When you're integrating two software systems, you'll usually encounter two patterns: REST APIs (where you ask for data or trigger an action) and webhooks (where one system notifies another when something happens). Both are valid; they're designed for different scenarios, and choosing the wrong one creates unnecessary complexity.
The fundamental difference
REST API (polling/request-response): Your system asks another system for information or instructs it to do something. You initiate the communication. The other system responds. The relationship is client → server.
Example: "What are the open orders in my e-commerce system right now?" → The e-commerce API returns the current list of open orders.
Webhook (event-driven push): Another system notifies your system when something specific happens. They initiate the communication when an event occurs. The relationship is server → your endpoint.
Example: "When a payment is confirmed, send a notification to https://your-system.com/webhooks/payment-confirmed."
The simplest way to remember the difference: REST APIs work like a phone call you make to ask for information. Webhooks work like a notification pushed to you when something happens.
When to use REST APIs
Reading data on demand. When you need to retrieve current information in response to a user action or a scheduled process — a nightly sync, a user clicking "refresh", generating a report — REST APIs are the right choice. You control when the request happens.
Triggering actions. When you want to tell another system to do something — create a record, send an email, start a process — REST APIs are typically the mechanism. The action is triggered by you, at a time you control.
Polling. When you need to check for changes but the other system doesn't support webhooks, you periodically call a REST API and compare the result to what you saw last time. This is less efficient than webhooks (you're making calls even when nothing has changed) but is sometimes the only option with older systems.
When you need to query with parameters. REST APIs let you specify exactly what you want: "Give me orders created in the last 24 hours, with status 'pending', sorted by value descending." Webhooks push events as they happen — you get what the system decides to send you.
When to use webhooks
Real-time event handling. When you need to react immediately when something happens — a payment is confirmed, a user signs up, a task is completed — webhooks are the right choice. Polling APIs for this introduces latency (up to the polling interval) and wastes API quota.
Reducing API load. If you're polling an API every minute to check for new orders, you're making 1,440 API calls per day even when there are no new orders. A webhook delivers events only when they occur.
Audit trails and event sourcing. Webhooks provide a natural event stream that can be stored and replayed. This is useful for building audit logs, debugging, and event sourcing architectures.
Third-party system notifications. Payment processors (Stripe, Mollie), communication platforms (Twilio, SendGrid), and most modern SaaS tools offer webhooks as the primary mechanism for notifying your system of events. This is increasingly the default pattern for modern API design.
When webhooks don't work
The other system doesn't support them. Older enterprise systems, legacy ERPs, and some government APIs may not support webhooks. In this case, polling is your only option.
You need to query historical data. Webhooks are real-time. They can't give you all events that happened before you registered the webhook. For historical data, you need a REST API.
Webhook delivery is unreliable. Webhook delivery is usually reliable, but not guaranteed. Most webhook providers retry on failure, but there are edge cases (network partition, your endpoint is down for maintenance) where events can be missed. For critical workflows, you may need a combination: webhooks for real-time notification and REST API polling as a reconciliation mechanism.
You're behind a firewall. Webhooks require your endpoint to be publicly accessible on the internet. If your system runs behind a corporate firewall without a public endpoint, you can't receive webhooks. REST API polling, or a relay/gateway pattern, is the alternative.
Building reliable webhook receivers
If you're on the receiving end of webhooks, there are a few implementation patterns that prevent common failures:
Always return 200 immediately. Your webhook receiver should acknowledge receipt (HTTP 200) and then process the event asynchronously. If processing takes time and your endpoint doesn't respond within the sender's timeout window, they'll retry — which can cause duplicate processing.
Handle duplicates. Even well-implemented webhook systems can deliver the same event more than once (usually during retries). Your processing logic should be idempotent — processing the same event twice should produce the same result as processing it once. This typically means checking if you've seen an event ID before processing it.
Validate signatures. Most webhook providers sign their payloads with a secret that you verify on receipt. Skip this and your webhook endpoint will accept requests from anyone who knows its URL.
Log everything. Store raw webhook payloads before processing them. When something goes wrong — and something will go wrong — the stored payload lets you debug and replay events without waiting for the sender to resend.
Queue and process asynchronously. For high-volume or slow-processing webhooks, put events on a queue (Redis, SQS, RabbitMQ) and process them with a separate worker. This keeps your webhook receiver fast and prevents timeouts.
REST + webhooks together
The most robust integration architectures use both patterns together:
- Webhooks for real-time event notification ("a new order was placed").
- REST API for initial data load and reconciliation ("give me all orders from the last 24 hours to catch anything I might have missed").
This hybrid approach gets you the responsiveness of webhooks with the reliability of periodic reconciliation. It's more engineering work upfront but significantly more robust in production.
Choosing between REST APIs and webhooks is not a matter of one being better — it's about which pattern fits the specific integration scenario. Most production integrations end up using both.
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