Understanding Webhooks and Callbacks

Webhooks and callbacks are key mechanisms in event-driven applications that allow services to communicate with external systems asynchronously. While often grouped under the same umbrella, they differ in fundamental ways, particularly in how the receiving endpoint is defined.

This article breaks down two top-level patterns — Webhooks and Callbacks — and categorizes common behavior types seen in both.

Delivery Mechanisms

Webhooks (Statically Defined URLs)

Webhooks are requests made to a statically defined URL that is pre-registered with the sending service. The sender determines when to send the request, and the recipient listens passively for those events.

In some cases, these URLs can be temporarily overridden or modified by supplying a dynamic callback URL as part of an API call — though the infrastructure still follows a webhook model.

Examples: GitHub, Stripe, Postmark, Twilio, Shopify, Slack (Events API)

Callbacks (Dynamically Supplied URLs)

Callbacks are URLs provided at runtime — often as part of an API call — and invoked when a long-running or asynchronous process completes. These are more explicitly under the control of the consumer.

Examples: AWS Step Functions, workflow engines, certain custom integrations

Event Types

These behaviors can be found in either delivery mechanism, though some are more typical in one than the other:

Notification

Sends an event payload without expecting more than a 2xx response.

Examples: GitHub, Stripe, Slack, GitLab, Shopify

Instruction Fetching

Sends a request and expects a structured response to control behavior.

Examples: Twilio (TwiML), Vonage (NCCO)

Verification / Challenge

Sent to validate or test the receiving endpoint. May include a token to echo back or simply require a 2xx response.

Examples: GitHub, Slack, Facebook, Zoom

Quick Reference Matrix

Delivery Mechanism	Behavior Type	Description	Examples
Webhook	Notification	Event data is pushed to a statically configured URL. No response beyond 2xx expected.	GitHub, Stripe, Slack, Postmark, GitLab, Shopify
Webhook	Instruction Fetching	Webhook expects a structured response that influences system behavior.	Twilio (TwiML), Vonage (NCCO)
Webhook	Verification / Challenge	Used to validate the webhook endpoint (e.g., challenge tokens, ping events).	GitHub (ping), Slack, Facebook, Zoom
Callback	Async Completion	Client supplies a callback URL dynamically; platform calls it when ready.	AWS Step Functions, custom integrations

For developers consuming webhooks or callbacks, recognizing these patterns helps with planning how to receive, process, and react to incoming events — whether you're building reactive UIs, triggering workflows, or persisting system state.

If you're designing an API or integrating with a third-party system, being deliberate about which pattern and event type you use — and documenting it clearly — not only improves developer experience but also helps you design for failure, scale, and long-term maintainability.

When to Use Webhooks Tutorial