What Are Webhooks and How to Build Them Reliably

TL;DR: A webhook is an automated HTTP request one system sends to another the moment an event happens, so you don't have to keep asking "anything new yet?". Building them reliably comes down to a few disciplines: verify signatures, respond fast and process asynchronously, make handlers idempotent, and retry with backoff. Get those right and webhooks become the cheapest, most real-time way to integrate systems.

What are webhooks, in plain terms?

The clearest way to understand what are webhooks is a simple contrast. A normal API works like you calling a shop to ask "has my order shipped?" over and over. A webhook works like the shop calling you back the instant it ships. Technically, a webhook is an HTTP POST request that a source system (say Stripe, GitHub or your own service) sends to a URL you control, carrying a payload that describes an event — a payment succeeding, a pull request merging, a user signing up.

You register that URL once (an "endpoint"), tell the provider which events you care about, and from then on the provider pushes data to you as things happen. Because the flow is event-driven rather than request-driven, people also call them reverse APIs, HTTP callbacks, or event notifications. The concept is deliberately boring — it is just an HTTP request — and that simplicity is exactly why webhooks are everywhere.

How do webhooks work, step by step?

A single webhook delivery usually follows the same lifecycle:

1. An event occurs in the source system (a charge is captured).
2. The provider builds a payload — typically JSON — describing what happened, and often signs it.
3. The provider sends an HTTP POST to your registered endpoint.
4. Your endpoint validates and acknowledges the request, ideally within a second or two, by returning a 2xx status.
5. Your system does the real work — update a database, send an email, kick off a workflow — usually on a background queue.
6. If you don't return 2xx, the provider retries later according to its own schedule.

The trap most teams fall into is doing step 5 during step 4. If your handler runs heavy logic before responding, a slow database or a downstream API can push you past the provider's timeout, the delivery is marked failed, it gets retried, and now you may process the same event twice. The reliable pattern is to acknowledge first, process second.

Why use webhooks instead of polling?

Before webhooks, integrations polled: a cron job asked an API "what changed?" every minute. Polling still has its place, but for most event-driven needs webhooks win on cost, freshness and load.

Factor	Polling	Webhooks
Latency	Up to your poll interval (seconds to minutes)	Near real time
Wasted requests	Most polls return "nothing new"	Traffic only when events happen
Who initiates	You call them repeatedly	They call you on demand
Failure handling	Next poll simply catches up	Needs retries + idempotency
Best for	Batch syncs, rate-limited APIs, backfills	Real-time reactions to discrete events

A pragmatic architecture often uses both: webhooks for immediate reactions, plus a periodic reconciliation poll as a safety net to catch anything a webhook missed. Webhooks are fast but not guaranteed; a nightly sync closes the gap.

How do you build webhooks reliably?

This is where good engineering separates a demo from production. Five practices matter most.

1. Verify every request with a signature

Your endpoint is a public URL, so anyone can POST to it. Never trust a payload just because it arrived. Reputable providers sign each request — usually an HMAC of the raw body using a shared secret, sent in a header. Recompute the signature on your side and reject anything that doesn't match. Verify against the raw request body, not a re-serialized version, or the hashes won't line up.

2. Respond fast, process asynchronously

Acknowledge with a 2xx as soon as you've validated and safely stored the event, then hand the real work to a background queue or job runner. This keeps you under provider timeouts and means a slow downstream service can't cause phantom retries.

// Sketch of the acknowledge-first pattern (Node/TypeScript)
app.post('/webhooks/stripe', async (req, res) => {
  const valid = verifySignature(req.rawBody, req.headers['stripe-signature']);
  if (!valid) return res.status(400).send('bad signature');

  await queue.add('process-event', { id: req.body.id, payload: req.body });
  return res.status(200).send('ok'); // acknowledge immediately
});

3. Make handlers idempotent

Assume every event can arrive more than once — because of retries, network hiccups, or at-least-once delivery from the provider. Store each event's unique ID and check it before acting, so reprocessing the same event is a no-op. This is the single most important habit for webhook correctness, and it's the same principle behind safe API design in general. We go deeper in What Is Idempotency in API Design (and Why It Matters).

4. Retry with backoff and a dead-letter path

If you are the one sending webhooks, retry failed deliveries with exponential backoff (for example after 1, 5, 30 minutes, then hourly) and stop after a sensible window. Give consumers a way to see failed deliveries and replay them. If you are receiving, remember the provider is doing this to you — which is exactly why idempotency matters.

5. Log, monitor and alert

Persist every received event with its status. You want to answer "did we get it, and did we process it?" without guessing. Alert on rising failure rates and on the age of the oldest unprocessed event. Rate limiting also belongs here so a burst of events can't overwhelm you — see API Rate Limiting Explained.

When should you not use webhooks?

Webhooks are not a universal answer. Skip or supplement them when:

• You need guaranteed ordering. Deliveries can arrive out of order; if sequence matters, include timestamps or version numbers and reason about state, don't assume order.
• You need a strict audit of every state. Pair webhooks with periodic reconciliation.
• The receiver can't expose a public HTTPS endpoint. In locked-down environments, polling or a message queue may be simpler.
• You need bidirectional, low-latency streams (like live chat). That's a job for WebSockets or server-sent events, not webhooks.

A good rule of thumb: webhooks for "tell me when something happened," polling for "let me make sure nothing was missed," and WebSockets for "keep a live connection open."

Who should build your webhook infrastructure?

Webhooks look trivial in a tutorial and get subtle in production — signature verification, idempotency, retries, replay, and observability are where reliability is won or lost. This is squarely a backend and QA concern, which is why teams that treat testing as a first-class activity tend to ship steadier integrations. At CodeVix Labs, our QA-first, Node.js and TypeScript engineering practice builds exactly this kind of resilient integration layer for clients worldwide. If you're weighing whether to build it in-house or bring in help, our guides on choosing between in-house, agency and freelance developers and our engineering services are good starting points, and you can see delivered work on our portfolio.

Frequently asked questions

What is the difference between a webhook and an API?

An API is a general interface you call when you want data; a webhook is a specific pattern where a system calls you automatically when an event happens. A webhook is delivered over HTTP, so it uses APIs — it's a push-based use of one, rather than a separate technology.

Are webhooks secure?

They can be, if you enforce discipline: serve the endpoint over HTTPS, verify the provider's signature on every request, reject unsigned or mismatched payloads, and never trust the body blindly. Treat the signing secret like any other credential and rotate it if exposed.

What happens if my server is down when a webhook fires?

Most reputable providers retry failed deliveries for a period, so a brief outage usually isn't fatal. But delivery is not guaranteed forever, which is why a periodic reconciliation job that re-syncs state is a smart backstop for anything financially or legally important.

How do I test webhooks during development?

Use a tunneling tool to expose your local server to the internet, or the provider's CLI and dashboard, which usually let you send test events and inspect delivery logs. Always write an automated test that replays a real captured payload so you catch signature and idempotency regressions before they reach production.