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Build a Notification Fanout Service

Notification Fanout Service — a complete walkthrough of notification delivery, covering event receiver, channel router, delivery queue, and testing.

notification_fanout_service/
├── src/index.ts
├── src/event-receiver.ts
├── src/channel-router.ts
├── tests/notification_fanout_service.test.ts
└── package.json

// Event receiver: notification delivery core
export function eventreceiver(input: EventreceiverInput): EventreceiverResult {
  // Build the event receiver that anchors notification delivery and defines the core data shape.
  return processEventreceiver(input);
}

// Channel router — handles notification delivery Add the channel router so
export class Channelrouter {
  constructor(private opts: ChannelrouterOptions) {}
  async run(ctx: Context): Promise<ChannelrouterOutput> {
    return this.execute(ctx);
  }
}

// Delivery queue: notification delivery stage
export const deliveryqueue = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Rate per channel for notification delivery
export async function rateperchannel(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Dead letter store: notification delivery finalization
export function deadletterstore(state: State): Result {
  validate(state);
  return finalize(state);
}

Project overview

This project builds a notification delivery system whose reason for existing is expanding a single event into per-channel deliveries without a slow channel blocking the fast ones. The temptation in notification delivery is to bolt the interesting part onto a working prototype and discover the hard cases in production; this build inverts that order and treats a slow email channel that backs up its queue and delays notifications that should have been instant on push as the first thing designed, not the last.

You will implement Event receiver, Channel router, Delivery queue, Rate per channel, and Dead letter store, each as a module with one responsibility and a test that proves it in isolation. The notification delivery pipeline is then the wiring that composes them, and the integration test is where the composition is shown to survive a slow email channel that backs up its queue and delays notifications that should have been instant on push.

The walkthrough is written for a developer who has used a notification delivery system but has not had to make one honest. It favors explicit data shapes over clever abstractions, and it names the failure each design choice prevents rather than presenting the choice as a style preference.

Each phase ends with something observable — a event receiver that returns the right shape, a channel router that rejects bad input, an integration test that goes green — so you can stop at any point and still have a notification delivery system that works for the cases covered so far.

What you will build

The finished notification delivery system exposes five capabilities, each with a focused test:

• Event receiver
• Channel router
• Delivery queue
• Rate per channel
• Dead letter store

Event receiver is the foundation that every later stage assumes; Channel router is where notification delivery correctness lives; Delivery queue handles a slow email channel that backs up its queue and delays notifications that should have been instant on push; Rate per channel makes the system observable; and Dead letter store is the graceful-degradation layer that keeps overload from becoming corruption.

The build keeps the notification delivery logic in ordinary modules you can read in one sitting, and pulls in dependencies only for the parts that are genuinely tedious to hand-roll. That keeps the diff between the tutorial and a real codebase small, and keeps the failure modes visible in code rather than hidden behind a framework.

Architecture and data model

The system is organized around Event receiver, Channel router, and Delivery queue, with Rate per channel tying them together. Data flows as a fanout that turns one event into many deliveries, each queued per channel with its own rate limit and retry policy.

Each component is a module with a pure interface where practical: the same input produces the same output, and side effects are pushed to the edges. That discipline is what makes the phases independently testable, because a test for Delivery queue does not need to stand up Event receiver and Channel router to exercise the interesting branch.

The data model is deliberately explicit. A fanout that turns one event into many deliveries, each queued per channel with its own rate limit and retry policy Keeping the shape written down — in a schema, a type, or a fixture — means a change to the shape is a visible diff rather than a silent runtime shift, and it means the integration test can assert against a concrete shape instead of guessing.

Configuration and code are kept apart: the configuration says what notification delivery should do, the modules say how. A reviewer can change a threshold or a policy without reading the implementation, which keeps the risky changes small and lets the diff stand as the review.

The failure path through Delivery queue is designed before the happy path is finished, because a slow email channel that backs up its queue and delays notifications that should have been instant on push is the case that distinguishes a notification delivery system from a demo. Every component declares what it does when its inputs are wrong, and the integration test asserts each of those declarations.

Prerequisites and dependencies

Have these ready before you install anything, so the notification delivery setup is not blocked by a missing tool:

• Node.js 20 LTS or later
• Familiarity with notification delivery fundamentals
• A local environment to run the test suite

The runtime dependencies are chosen to keep the notification delivery datapath legible:

• fastify (^5.0.0) — Fanout API
• redis (^4.7.0) — Delivery queues
• pino (^9.0.0) — Delivery logging

# setup for notification-fanout-service
pnpm init
pnpm add fastify redis pino
pnpm add -D typescript tsx vitest

Versions are pinned to a major range so the notification delivery install is reproducible; commit the lockfile. The event receiver reads its operational parameters from the environment and validates them at startup, so a missing value fails the boot rather than the first notification delivery request.

Directory structure

Keep each notification delivery component in its own file so it can be tested without starting the others:

notification_fanout_service/
├── src/index.ts
├── src/event-receiver.ts
├── src/channel-router.ts
├── tests/notification_fanout_service.test.ts
└── package.json

Phase 1: Event receiver

Build the event receiver that anchors notification delivery and defines the core data shape.

// Event receiver: notification delivery core
export function eventreceiver(input: EventreceiverInput): EventreceiverResult {
  // Build the event receiver that anchors notification delivery and defines the core data shape.
  return processEventreceiver(input);
}

After this phase the event receiver works in isolation and is covered by a unit test. The event receiver defines the data shape that every later stage assumes, so getting it right first means the later phases compile against a stable contract.

Treat the event receiver as the single source of truth for the notification delivery data shape. Validate inputs at the boundary and reject early, so a malformed value never reaches Channel router or Delivery queue where its effect would be harder to localize.

Phase 2: Channel router

Add the channel router so notification delivery can expanding a single event correctly.

// Channel router — handles notification delivery Add the channel router so
export class Channelrouter {
  constructor(private opts: ChannelrouterOptions) {}
  async run(ctx: Context): Promise<ChannelrouterOutput> {
    return this.execute(ctx);
  }
}

After this phase the channel router works in isolation and is covered by a unit test. This is where notification delivery correctness lives: the channel router is what makes expanding a single event into per-channel deliveries without a slow channel blocking the fast ones true, and its test is the proof.

Keep the channel router pure where you can. A function that takes an input and returns an output is trivial to test and impossible to mock wrong; a class with hidden state is the opposite. Push side effects to the caller and keep the channel router about the transformation.

Phase 3: Delivery queue

Wire the delivery queue to handle a slow email channel without losing data.

// Delivery queue: notification delivery stage
export const deliveryqueue = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

After this phase the delivery queue works in isolation and is covered by a unit test. The delivery queue is the difference between a notification delivery system that works on the happy path and one that survives a slow email channel that backs up its queue and delays notifications that should have been instant on push.

Design the failure before the success. Decide what delivery queue does when a slow email channel that backs up its queue and delays notifications that should have been instant on push: reject, retry, shed, or degrade — and write that test first. A notification delivery system whose failure behavior is unspecified will pick one at random under pressure, and it is usually the wrong one.

Phase 4: Rate per channel

Integrate the rate per channel to make notification delivery observable and testable end to end.

// Rate per channel for notification delivery
export async function rateperchannel(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

After this phase the rate per channel works in isolation and is covered by a unit test. Observability is not a finishing touch; it is how you know the notification delivery system is doing what it claims in production.

Emit one structured event per meaningful action, with enough context to localize a failure without a second round-trip. The rate per channel is what turns “something is slow” into “the channel router is slow for inputs matching this shape,” which is the difference between an alert and an investigation.

Phase 5: Dead letter store

Finalize the dead letter store so the notification delivery system degrades gracefully under overload.

// Dead letter store: notification delivery finalization
export function deadletterstore(state: State): Result {
  validate(state);
  return finalize(state);
}

After this phase the dead letter store works in isolation and is covered by a unit test. The dead letter store is what keeps overload from becoming corruption: when the notification delivery system is asked for more than it can do, it degrades in a defined way instead of failing in an undefined one.

Pick a degradation strategy for dead letter store — shed, queue, or backpressure — and make it explicit in code and in a test. Leaving the behavior to chance is how a notification delivery traffic spike becomes a data loss incident.

Testing

Each phase has a focused test: event receiver, channel router, and delivery queue are exercised in isolation, then an integration test drives the full notification delivery pipeline and asserts the a slow email channel case is handled.

Start with unit tests that pin each component’s contract: the event receiver returns the documented shape, the channel router transforms correctly, and the delivery queue fails in the documented way. These tests are fast and they localize regressions to the component that broke.

Then add an integration test that composes the components and drives a realistic notification delivery scenario end to end. Assert the happy path first, then deliberately trigger a slow email channel that backs up its queue and delays notifications that should have been instant on push and assert the system degrades rather than corrupts. Because the components are pure where practical, the integration test stays deterministic rather than flaky.

A notification delivery suite must also prove the negative: bad input is rejected at the boundary, overload is shed rather than queued indefinitely, and a partial failure does not leave the system in an inconsistent state. These are the tests that catch the regressions a happy-path suite never would.

The entire notification delivery suite runs with no external service; every dependency is either in-process or stubbed, so every case is reproducible in CI. That is what lets the delivery queue failure-mode assertions live as tests rather than as a manual checklist that drifts out of date.

# run the notification delivery test suite
pnpm vitest run

Deployment and operations

Run at least two notification delivery instances behind a load balancer and share state in Redis. Set resource limits so notification delivery overload surfaces as backpressure rather than failure.

Run at least two stateless instances behind a load balancer, and move any shared state into a shared store so the notification delivery behavior is global rather than per instance. Size limits so the system sheds load before it runs out of memory, because a notification delivery system that OOMs under load fails in a way that is hard to reason about.

Drain on deploy: stop accepting new work, let in-flight notification delivery operations finish, then exit. Because the system holds no state that cannot be reconstructed, a rollout is a matter of cycling instances one at a time while watching the error rate.

When a dependency is unavailable, decide explicitly whether the notification delivery system fails closed or degrades. Leaving that choice to chance is how a dependency outage becomes a silent data loss, so make the choice visible in code and in a test.

Security considerations

Validate all notification delivery inputs at the boundary, reject oversized payloads before buffering them, and keep secrets in the environment rather than the repository.

The notification delivery system is a choke point, so it is the right place to reject malformed input before it reaches channel router or delivery queue. Validate the shape and the size at the edge, and let the inner components assume what they receive is well-formed.

Never log secrets or full payloads; log enough to localize a failure without recording data that should not be on disk. Treat any external call as untrusted and validate its response before using it, because a notification delivery system that trusts its inputs blindly is one malformed response away from corruption.

Keep dependencies pinned and run an audit in CI. A transitive dependency with a known vulnerability is still a vulnerability in your notification delivery system, and the lockfile is what makes the audit meaningful.

Observability

Emit one structured event per notification delivery operation with the inputs’ shape, the outcome, and a correlation id. Track error rate and latency per component and alert on a step change rather than a single spike.

The correlation id is the thread that ties a user-visible symptom to the notification delivery decision and to the component log entry for the same operation. Without it, an investigation starts from a timestamp and a guess.

Watch the delivery queue failure rate separately from the overall error rate. A jump in delivery queue failures usually means a slow email channel that backs up its queue and delays notifications that should have been instant on push, not a bug in channel router, and the two have different owners and different fixes.

Chart the share of notification delivery operations that shed, queue, or succeed. A change in the mix points at the cause faster than an aggregate latency can, because each class has a different root.

Troubleshooting

A slow email channel that backs up its queue and delays notifications that should have been instant on push. Reproduce with a minimal fixture, then add a guard test so the notification delivery system rejects the condition loudly.

Performance regresses under notification delivery load. Profile with the sampling analyzer, locate the hot path in channel router, and add a targeted benchmark to lock the regression.

Keep the notification delivery dashboards separate from dependency dashboards. When an operation fails, the correlation id lets you decide in seconds whether the fault is event receiver, channel router, or delivery queue, instead of guessing from a timeout.

Limitations and next steps

• Single-region; multi-region replication is out of scope.
• The event receiver is in-process and not horizontally shared by default.
• No auth layer; assume deployment behind an edge that enforces identity.

The notification delivery system is single-region by design; extending to multi-region means making event receiver shareable across regions, which is a real change to the consistency model rather than a config flag. The delivery queue is in-process, which is correct for one instance but must move to a shared store before scaling out.

There is no authentication or authorization layer; the system assumes it is deployed behind an edge that enforces identity. Adding one is straightforward but out of scope for this build, and doing it here would distract from the notification delivery correctness that is the point.

The natural next step, once the notification delivery pipeline is trusted in production, is to add a short-TTL cache for idempotent reads and a richer rate per channel that exports histograms rather than counters. Both are additive and neither changes the contracts this build establishes.