quayside

Generic idempotency for Node.js — execute any operation exactly once per key, with pluggable storage, explicit concurrency semantics, and first-class observability.

The quay is where cargo lands once: unloaded, registered, never processed twice. quayside is that quay for your operations — REST handlers, queue consumers, cron jobs, workers and CLI commands all get the same guarantee: run this function once per key; if it already ran, return the stored result; if it is running right now, don't run it again.

import { Idempotency } from 'quayside'
import { RedisStorage } from 'quayside/redis'

const idempotency = new Idempotency({
  storage: new RedisStorage(redis),
  resultTtl: '24h',   // how long a completed result stays replayable
  lockTtl: '30s'      // how long a crashed execution blocks the key
})

const result = await idempotency.execute('invoice:123', async () => {
  return createPayment()   // runs once; every later call replays the result
})

Table of contents

• Why another idempotency library?
• Install
• Quick start
• Core concepts
• Storage
• HTTP adapters
• NestJS
• Observability
• Errors
• API
• Documentation
• Development

Why another idempotency library?

Every company eventually hand-rolls this. Behind the one-liner hide lock acquisition, concurrent execution, replay, TTLs, crash recovery, rollback, result serialization and multi-storage support — and the existing libraries each solve a slice:

	quayside	steadykey	Powertools	@node-idempotency	express-idempotency
Fencing tokens — a holder that lost its lock can never overwrite the new holder's result	✅ enforced in-store, race-tested	❌ README admits the operation may run twice	➖	❌	❌
Framework-agnostic core (consumers, jobs, CLI)	✅	✅	➖ Lambda-shaped	➖ HTTP semantics baked in	❌ Express only
State machine with crash recovery (lockTtl)	✅	✅ lease, unfenced	✅	➖ partial	❌ replay only
Separate lock TTL vs result TTL	✅	➖	✅	❌	❌
Payload fingerprint (same key + different body ⇒ error)	✅ constant-time	➖ payload is the key	✅	✅	➖ deep-equal, HTTP only
Concurrency policy: reject and wait	✅ + storage-assisted wake-up	➖ poll-only wait	➖ throws only	➖ throws only	❌
Storage: memory / Redis / Postgres / MySQL / DynamoDB	✅ all five, one contract suite	✅ many, thin contract	➖ DynamoDB-first	➖ memory, redis	➖ plugin-ish
HTTP adapters with IETF draft semantics (status/header replay, 409/422)	✅ Express · Fastify · Hono	➖ middleware, thinner	n/a	✅	➖ stale
NestJS module + @Idempotent() decorator	✅	❌	✅ (Lambda)	✅	❌
Typed events + pluggable metrics collector	✅ native, correlationId	➖ onHit/onMiss callbacks	➖ CloudWatch-shaped	❌	❌
Runtime dependencies	0	0	several	several	several

Design principles:

• The core knows nothing about HTTP. execute(key, fn) is the primitive; HTTP is one adapter among many, and the raw core works in any framework from day 1.
• Semantics borrowed from the best. The state machine and payload-fingerprint validation follow AWS Powertools and Stripe — the two implementations that got the hard cases right.
• Atomicity lives in the storage, never in JavaScript. Fenced transitions are Lua scripts on Redis, token-conditional statements on SQL and condition expressions on DynamoDB; a stale holder's late write fails, it never overwrites.
• Never magic. Values that cannot be stored faithfully raise errors instead of being silently dropped; keys are rejected instead of truncated; failures degrade loudly.
• Zero runtime dependencies, in the core and in every adapter — storage clients and frameworks are bring-your-own, typed structurally.

Every claim above is enforced by the test suite — including a 50-way concurrency race, SIGKILL crash recovery and a split-brain fencing test against real Redis, Postgres, MySQL and DynamoDB servers (Testcontainers).

Install

npm install quayside

Works with both module systems:

import { Idempotency } from 'quayside'          // ESM
const { Idempotency } = require('quayside')     // CJS

Requires Node.js >= 22.

Quick start

import { Idempotency } from 'quayside'
import { MemoryStorage } from 'quayside/memory'

const idempotency = new Idempotency({ storage: new MemoryStorage() })

// 1. The primitive: execute once per key
const payment = await idempotency.execute('invoice:123', () => createPayment())

// 2. Payload fingerprint: same key + different body => IdempotencyKeyReuseError
await idempotency.execute(
  { key: req.headers['idempotency-key'] as string, payload: req.body },
  () => createPayment(req.body)
)

// 3. No client key? Derive one from the payload (consumers, jobs)
await idempotency.execute(
  { payload: message.value, ignoreFields: ['meta.timestamp', 'requestId'] },
  () => processOrder(message.value)
)

// 4. Decorate once, call everywhere
const createOnce = idempotency.wrap(createPayment, {
  key: (input) => `payment:${input.invoiceId}`
})

// 5. Replay metadata, inspection, invalidation
const { value, replayed, storedAt } = await idempotency.executeWithMetadata('invoice:123', fn)
await idempotency.get('invoice:123')
await idempotency.invalidate('invoice:123')

The same execute call works verbatim inside an Express route, a Fastify handler, a Hono handler, a Kafka/RabbitMQ consumer, a cron job or a CLI command. Adapters are protocol sugar, not a requirement.

Core concepts

The atomic write is the lock

IN_PROGRESS is written atomically (create-if-absent) before your function runs — that write is the lock; there is no separate locking step. On success the record transitions to COMPLETED guarded by a fencing token: a holder that lost its lock (GC pause, slow I/O, expired TTL) gets FencingError from the store itself and can never overwrite the new holder's result.

Two TTLs, not one

Conflating these is the single most common bug in homemade implementations:

• lockTtl (default 30s) bounds crash recovery: if the process dies mid-flight, the key unblocks when the lock expires. Long-running functions heartbeat with ctx.extend().
• resultTtl (default 24h, Stripe's convention) is the replay window for completed results.

Intent first, content as validation

The explicit key names the intent (an Idempotency-Key header, an invoice id). The payload fingerprint — a canonical, type-tagged, locale-independent hash — exists to validate it: the same key with a different payload fails with IdempotencyKeyReuseError instead of silently replaying a result for another request. When no client sends a key (queue consumers, jobs), deriving the key from the payload is an opt-in convenience with ignoreFields/pickFields for volatile fields.

Concurrency is a policy

When a call finds the key IN_PROGRESS: onConflict: 'reject' (default, HTTP-safe) throws ConcurrentExecutionError immediately; 'wait' blocks until the winner finishes and replays its outcome, bounded by waitTimeout over the whole call (a waiter that takes over a dead holder's key and then loses the race keeps the same budget) — ideal for consumers and workers. Waiters poll with exponential backoff, and storages that support notifications (Redis keyspace events) wake them early.

Failures are not idempotent

A rejection deletes the record; retries run fresh. persistFailures: true opts into storing and replaying the error for non-retryable business failures — the replay preserves name, message, stack, own enumerable properties (code, statusCode, ...) and the cause chain, so check error.code on replay, not instanceof.

Fail closed, bypass loudly

If the storage is unreachable, execute throws StorageUnavailableError instead of running without the guarantee. onStorageError: 'open' flips the trade-off — the function runs unguarded and every bypass emits a storage-bypass event, so degradation is always observable. Fail-open covers every storage interaction (acquire, completion write, a poll that fails mid-wait, a ctx.extend heartbeat), but not a storage that answers with a record the contract cannot describe: that raises StorageCorruptError, because a data bug decodes the same way on every attempt and running unguarded would last as long as the record rather than as long as an outage.

Keys are never truncated

Namespace and key segments are percent-encoded before composition (a client-supplied key cannot impersonate another namespace) and composed keys longer than maxKeyLength (default 512) are rejected with IdempotencyKeyInvalidError — truncation would silently alias two keys into one record. The value came from the caller, so the HTTP adapters answer 400, never a 5xx.

Deep dive on all of the above: docs/core.md.

Storage

import { MemoryStorage } from 'quayside/memory'     // tests and development
import { RedisStorage } from 'quayside/redis'       // SET NX PX + fenced Lua transitions
import { PostgresStorage } from 'quayside/postgres' // ON CONFLICT DO NOTHING + token-conditional updates
import { MysqlStorage } from 'quayside/mysql'       // INSERT IGNORE equivalent
import { DynamoStorage } from 'quayside/dynamodb'   // conditional PutItem + token-conditional updates

Bring your own client — any ioredis instance or @pinceladasdaweb/redis RedisClient, any pg Pool, any mysql2/promise Pool, any DynamoDBClient (its command classes are passed in, so the AWS SDK is never bundled). The SQL adapters ship CREATE TABLE migrations (migrate() or an exported DDL string) and clean up expired rows lazily — no cron required, with an optional sweep() for bulk housekeeping. DynamoDB ships the same migrate() plus exported table and TTL definitions for CDK/Terraform, and treats its native TTL as a garbage collector only: expiry is decided on read, because the service deletes expired items on its own schedule (docs/dynamodb.md).

Every adapter passes the same storage-contract suite against a real server, including the three invariants that protect correctness: expired-but-not-purged records read as absent, an expired record is reclaimed in place by acquire (a crashed holder can never wedge its key), and keys are stored faithfully or rejected — never truncated. Custom adapters implement one interface and inherit the suite: see docs/sql.md, docs/dynamodb.md and tests/contract.

HTTP adapters

All HTTP semantics live in one shared kernel implementing the IETF Idempotency-Key draft; each adapter is thin glue:

import { ExpressMiddleware } from 'quayside/express'
import { FastifyPlugin } from 'quayside/fastify'
import { HonoMiddleware } from 'quayside/hono'

app.use(ExpressMiddleware(idempotency, { enforce: true }))

• Faithful replay: original status + selected headers + body, with Idempotency-Replayed: true. A 201 + Location replays as 201 + Location, never a generic 200.
• Error mapping: 409 + Retry-After while the first request runs, 422 on key reuse with a different payload, 503 fail-closed, optional 400 for missing keys.
• Safe caching: binary, oversized (maxBodyBytes, default 1 MiB) and 5xx responses are served but never stored — and the endpoint keeps its concurrency protection on every attempt.
• Principal scoping: a bare header key is shared by every caller on the same storage — on multi-tenant endpoints, derive the key from the authenticated principal with the key option (the request facts carry the framework's native request as raw), so the same header value never collides across callers.

Options, cacheability rules and the key-scoping pattern: docs/http.md. Adding another framework is an afternoon of work: docs/writing-an-adapter.md.

NestJS

import { QuaysideModule, Idempotent } from 'quayside/nestjs'

QuaysideModule.forRoot({ storage: new RedisStorage(redis) })

@Post('/payments')
@Idempotent({ ttl: '24h' })
createPayment () { ... }

Module (forRoot/forRootAsync), interceptor and decorator, with per-route key extractors, TTL overrides and enforce. Peer dependencies are @nestjs/common and rxjs only — already present in any Nest application. Full guide: docs/nestjs.md.

Observability

Every execution emits typed events carrying a correlationId:

const idempotency = new Idempotency({
  storage,
  onEvent: (event) => log.debug(event),
  // acquired | replayed | conflict | completed | failed | expired-recovery | storage-bypass
  metrics: {
    onReplayed: (event) => replayCounter.inc(),
    onConflict: (event) => conflictCounter.inc(),
    onStorageBypass: (event) => bypassCounter.inc()  // watch this one
  }
})

Listener failures never affect execution semantics — a throwing listener surfaces as a process warning instead of failing the operation or being silently swallowed. Ready-made integrations follow the same shape as breakwater's:

import { prometheusMetrics } from 'quayside/prometheus' // counters + duration histogram
import { otelSpans } from 'quayside/otel'               // spans with replay/conflict attributes

new Idempotency({ storage, metrics: prometheusMetrics() })

Terminal events carry durationMs — a replayed duration approximates the time a waiter spent blocked. Details and the PromQL replay-ratio query: docs/observability.md.

Errors

All errors extend QuaysideError and carry a stable code — codes are contract, message text is not:

Error	code	HTTP mapping
ConcurrentExecutionError	IDEMPOTENCY_IN_PROGRESS	409
IdempotencyKeyInvalidError	IDEMPOTENCY_KEY_INVALID	400
IdempotencyKeyReuseError	IDEMPOTENCY_KEY_REUSE	422
WaitTimeoutError	IDEMPOTENCY_WAIT_TIMEOUT	409
FencingError	IDEMPOTENCY_FENCING	500
SerializationError	IDEMPOTENCY_SERIALIZATION	500
StorageCorruptError	IDEMPOTENCY_STORAGE_CORRUPT	500
StorageUnavailableError	IDEMPOTENCY_STORAGE_UNAVAILABLE	503

API

new Idempotency(options)

Option	Default	Meaning
storage	— (required)	Any IdempotencyStorage implementation
resultTtl	'24h'	Replay window for completed results
lockTtl	'30s'	How long an in-progress record survives without completion
onConflict	'reject'	'reject' throws immediately; 'wait' blocks until the winner finishes
waitTimeout	'10s'	Upper bound for onConflict: 'wait', measured over the whole call
namespace	—	Key prefix isolating domains that share one storage
maxKeyLength	512	Longest composed storage key; longer keys are rejected
codec	JSON	Serialization of stored results and persisted failures (Codec interface for superjson/msgpack/encrypt-at-rest users)
persistFailures	false	Store and replay failures instead of allowing retries
onStorageError	'closed'	'open' runs without the guarantee and emits storage-bypass
onEvent / metrics	—	Typed event listener / metrics collector
clock	wall clock	{ now(), sleep(ms) } time source — inject a manual clock for deterministic TTL and backoff tests

Durations accept ms numbers or strings: '500ms', '30s', '10m', '24h', '7d'.

The object input form takes { key, payload, ignoreFields, pickFields, resultTtl } — resultTtl overrides the replay window for that call alone, so a per-route TTL never means building a second engine.

Methods: execute(input, fn) · executeWithMetadata(input, fn) · wrap(fn, { key }) · get(key) · invalidate(key). The fn receives a context: { key, replayed, signal, extend(ttl), doNotStore() } — doNotStore() keeps the lock's protection for concurrent callers while giving up the replay window, so an outcome that must not be served twice leaves no record behind.

Documentation

• Core semantics — state machine, TTLs, fingerprints, serialization rules, wait policy
• Observability — events, Prometheus metrics, OpenTelemetry spans
• SQL storage — migrations, lazy expiry, sweep()
• DynamoDB storage — table shape, why the native TTL is not the expiry, costs
• HTTP adapters — options, error mapping, cacheability rules
• NestJS — module, interceptor, decorator
• Writing an adapter — Koa as the worked example
• Benchmarks — what the guarantee costs per storage

Recipes — the non-HTTP callers are the headline:

• RabbitMQ consumers — key from messageId, outcomes mapped to ack/retry/DLQ
• SQS consumers — dedup beyond FIFO's 5-minute window, on standard queues too
• BullMQ processors — content/intent idempotency beyond jobId dedup
• Cron, workers and CLIs — one run per schedule slot, across replicas
• breakwater — resilience policies around storage calls

Development

npm install
npm run hooks             # once per clone: lint + commit-message hooks
npm test                  # unit tests, no external services needed
npm run test:integration  # real Redis/Postgres/MySQL via Testcontainers (needs Docker)
npm run test:mutation     # Stryker mutation testing over the unit suite
npm run examples          # runnable, self-asserting examples

Requirements

• Node.js >= 22
• Ships both ESM and CJS builds

License

MIT