alone if present, so two instances starting together share the position
   * rather than fighting over it (research R8). */
  async function connection_(): Promise<NatsConnection> {
    if (connection && !connection.isClosed()) return connection;
    const nc = await connect({ servers: url });
    await ensureStream(nc);
    const jsm = await nc.jetstreamManager();
    const exists = await jsm.consumers
      .info(STREAM, durable)
      .then(() => true)
      .catch(() => false);
    if (!exists) {
      await jsm.consumers.add(STREAM, {
        durable_name: durable,
        ack_policy: AckPolicy.Explicit,
        ack_wait: ACK_WAIT_NS,
        max_deliver: MAX_DELIVER,
        max_ack_pending: MAX_ACK_PENDING,
        filter_subject: filterSubject,
        ...(fromNewOnly ? { deliver_policy: DeliverPolicy.New } : {}),
      });
    }
    connection = nc;
    return nc;
  }
 
  function parse(message: JsMsg): OutboxEvent | null {
    try {
      const result = outboxEventSchema.safeParse(
        JSON.parse(new TextDecoder().decode(message.data)),
      );
      return result.success ? result.data : null;
    } catch {
      return null;
    }
  }
 
  async function pollOnce(): Promise<{
    handled: number;
    duplicates: number;
    retried: number;
  }> {
    const nc = await connection_();
    const consumer = await nc.jetstream().consumers.get(STREAM, durable);
    const messages = await consumer.fetch({
      max_messages: batch,
      expires: 1_000,
    });
 
    let handled = 0;
    let duplicates = 0;
    let retried = 0;
 
    for await (const message of messages) {
      const parsed = parse(message);
      let result: ClaimResult = "duplicate";
      const outcome = await decideOutcome({
        parsed,
        attempt: message.info.deliveryCount,
        claim: async (effect) => {
          result = await claimEvent(db, durable, parsed!.id, effect);
          return result;
        },
        handler,
      });
 
      if (outcome === "terminate") {
        // Stops the redelivery for good. The chapter says out loud that nothing
        // catches what lands here.
        message.term();
        logger.log("error", "consumer.unparseable", {
          consumer: durable,
          stream_sequence: message.seq,
        });
        continue;
      }
      if (outcome === "retry") {
        message.nak();
        retried += 1;
        continue;
      }
      message.ack();
      if (result === "duplicate") duplicates += 1;
      else handled += 1;
    }
 
    return { handled, duplicates, retried };
  }
 
  async function run(): Promise<void> {
    while (running) {
      try {
        const { handled, duplicates, retried } = await pollOnce();
        if (handled + duplicates + retried > 0) {
          // Counts, never payloads. A message body in a log line is a tenant's
          // data in an operator's terminal (NFR-SEC-06).
          logger.log("info", "consumer.batch", {
            consumer: durable,
            handled,
            duplicates,
            retried,
          });
          continue;
        }
      } catch (error) {
        logger.log("error", "consumer.poll_failed", {
          consumer: durable,
          error: String(error),
        });
      }
      await new Promise((resolve) => setTimeout(resolve, 200));
    }
  }
 
  return {
    start() {
      if (running) return;
      running = true;
      loop = run();
    },
    async stop() {
      running = false;
      await loop;
      if (connection && !connection.isClosed()) await connection.drain();
      connection = null;
    },
    pollOnce,
  };
}

import type { Logger } from "@relay/service-kit";
 
import type { EventHandler } from "./handler";
 
// The first consumer (chapter 3.4) — and it is a SCAFFOLD, with a named
// retirement, not a feature.
//
// Every consumer the SAD names belongs to a later chapter: the webhook
// dispatcher (3.5), the analytics ingester and the media worker (Part 4), the
// dashboard's live stream (Part 5). Giving this one a job would mean either
// stealing 3.5's subject or inventing product nobody asked for, and Principle
// VII forbids the second.
//
// So it does the smallest real thing: it observes that an event arrived. The
// EFFECT — the row in `consumed_events` — is written by the runtime's claim, in
// the same transaction, which is the entire mechanism this chapter exists to
// demonstrate. This handler's body is nearly empty on purpose, and that
// emptiness is the point: what makes the consumer correct is the runtime around
// it, not the code inside it.
//
// RETIREMENT: chapter 3.5 replaces this with the webhook dispatcher, which is a
// handler with the same signature and a great deal more to do.
export function createRecorder(logger: Logger): EventHandler {
  return async (event, context) => {
    // Identifiers and counts only — never `event.data.text`. A tenant's message
    // body has no business in the platform's own logs (NFR-SEC-06).
    logger.log("info", "event.recorded", {
      event_id: event.id,
      type: event.type,
      environment_id: event.environment_id,
      attempt: context.attempt,
    });
  };
}

round 0: got seq=1 deliveryCount=1 redelivered=false
round 0: got seq=1 deliveryCount=2 redelivered=true
round 1: got seq=1 deliveryCount=3 redelivered=true
pending=0 ack_pending=0 redelivered=1

import { Inject, Injectable, Module, type OnModuleDestroy } from "@nestjs/common";
 
import { createLogger } from "@relay/service-kit";
 
import { createRecorder } from "./recorder";
import { createConsumerRuntime, type ConsumerRuntime } from "./runtime";
 
// The consumer's home (chapter 3.4). It runs INSIDE the api service, and that
// is a constraint rather than a convenience.
//
// Its deduplication ledger is a Postgres write, and ADR-04 makes the api the
// only service that writes to Postgres — the SAD applies that strictly enough
// that the media worker transitions state through an internal route precisely
// so it never touches the database. A consumer deployed as its own service
// would be a second writer.
//
// So it sits here, exactly as chapter 3.3's outbox relay does under ADR-06's
// "a small loop inside the API service initially, promotable to its own
// deployment". What that costs is named rather than discovered: chapter 3.5's
// dispatcher IS meant to be its own service, and it will need either an
// internal route for its ledger or an explicit ADR amendment (research R5).
 
export const EVENT_CONSUMER = "EVENT_CONSUMER";
 
/** The durable name. It is a POSITION in the stream, shared by every instance
 * using it — which is what lets two api processes divide the work instead of
 * each receiving everything (research R8). */
export const RECORDER_DURABLE = "recorder";
 
/** On by default: an event spine nobody reads is what chapter 3.3 left behind.
 * `RELAY_EVENT_CONSUMER=off` exists for suites that want a quiet database —
 * 3.3 learned the hard way that a background loop mutating a table two other
 * test files assert on is a race between test files, not a property. */
export function consumerEnabled(): boolean {
  return (process.env.RELAY_EVENT_CONSUMER ?? "on").toLowerCase() !== "off";
}
 
@Injectable()
export class EventConsumerService implements OnModuleDestroy {
  constructor(@Inject(EVENT_CONSUMER) private readonly runtime: ConsumerRuntime) {}
 
  start(): void {
    if (consumerEnabled()) this.runtime.start();
  }
 
  async onModuleDestroy(): Promise<void> {
    await this.runtime.stop();
  }
}
 
@Module({
  providers: [
    {
      provide: EVENT_CONSUMER,
      useFactory: (): ConsumerRuntime => {
        const logger = createLogger("consumer");
        return createConsumerRuntime({
          durable: RECORDER_DURABLE,
          handler: createRecorder(logger),
          logger,
        });
      },
    },
    EventConsumerService,
  ],
  exports: [EVENT_CONSUMER, EventConsumerService],
})
export class ConsumerModule {}

@@ -4,6 +4,7 @@ import { NestFactory } from "@nestjs/core";
 import { createLogger } from "@relay/service-kit";
 
 import { AppModule } from "./app.module";
+import { EventConsumerService } from "./consumer/consumer.module";
 import { OutboxRelayService } from "./outbox/outbox.module";
 
 // Nest's own banner logger stays off: this workspace already decided what a
@@ -18,6 +19,10 @@ async function bootstrap(): Promise<void> {
   // accumulating in Postgres instead of preventing the api from serving writes
   // (chapter 3.3, research R9).
   app.get(OutboxRelayService).start();
+  // And the first thing that reads what the relay publishes (chapter 3.4).
+  // Same placement, same reason, same lazy connection: an unreachable broker
+  // leaves the api serving writes.
+  app.get(EventConsumerService).start();
   // Nest calls onModuleDestroy on shutdown hooks; without this the relay's loop
   // would outlive the process's intent to stop.
   app.enableShutdownHooks();

@@ -10,6 +10,7 @@ import { AuthenticateMiddleware } from "./auth/authenticate.middleware";
 import { HealthController } from "./health.controller";
 import { InternalModule } from "./internal/internal.module";
 import { MessagesModule } from "./messages/messages.module";
+import { ConsumerModule } from "./consumer/consumer.module";
 import { OutboxModule } from "./outbox/outbox.module";
 import { TenancyModule } from "./tenancy/tenancy.module";
 import { LOGGER, apiLogger } from "./logger";
@@ -27,6 +28,7 @@ import { RequestContextMiddleware } from "./request-context.middleware";
     InternalModule,
     TenancyModule,
     OutboxModule,
+    ConsumerModule,
   ],
   controllers: [HealthController],
   providers: [

$ pnpm --filter @relay/api test:integration src/consumer/consumer.itest.ts
✓ invariant 1: the stream's settings read back exactly as configured
✓ invariant 2: applying the configuration twice is a no-op, not an error
✓ invariant 3: an event is delivered, handled once, and acknowledged
✓ invariant 4: a kill between handling and acknowledgement is redelivered — and handled once (SC-003)
✓ invariant 5: deduplication survives a restart
✓ invariant 6: two instances sharing a durable name divide the work
✓ invariant 7: a handler that always throws stops being retried
✓ invariant 8: an unparseable payload is terminated on the first attempt
✓ invariant 9: a consumer stopped for N publishes receives all N on restart
✓ invariant 12: a consumer log line carries counts, never payloads
Tests  10 passed (10)

import "reflect-metadata";
 
import { spawn } from "node:child_process";
import { randomUUID } from "node:crypto";
import { join } from "node:path";
 
import { connect } from "nats";
import { createLogger, type Logger } from "@relay/service-kit";
import { subjectFor } from "@relay/protocol";
import { afterAll, beforeAll, describe, expect, it } from "vitest";
 
import { createDb, createPool, type Db } from "../db/client";
import { claimEvent, timesHandled } from "../db/repository";
import { ensureStream } from "../outbox/jetstream.publisher";
import { createConsumerRuntime } from "./runtime";
import type { EventHandler } from "./handler";
 
// The consumer, against a real broker and a real database (chapter 3.4).
//
// Every durable name here is unique per run. A durable consumer is a POSITION
// in a shared stream that already holds tens of thousands of events from earlier
// chapters — two runs sharing a name would inherit each other's progress, and
// the second would look mysteriously empty. This is the same lesson 2.6 learned
// about Redis subjects and 3.3 about the outbox table: a shared store needs a
// per-run handle, because the isolation every other suite gets from a tenant
// column is not available here.
 
const silent: Logger = createLogger("consumer-itest", () => {});
 
const ENV = () => randomUUID();
 
/** Publish one event straight onto the stream, the way the relay would. */
async function publish(
  environmentId: string,
  overrides: Record<string, unknown> = {},
): Promise<string> {
  const nc = await connect({
    servers: process.env.RELAY_NATS_URL ?? "nats://localhost:4222",
  });
  const id = randomUUID();
  const payload = {
    id,
    type: "message.created",
    environment_id: environmentId,
    occurred_at: new Date().toISOString(),
    data: {
      id: randomUUID(),
      channel_id: randomUUID(),
      seq: 1,
      user: "tuan",
      text: "B2, north ramp",
      created_at: new Date().toISOString(),
    },
    ...overrides,
  };
  await nc
    .jetstream()
    .publish(
      subjectFor("message.created", environmentId),
      new TextEncoder().encode(JSON.stringify(payload)),
      { msgID: id },
    );
  await nc.drain();
  return id;
}
 
/** Publish something that is not an event at all. */
async function publishGarbage(environmentId: string): Promise<void> {
  const nc = await connect({
    servers: process.env.RELAY_NATS_URL ?? "nats://localhost:4222",
  });
  await nc
    .jetstream()
    .publish(
      subjectFor("message.created", environmentId),
      new TextEncoder().encode("{ this is not an event }"),
    );
  await nc.drain();
}
 
/** A runtime whose durable name is unique to this test, filtered to one
 * environment's subject so the stream's existing backlog stays out of the way. */
function runtimeFor(
  db: Db,
  durable: string,
  handler: EventHandler,
  logger: Logger = silent,
  environmentId?: string,
) {
  return createConsumerRuntime({
    durable,
    handler,
    logger,
    db,
    // Scoped to one environment's subject. Without it every test here would
    // replay the ~13,000 events earlier chapters left in the stream before
    // reaching its own — which is what `limits` retention means, and is exactly
    // the behaviour invariant 9 asserts on deliberately.
    ...(environmentId
      ? { filterSubject: subjectFor("message.created", environmentId) }
      : {}),
  });
}
 
/** Durables this suite created through a CHILD process rather than directly.
 * The walk names its own — `walk-<uuid>` — so the suite cannot predict them and
 * a prefix sweep would delete a reader's walk running alongside it. It records
 * what it spawned instead, and cleans exactly that. */
const spawnedDurables: string[] = [];
 
/** Run the walk in its kill mode and SIGKILL it the moment it says it is in the
 * gap between the committed effect and the acknowledgement. */
async function killInTheGap(): Promise<{
  durable: string;
  eventId: string;
  environmentId: string;
}> {
  const script = join(
    __dirname,
    "..",
    "..",
    "..",
    "..",
    "scripts",
    "consumer-walk.mjs",
  );
  return new Promise((resolve, reject) => {
    const child = spawn(
      "node",
      [script, "--kill-before-ack", "--pause=30000"],
      { env: { ...process.env }, stdio: ["ignore", "pipe", "pipe"] },
    );
    let out = "";
    let killed = false;
    const timer = setTimeout(() => {
      child.kill("SIGKILL");
      reject(new Error(`no marker within 60s; output was:\n${out}`));
    }, 60_000);
    child.stdout.on("data", (chunk: Buffer) => {
      out += chunk.toString();
      if (!killed && out.includes("MARKER kill-me-now")) {
        killed = true;
        child.kill("SIGKILL");
      }
    });
    child.stderr.on("data", (chunk: Buffer) => (out += chunk.toString()));
    child.on("exit", () => {
      clearTimeout(timer);
      if (!killed) return reject(new Error(`child finished early:\n${out}`));
      const durable = /durable consumer\s+(\S+)/.exec(out)?.[1];
      const eventId = /published\s+(\S+)/.exec(out)?.[1];
      const environmentId = /environment\s+([0-9a-f-]{36})/.exec(out)?.[1];
      if (!durable || !eventId) {
        return reject(new Error(`could not read the walk's output:\n${out}`));
      }
      spawnedDurables.push(durable);
      resolve({ durable, eventId, environmentId: environmentId ?? "" });
    });
  });
}
 
describe("the consumer", () => {
  let db: Db;
 
  beforeAll(async () => {
    db = createDb(createPool());
    const nc = await connect({
      servers: process.env.RELAY_NATS_URL ?? "nats://localhost:4222",
    });
    await ensureStream(nc);
    await nc.drain();
  }, 60_000);
 
  afterAll(async () => {
    await db.execute(`DELETE FROM consumed_events WHERE consumer LIKE 'itest-%'`);
    for (const durable of spawnedDurables) {
      await db.execute(
        `DELETE FROM consumed_events WHERE consumer = '${durable}'`,
      );
    }
    // And the durable consumers themselves. A durable is server-side state that
    // outlives the process that made it: without this, every run of this suite
    // left another handful behind on a shared broker, and `stream-info.mjs`
    // found twelve of them the first time it looked. Per-run names keep runs
    // independent; they do not clean up after themselves.
    //
    // Both kinds go: the ones this process named `itest-…`, and the `walk-…`
    // ones its child processes named for themselves. Missing the second kind is
    // how the first count reached twelve.
    const nc = await connect({
      servers: process.env.RELAY_NATS_URL ?? "nats://localhost:4222",
    });
    const jsm = await nc.jetstreamManager();
    for await (const info of jsm.consumers.list("EVENTS")) {
      if (info.name.startsWith("itest-") || spawnedDurables.includes(info.name)) {
        await jsm.consumers.delete("EVENTS", info.name).catch(() => undefined);
      }
    }
    await nc.drain();
  }, 60_000);
 
  it("invariant 1: the stream's settings read back exactly as configured", async () => {
    const nc = await connect({
      servers: process.env.RELAY_NATS_URL ?? "nats://localhost:4222",
    });
    const info = await (await nc.jetstreamManager()).streams.info("EVENTS");
    const c = info.config;
    expect(c.subjects).toEqual(["events.>"]);
    // NFR-REL-08's floor is 24 hours; the chapter chose seven days so a Friday
    // outage survives the weekend.
    expect(c.max_age).toBe(7 * 24 * 60 * 60 * 1_000_000_000);
    expect(c.max_bytes).toBe(1024 * 1024 * 1024);
    expect(c.discard).toBe("old");
    // Immutable once created, and both already right because 3.3 chose them.
    expect(c.retention).toBe("limits");
    expect(c.storage).toBe("file");
    await nc.drain();
  });
 
  it("invariant 2: applying the configuration twice is a no-op, not an error", async () => {
    const nc = await connect({
      servers: process.env.RELAY_NATS_URL ?? "nats://localhost:4222",
    });
    const before = await (await nc.jetstreamManager()).streams.info("EVENTS");
    await ensureStream(nc);
    await ensureStream(nc);
    const after = await (await nc.jetstreamManager()).streams.info("EVENTS");
    // Same settings, and — the part that matters on a stream holding tens of
    // thousands of events — nothing lost.
    expect(after.config.max_age).toBe(before.config.max_age);
    expect(after.state.messages).toBeGreaterThanOrEqual(before.state.messages);
    await nc.drain();
  });
 
  it("invariant 3: an event is delivered, handled once, and acknowledged", async () => {
    const environmentId = ENV();
    const durable = `itest-basic-${Date.now()}`;
    const seen: string[] = [];
    const eventId = await publish(environmentId);
 
    const runtime = runtimeFor(
      db,
      durable,
      async (event) => {
        seen.push(event.id);
      },
      silent,
      environmentId,
    );
    for (let i = 0; i < 20 && !seen.includes(eventId); i++) {
      await runtime.pollOnce();
    }
    await runtime.stop();
 
    expect(seen.filter((id) => id === eventId)).toHaveLength(1);
    expect(await timesHandled(db, durable, eventId)).toBe(1);
  }, 120_000);
 
  it("invariant 4: a kill between handling and acknowledgement is redelivered — and handled once (SC-003)", async () => {
    // The chapter's centrepiece. The walk claims the event (which commits the
    // effect), prints its marker, and is SIGKILLed before it acknowledges.
    // The broker is entitled to redeliver — it never heard an acknowledgement —
    // and the ledger is what makes the redelivery safe.
    //
    // A real signal from the parent, not a thrown exception: an exception runs
    // the error path, and a crash does not (research R7, the shape 3.3 used).
    const { durable, eventId, environmentId } = await killInTheGap();
 
    // What the kill left behind: handled once, never acknowledged.
    expect(await timesHandled(db, durable, eventId)).toBe(1);
 
    // Now let a runtime pick up where the corpse left off. The broker redelivers
    // after ack_wait; the ledger refuses the claim; the message is acknowledged
    // because it genuinely has been handled.
    let redeliveries = 0;
    let handlerRuns = 0;
    const runtime = createConsumerRuntime({
      durable,
      db,
      logger: silent,
      filterSubject: subjectFor("message.created", environmentId),
      handler: async () => {
        handlerRuns += 1;
      },
    });
    for (let i = 0; i < 90; i++) {
      const { duplicates } = await runtime.pollOnce();
      redeliveries += duplicates;
      if (redeliveries > 0) break;
      await new Promise((r) => setTimeout(r, 500));
    }
    await runtime.stop();
 
    // Redelivered, recognised, and NOT handled a second time.
    expect(redeliveries).toBeGreaterThan(0);
    expect(handlerRuns).toBe(0);
    expect(await timesHandled(db, durable, eventId)).toBe(1);
 
    await db.execute(`DELETE FROM consumed_events WHERE consumer = '${durable}'`);
  }, 180_000);
 
  it("invariant 5: deduplication survives a restart", async () => {
    // The ledger is in Postgres precisely so that a process restart does not
    // reset it. A second runtime with the same durable name gets the same
    // answer the first one would have.
    const durable = `itest-restart-${Date.now()}`;
    const eventId = randomUUID();
 
    expect(await claimEvent(db, durable, eventId, async () => {})).toBe(
      "handled",
    );
    expect(await claimEvent(db, durable, eventId, async () => {})).toBe(
      "duplicate",
    );
    expect(await timesHandled(db, durable, eventId)).toBe(1);
  });
 
  it("invariant 6: two instances sharing a durable name divide the work", async () => {
    // The ordinary deployment. A durable consumer is one position in the stream,
    // so two api processes pulling from it share the work — the property the
    // broker provides here that `SKIP LOCKED` provides for the outbox.
    const durable = `itest-shared-${Date.now()}`;
    const byA: string[] = [];
    const byB: string[] = [];
    const ids = [
      await publish(ENV()),
      await publish(ENV()),
      await publish(ENV()),
    ];
 
    const a = runtimeFor(db, durable, async (e) => void byA.push(e.id));
    const b = runtimeFor(db, durable, async (e) => void byB.push(e.id));
    for (let i = 0; i < 400; i++) {
      await Promise.all([a.pollOnce(), b.pollOnce()]);
      if (ids.every((id) => byA.includes(id) || byB.includes(id))) break;
    }
    await a.stop();
    await b.stop();
 
    for (const id of ids) {
      // Exactly one of them handled it, and the ledger agrees.
      const handledBoth =
        byA.filter((x) => x === id).length + byB.filter((x) => x === id).length;
      expect(handledBoth).toBe(1);
      expect(await timesHandled(db, durable, id)).toBe(1);
    }
  }, 120_000);
 
  it("invariant 7: a handler that always throws stops being retried", async () => {
    // `max_deliver` is 5. After that the broker stops delivering and the message
    // leaves the consumer's view — measured in research R4, and the honest
    // answer this chapter gives rather than a dead-letter path that does not
    // exist yet.
    const environmentId = ENV();
    const durable = `itest-poison-${Date.now()}`;
    const eventId = await publish(environmentId);
    let attempts = 0;
 
    const runtime = runtimeFor(
      db,
      durable,
      async (event) => {
        if (event.id === eventId) {
          attempts += 1;
          throw new Error("this handler never succeeds");
        }
      },
      silent,
      environmentId,
    );
    for (let i = 0; i < 60 && attempts < 6; i++) {
      await runtime.pollOnce();
      await new Promise((r) => setTimeout(r, 50));
    }
    await runtime.stop();
 
    expect(attempts).toBeGreaterThan(0);
    expect(attempts).toBeLessThanOrEqual(5);
    // And nothing was recorded as handled: a failed handler rolls its claim back
    // with it, which is what makes the retry a real retry.
    expect(await timesHandled(db, durable, eventId)).toBe(0);
  }, 180_000);
 
  it("invariant 8: an unparseable payload is terminated on the first attempt", async () => {
    // Retrying malformed bytes five times changes nothing about them. The
    // runtime terminates the message instead of burning the budget and dropping
    // it anyway — and says so in a log line carrying no payload.
    const environmentId = ENV();
    const durable = `itest-garbage-${Date.now()}`;
    const lines: string[] = [];
    const noisy = createLogger("consumer-itest", (line) =>
      lines.push(typeof line === "string" ? line : JSON.stringify(line)),
    );
    await publishGarbage(environmentId);
    const marker = await publish(environmentId);
 
    let sawMarker = false;
    const runtime = runtimeFor(
      db,
      durable,
      async (event) => {
        if (event.id === marker) sawMarker = true;
      },
      noisy,
      environmentId,
    );
    for (let i = 0; i < 20 && !sawMarker; i++) await runtime.pollOnce();
    await runtime.stop();
 
    expect(sawMarker).toBe(true);
    const unparseable = lines.filter((l) => l.includes("consumer.unparseable"));
    expect(unparseable.length).toBe(1);
    expect(unparseable.join("")).not.toContain("this is not an event");
  }, 180_000);
 
  it("invariant 9: a consumer stopped for N publishes receives all N on restart", async () => {
    // What `limits` retention means: the stream holds messages whether or not
    // anybody is reading. The backlog waits.
    const durable = `itest-catchup-${Date.now()}`;
    const seen: string[] = [];
    const runtime = runtimeFor(db, durable, async (e) => void seen.push(e.id));
 
    // Get to the head of the stream first, so "everything published while away"
    // is measurable rather than lost in twelve thousand older events.
    for (let i = 0; i < 800; i++) {
      const { handled, duplicates } = await runtime.pollOnce();
      if (handled + duplicates === 0) break;
    }
    await runtime.stop();
 
    const published = [
      await publish(ENV()),
      await publish(ENV()),
      await publish(ENV()),
    ];
 
    const restarted = runtimeFor(db, durable, async (e) => void seen.push(e.id));
    for (let i = 0; i < 100; i++) {
      await restarted.pollOnce();
      if (published.every((id) => seen.includes(id))) break;
    }
    await restarted.stop();
 
    for (const id of published) expect(seen).toContain(id);
  }, 240_000);
 
  it("invariant 12: a consumer log line carries counts, never payloads", async () => {
    const environmentId = ENV();
    const durable = `itest-logs-${Date.now()}`;
    const lines: string[] = [];
    const noisy = createLogger("consumer-itest", (line) =>
      lines.push(typeof line === "string" ? line : JSON.stringify(line)),
    );
    const eventId = await publish(environmentId, {
      data: {
        id: randomUUID(),
        channel_id: randomUUID(),
        seq: 1,
        user: "tuan",
        text: "a secret worth keeping out of logs",
        created_at: new Date().toISOString(),
      },
    });
 
    let seen = false;
    const runtime = runtimeFor(
      db,
      durable,
      async (event) => {
        if (event.id === eventId) seen = true;
      },
      noisy,
      environmentId,
    );
    for (let i = 0; i < 20 && !seen; i++) await runtime.pollOnce();
    runtime.start();
    await new Promise((r) => setTimeout(r, 300));
    await runtime.stop();
 
    expect(lines.join("\n")).not.toContain("a secret worth keeping out of logs");
  }, 180_000);
});

// The chapter 3.4 walk: a redelivery, made to happen on purpose.
//
//   node scripts/consumer-walk.mjs                     # consume normally
//   node scripts/consumer-walk.mjs --kill-before-ack   # die in the gap
//   node scripts/consumer-walk.mjs --resume=walk-1234  # pick the corpse back up
//   node scripts/consumer-walk.mjs --from=all --limit=50
//
// The interesting pair is the middle two, run in that order. The kill mode does
// by hand what the runtime does in a loop — fetch, claim (which commits the
// effect), acknowledge — and prints `MARKER kill-me-now` between the commit and
// the acknowledgement, where it dies. A parent watching stdout can SIGKILL it
// there (the integration suite does); left alone it SIGKILLs itself, so the
// demonstration is one command.
//
// Then `--resume` reuses that durable name — which is a POSITION, not a label —
// and receives the same event again, because the broker never heard an
// acknowledgement. The ledger recognises it, and the effect does not happen
// twice. That is the chapter.
import { randomUUID } from "node:crypto";
 
import { connect, AckPolicy } from "../services/api/node_modules/nats/lib/src/mod.js";
import { subjectFor } from "../packages/protocol/dist/index.js";
import { createDb, createPool } from "../services/api/dist/db/client.js";
import { claimEvent, timesHandled } from "../services/api/dist/db/repository.js";
 
const arg = (name, fallback) => {
  const hit = process.argv.find((a) => a.startsWith(`--${name}=`));
  return hit ? hit.slice(name.length + 3) : fallback;
};
const flag = (name) => process.argv.includes(`--${name}`);
 
const URL_ = process.env.RELAY_NATS_URL ?? "nats://127.0.0.1:4222";
const LIMIT = Number(arg("limit", "5"));
const PAUSE_MS = Number(arg("pause", "400"));
const KILL_MODE = flag("kill-before-ack");
const FROM_ALL = arg("from", "new") === "all";
const RESUME = arg("resume", "");
 
const show = (label, value) => console.log(`${label.padEnd(26)} ${value}`);
 
const db = createDb(createPool());
const nc = await connect({ servers: URL_ });
const jsm = await nc.jetstreamManager();
const js = nc.jetstream();
 
// A durable per run: a durable name is a POSITION, and reusing one would make
// this walk inherit the last run's progress. Which is precisely what --resume
// wants, so it says the name out loud instead.
const durable = RESUME || `walk-${randomUUID().slice(0, 8)}`;
const environmentId = randomUUID();
 
if (!RESUME) {
  await jsm.consumers.add("EVENTS", {
    durable_name: durable,
    ack_policy: AckPolicy.Explicit,
    ack_wait: 30 * 1e9,
    max_deliver: 5,
    filter_subject: FROM_ALL ? "events.>" : subjectFor("message.created", environmentId),
  });
}
show("durable consumer", durable);
if (!RESUME) show("environment", environmentId);
 
if (!FROM_ALL && !RESUME) {
  // Publish one event for this walk to find, the way the relay would.
  const id = randomUUID();
  await js.publish(
    subjectFor("message.created", environmentId),
    new TextEncoder().encode(
      JSON.stringify({
        id,
        type: "message.created",
        environment_id: environmentId,
        occurred_at: new Date().toISOString(),
        data: {
          id: randomUUID(),
          channel_id: randomUUID(),
          seq: 1,
          user: "tuan",
          text: "B2, north ramp",
          created_at: new Date().toISOString(),
        },
      }),
    ),
    { msgID: id },
  );
  show("published", id);
}
 
const consumer = await js.consumers.get("EVENTS", durable);
 
/** One fetch, unless we are waiting for a redelivery — which cannot arrive
 * before `ack_wait` has elapsed on the delivery nobody acknowledged. Thirty
 * seconds of nothing happening is the guarantee working, not a hang. */
async function nextBatch() {
  const deadline = Date.now() + (RESUME ? 45_000 : 0);
  let announced = false;
  for (;;) {
    const batch = await consumer.fetch({ max_messages: LIMIT, expires: 2_000 });
    const messages = [];
    for await (const message of batch) messages.push(message);
    if (messages.length > 0 || Date.now() >= deadline) return messages;
    if (!announced) {
      announced = true;
      show("waiting", "nothing yet — the broker redelivers after ack_wait (30s)");
    }
  }
}
 
let handled = 0;
for (const message of await nextBatch()) {
  const event = JSON.parse(new TextDecoder().decode(message.data));
  show("delivered", `${event.id} attempt=${message.info.deliveryCount} redelivered=${message.redelivered}`);
 
  // The claim and the effect commit together (chapter 3.4). After this line the
  // work has HAPPENED, durably, and the broker still believes it has not.
  const result = await claimEvent(db, durable, event.id, async () => {});
  show("claim", result);
  handled += result === "handled" ? 1 : 0;
 
  if (KILL_MODE) {
    show("times handled", await timesHandled(db, durable, event.id));
    console.log("MARKER kill-me-now");
    // The window a parent uses to kill this process from outside — which is what
    // the integration suite does, watching stdout for the marker.
    await new Promise((r) => setTimeout(r, PAUSE_MS));
    // Nobody killed it, so it kills itself. SIGKILL to its own pid is a real
    // uncatchable death, not a tidy `process.exit()`: no flush, no `finally`,
    // no drain. The work above HAPPENED and was never acknowledged, which is
    // exactly the state a redelivery is for. Run by hand, this is the whole
    // demonstration in one command.
    process.kill(process.pid, "SIGKILL");
  }
 
  if (RESUME) show("times handled", await timesHandled(db, durable, event.id));
 
  message.ack();
  show("acknowledged", event.id);
}
 
if (FROM_ALL) show("handled in this batch", handled);
await nc.drain();
process.exit(0);

// What the stream actually holds and how it is actually configured
// (chapter 3.4). The chapter quotes the broker, not the config file — a
// configuration that was written is not the same as a configuration that was
// applied, and the difference is exactly what this chapter is about.
//
//   docker compose up -d --wait nats
//   RELAY_NATS_URL=nats://localhost:14222 node scripts/stream-info.mjs
import { connect } from "../services/api/node_modules/nats/lib/src/mod.js";
 
const url = process.env.RELAY_NATS_URL ?? "nats://127.0.0.1:4222";
const nc = await connect({ servers: url });
const jsm = await nc.jetstreamManager();
 
const seconds = (ns) => (ns === 0 ? "unlimited" : `${ns / 1e9}s`);
const bytes = (n) => (n === -1 ? "unlimited" : `${(n / 1024 ** 3).toFixed(2)} GiB`);
const show = (label, value) => console.log(`${label.padEnd(20)} ${value}`);
 
const info = await jsm.streams.info("EVENTS");
const c = info.config;
 
console.log("stream EVENTS");
show("  messages", info.state.messages);
show("  bytes", `${(info.state.bytes / 1024 ** 2).toFixed(1)} MiB`);
show("  consumers", info.state.consumer_count);
console.log("configuration");
show("  subjects", JSON.stringify(c.subjects));
show("  retention", `${c.retention}   (immutable once created)`);
show("  storage", `${c.storage}    (immutable once created)`);
show("  replicas", c.num_replicas);
show("  max_age", `${seconds(c.max_age)}   (NFR-REL-08 floor: 86400s)`);
show("  max_bytes", bytes(c.max_bytes));
show("  discard", `${c.discard}     (at the bound, drop the OLDEST)`);
show("  duplicate_window", `${seconds(c.duplicate_window)}   (the broker's dedupe, not ours)`);
 
console.log("consumers");
for await (const ci of jsm.consumers.list("EVENTS")) {
  show(
    `  ${ci.name}`,
    `pending=${ci.num_pending} ack_pending=${ci.num_ack_pending} redelivered=${ci.num_redelivered} max_deliver=${ci.config.max_deliver ?? "-"}`,
  );
}
 
await nc.drain();
process.exit(0);

× invariant 3: an event is delivered, handled once, and acknowledged
× invariant 4: a kill between handling and acknowledgement is redelivered — and handled once
× invariant 6: two instances sharing a durable name divide the work
3 failed | 7 passed

@@ -28,6 +28,8 @@
         "RELAY_NATS_URL",
         "RELAY_NATS_PORT",
         "RELAY_OUTBOX_RELAY",
+        "RELAY_EVENT_CONSUMER",
+        "RELAY_NATS_REPLICAS",
         "RELAY_E2E_API_PORT"
       ]
     },

@@ -342,6 +342,11 @@ export async function boot({ gateways = 2 } = {}): Promise<System> {
     // files, not a property of the system. The relay has its own suite, which
     // drives it explicitly.
     RELAY_OUTBOX_RELAY: "off",
+    // Chapter 3.4: no event consumer in these children either, for the reason
+    // the line above exists — this journey asserts message delivery, and a
+    // background consumer writing to a table 3.4's suite asserts on is a race
+    // between test files rather than a property of the system.
+    RELAY_EVENT_CONSUMER: "off",
   };
 
   const apiPort = Number(process.env.RELAY_E2E_API_PORT ?? 4100);