Webhooks (HTTP Callbacks)

About

A webhook is a lightweight, event-driven communication mechanism where a server proactively pushes information to another system via an HTTP(S) request whenever a specific event occurs. Instead of polling an API at regular intervals to check for updates, the receiving system simply waits for the sending system to notify it with a callback request.

At its core, a webhook works like this:

1. Consumer registers a callback URL (endpoint) with the provider system.

2. Provider triggers HTTP POST requests to that URL when events happen.

3. The payload of the POST request carries the event details, usually in JSON, XML, or form data.

Nature of Webhooks

• One-way push – initiated by the provider, no request from consumer needed at event time.

• Event-driven – tied to specific business or system events (e.g., “payment succeeded”, “user signed up”).

• Stateless – each webhook call is independent; no session is maintained between calls.

• Near real-time – latency depends on provider’s processing and network speed.

• Uses standard HTTP(S) – easy to integrate across different tech stacks.

Example Flow

• GitHub → Jenkins: GitHub sends a webhook to Jenkins when code is pushed, triggering a CI pipeline.

• Stripe → E-commerce App: Stripe sends a webhook when a payment is confirmed so the store can update order status.

Why Webhooks Matter in System Design ?

Webhooks solve a fundamental efficiency and scalability challenge in distributed systems: keeping components in sync without constant polling.

1. Efficiency over Polling

Without webhooks, a consumer system might need to send periodic poll requests (e.g., every 5 seconds) to check if a new event has happened.

• This wastes network bandwidth and processing power when no changes occur.

• Webhooks replace this with a push-based model, delivering updates only when necessary.

Example: Instead of an e-commerce app asking a payment gateway every few seconds if a payment is complete, the gateway sends a webhook immediately after the transaction is confirmed.

2. Near Real-Time User Experience

Webhooks enable instant reactions to events, improving responsiveness in applications.

• Notifications, chat systems, CI/CD triggers, and order tracking all benefit from millisecond-to-second latency delivery.

Example: When a customer updates their shipping address, a webhook instantly notifies the delivery partner to adjust the route.

3. Decoupled System Integration

Webhooks allow systems to communicate without being tightly coupled.

• The provider does not need to know how the consumer works internally.

• The consumer just needs to expose a publicly accessible HTTP endpoint.

This reduces dependency management and allows each system to evolve independently.

4. Cost Savings

• Fewer API calls mean lower operational costs, especially for services with API request charges.

• Reduced load on servers, freeing resources for other operations.

5. Broad Applicability

Webhooks are useful in many scenarios:

• E-commerce – Order status updates, payment confirmations.

• DevOps – CI/CD pipelines triggered by code changes.

• Social Media – Real-time activity feeds and notifications.

• IoT – Device alerts sent to cloud services.

Characteristics

Webhooks are event-driven HTTP requests sent from one application (the provider) to another (the consumer) when a predefined event occurs. They have a set of defining characteristics that influence how they are implemented and maintained.

1. Event-Driven

• Webhooks are triggered by specific events in the provider system.

• Events are predefined (e.g., payment_success, user_signup, file_uploaded).

• The consumer only receives updates when relevant activity occurs.

Example: A Git repository sends a webhook whenever a new commit is pushed to the main branch.

2. Push-Based Communication

• Unlike polling, the provider pushes data to the consumer over HTTP without waiting for a request.

• The provider calls the consumer’s endpoint directly, often with an HTTP POST request containing the event payload.

3. HTTP as the Transport Layer

• Webhooks use HTTP/HTTPS for delivery.

• Commonly POST is used (with JSON/XML payloads).

• Consumers expose an HTTP endpoint (URL) to receive the callback.

4. Asynchronous by Nature

• Webhooks operate asynchronously, meaning the consumer is notified after the event occurs without blocking the provider’s primary process.

• Delivery attempts may happen in parallel with other provider tasks.

5. Lightweight Payloads

• Payloads typically contain only relevant event data (often as JSON).

• Some providers send minimal payloads with a reference ID, requiring the consumer to make a follow-up API call for full details.

6. Security Considerations

• Authentication & Verification: To prevent spoofing, providers often use:

  • Secret tokens in headers

  • HMAC signatures for payload validation

• HTTPS is recommended to encrypt transmission.

7. Delivery Guarantees

• Most webhook systems follow at-least-once delivery:

  • If the consumer does not respond with 2xx, the provider retries delivery (sometimes with exponential backoff).

• Ordering is not guaranteed unless the provider specifically supports it.

8. Statelessness

• Each webhook request should be self-contained, not relying on previous requests.

• This ensures resilience if a consumer misses earlier events.

9. Error Handling and Retries

• Providers typically retry failed deliveries several times before marking them as failed.

• Consumers should respond quickly with a 2xx status code to acknowledge receipt.

Execution Flow

Webhooks work like a phone call between two systems - when something happens in the provider’s system, it “calls” the consumer’s system to tell it right away. Here’s the step-by-step flow in a clear, practical way:

1. Event Occurrence in the Provider System

• Definition: This is the triggering condition inside the provider application that kicks off the webhook process.

• Events can be:

  • System events (server logs an error, scheduled job completes)

  • User actions (customer places an order, form is submitted)

  • Third-party interactions (payment gateway confirms a transaction)

• Importance: Webhooks are event-driven - without a meaningful event, there’s no reason to send a webhook.

• Design Tip: Providers should clearly define a list of webhook-supported events in their documentation so consumers can subscribe only to relevant ones.

Example: In GitHub, the event push is triggered whenever code is pushed to a repository.

2. Event Captured & Translated into Payload

• Once the event occurs, the provider’s internal event-handling mechanism:

  1. Captures event details from the system’s core business logic.

  2. Formats the data into a standardized structure (usually JSON, sometimes XML or form-encoded).

  3. Adds metadata for debugging and security, such as:

     • Event timestamp

     • Event unique ID (useful for detecting duplicates)

     • Webhook API version (ensures consumer parses data correctly)

     • Digital signature (HMAC or similar) for verification

• Why This Matters: Without structured and predictable data, the consumer system risks incorrect parsing or misinterpretation.

Example payload snippet:

{
  "id": "evt_56f7a",
  "type": "payment_succeeded",
  "created": 1691405092,
  "data": {
    "orderId": "ORD-10045",
    "amount": 125.50,
    "currency": "USD"
  }
}

3. HTTP Request Sent to Consumer Endpoint

• Method: Webhooks are almost always sent as HTTP POST requests because:

  • POST allows sending structured bodies (JSON/XML)

  • It matches the semantics of “creating a new resource” (in this case, creating a new event in the consumer’s system)

• Headers: Critical for:

  • Authentication (e.g., Authorization: Bearer <token>)

  • Security (HMAC signature to detect tampering)

  • Content Negotiation (Content-Type: application/json)

• Why POST Instead of GET: GET is meant for retrieval and often cached; POST is more appropriate for sending new data.

• Reliability Considerations:

  • The provider must handle network failures gracefully.

  • Delivery should be idempotent - resending the same webhook shouldn’t create duplicate effects.

4. Consumer Endpoint Processes the Event

• Validation:

  • First, check authenticity by verifying the signature/token to ensure the request really came from the provider.

  • Also check timestamp freshness to prevent replay attacks.

• Parsing:

  • Convert JSON/XML into application objects.

  • Validate schema (using tools like JSON Schema).

• Business Logic Execution:

  • Based on the event type, execute the corresponding workflow.

  • Example: payment_succeeded → Mark order as paid, send invoice.

• Security Considerations:

  • Never trust incoming data blindly.

  • Apply rate limiting to avoid denial-of-service from malformed webhook floods.

5. Consumer Sends HTTP Response

• Purpose: This lets the provider know the webhook was received and processed successfully.

• Best Practice:

  • Always respond quickly (often <5 seconds).

  • If processing is long-running, acknowledge immediately and process asynchronously in the background.

• Typical Responses:

  • 200 OK → Accepted and processed

  • 202 Accepted → Accepted but will process later

  • 4xx → Consumer error (e.g., bad request)

  • 5xx → Temporary server failure

• Why Quick Responses Matter: If the provider times out waiting, it will assume the webhook failed and may retry unnecessarily.

6. Provider Handles Acknowledgment

• If Success (2xx):

  • The event is marked as “delivered” in the provider’s system.

  • Some providers log delivery metrics (latency, retries).

• If Failure (Non-2xx or Timeout):

  • Provider schedules a retry.

  • Retry policies often include:

    • Exponential backoff (1s, 2s, 4s, 8s…)

    • Retry limit (e.g., max 10 attempts)

    • Dead-letter queue if retries fail consistently.

• Importance of Idempotency: Retried webhooks should not cause duplicate actions. Consumers should check the event ID before processing.

7. Optional: Consumer Makes Follow-up API Call

• Why:

  • To reduce payload size, providers sometimes send only the event ID in the webhook.

  • Consumer then calls the provider’s API to fetch complete details.

• Trade-off:

  • Smaller payloads = faster delivery, but adds one more HTTP call.

• Example:

  { "id": "evt_56f7a", "type": "payment_succeeded" }

  → Consumer calls /events/evt_56f7a to retrieve full data.

Advantages

Webhooks bring several architectural and operational benefits that make them a preferred choice for event-driven integrations. Below are the key advantages, along with why they matter in real-world system design.

1. Real-Time Event Delivery

• What: Webhooks notify consumers immediately when an event occurs, without polling.

• Why It Matters:

  • Reduces latency - consumers react instantly.

  • Improves user experience in time-sensitive workflows (e.g., chat apps, order updates, payment confirmations).

• Example: Payment gateway instantly informs an e-commerce site that a payment was successful, allowing immediate order processing.

• Contrast with Polling: Instead of hitting the API every minute to “check” for updates, the provider tells us exactly when something changes.

2. Reduced Server Load & Bandwidth Usage

• What: Since consumers don’t have to poll repeatedly, fewer API calls are made.

• Why It Matters:

  • Saves infrastructure costs on both sides.

  • Makes integration more scalable.

• Example:

  • Polling: 10,000 clients × 60 requests/hour = 600,000 requests/hour.

  • Webhooks: 10,000 clients receive only relevant events - possibly 10,000 requests/day.

• Impact: Less noise in system logs, reduced congestion, and smaller attack surface.

3. Decoupled System Architecture

• What: Provider and consumer are loosely coupled - the provider only needs to send an HTTP request, without knowing how the consumer processes it.

• Why It Matters:

  • Systems can evolve independently.

  • Easier to integrate with multiple third-party systems without redesign.

• Example: A CRM system can send the same customer update webhook to multiple downstream analytics, billing, and email systems without changing internal business logic.

4. Better User Experience

• What: Users see changes reflected immediately in the UI.

• Why It Matters:

  • Reduces perceived system lag.

  • Increases trust, especially in financial or operational systems where delays feel risky.

• Example: GitHub shows a pull request status change in seconds because the CI service sends a webhook as soon as the build finishes.

5. Event-Driven Scalability

• What: Webhooks integrate naturally into event-driven architectures where services react to discrete events instead of continuously polling for changes.

• Why It Matters:

  • Efficient scaling - resources are used only when needed.

  • Works well with message queues, serverless triggers, and streaming systems.

• Example: A warehouse system can automatically trigger a restocking workflow when a “low inventory” webhook is received.

6. Flexible Consumer Implementation

• What: Consumers can process events however they like - synchronously, asynchronously, or store them for later.

• Why It Matters:

  • Gives integration partners control over processing logic.

  • Supports lightweight triggers or heavy-duty background workflows.

• Example:

  • Fast action: Send SMS immediately on webhook receive.

  • Slow action: Save to database and process in nightly batch.

7. Cost Efficiency for SaaS Providers

• What: Providers send updates only when necessary, minimizing compute cycles and API infrastructure costs.

• Why It Matters:

  • Especially important in high-scale SaaS, where millions of customers may need updates.

  • Aligns with pay-per-request billing models in cloud platforms (AWS Lambda, API Gateway).

• Example: Slack sends webhooks for message events instead of letting apps hammer its API for new messages.

8. Easier Debugging & Auditing

• What: Since each webhook delivery is an HTTP transaction, it can be logged, traced, and replayed.

• Why It Matters:

  • Providers can offer a delivery history for developers to troubleshoot.

  • Consumers can replay failed events from logs or dead-letter queues.

• Example: Stripe’s developer dashboard shows every webhook sent, its payload, HTTP status, and retry attempts.

9. Works Across Firewalls

• What: As long as the consumer exposes a public HTTPS endpoint, webhooks can work without complex network setups.

• Why It Matters:

  • Avoids the need for VPNs or persistent socket connections.

  • Widely compatible with microservices, legacy systems, and SaaS platforms.

• Example: A corporate ERP system behind a firewall can still receive webhooks via a reverse proxy or API gateway.

Limitations

While webhooks are powerful for real-time, event-driven communication, they are not a silver bullet. They introduce reliability, security, and operational challenges that architects and developers must carefully consider.

1. Delivery Reliability Challenges

• What: Webhooks rely on outbound HTTP requests from the provider to the consumer’s endpoint. If the consumer is temporarily unavailable, events can be lost unless retry logic exists.

• Why It Matters:

  • Network blips, server downtime, or DNS issues can cause missed events.

  • Not all providers implement durable retry queues.

• Example: If a payment provider sends a “payment completed” webhook but our server is down, we may never receive it unless they retry.

• Mitigation:

  • Use idempotent event handling.

  • Implement retry with exponential backoff on provider side.

  • Use message queues (e.g., RabbitMQ, Kafka) as buffers.

2. No Guaranteed Ordering

• What: Events might arrive out of order due to retries, network delays, or multiple processing nodes.

• Why It Matters:

  • Can cause incorrect state if consumer assumes sequential delivery.

• Example: “Order shipped” might arrive before “Order packed” if the latter was delayed in transmission.

• Mitigation:

  • Include timestamps and sequence numbers in webhook payloads.

  • Consumer should reorder or validate state before applying.

3. One-Way Communication

• What: Standard webhooks are push-only; the provider does not expect a payload from the consumer beyond a success/failure status.

• Why It Matters:

  • If we need additional data, we must call the provider’s API separately.

• Example: A “user created” webhook may not include full profile details - we need to make a GET API call to fetch them.

• Mitigation:

  • Use webhook as a trigger, then fetch complete data via API.

4. Security Concerns

• What: Webhooks expose an HTTP endpoint to the internet, making it a potential attack vector.

• Why It Matters:

  • Attackers can flood endpoints with fake requests.

  • Without proper authentication, our system might accept spoofed events.

• Example: A malicious actor sends a fake “payment success” event to trick our system.

• Mitigation:

  • Use HMAC signatures, mutual TLS, or OAuth 2.0 for validation.

  • Enforce IP whitelisting where possible.

  • Implement rate limiting and payload validation.

5. Limited Error Feedback

• What: Providers typically treat webhooks as “fire-and-forget” they don’t give detailed feedback on processing results.

• Why It Matters:

  • Provider only knows if the HTTP response was 2xx or not.

  • If the consumer logic fails after returning 200 OK, the provider won’t retry.

• Example: We acknowledge receipt but later fail to store data due to DB error - event is lost.

• Mitigation:

  • Process asynchronously after acknowledgment.

  • Log failures internally for recovery.

6. Scalability & Burst Traffic Issues

• What: High-volume providers can send bursts of events that overwhelm consumers.

• Why It Matters:

  • Consumer might experience latency spikes or crashes.

• Example: A ticketing system sending thousands of “ticket sold” events during peak sale time.

• Mitigation:

  • Use a load balancer or queue buffer in front of webhook processors.

  • Implement backpressure handling.

7. Difficult Local Development & Testing

• What: Webhooks require a publicly accessible URL, which complicates local testing.

• Why It Matters:

  • Developers must tunnel traffic from provider to local machine.

• Example: We can’t easily test Stripe webhook locally without using tools like ngrok.

• Mitigation:

  • Use request tunneling tools (ngrok, localtunnel).

  • Simulate webhook payloads using provider’s testing features.

8. Dependency on External Availability

• What: Consumers rely on the provider’s uptime and correctness of event sending.

• Why It Matters:

  • If provider experiences a bug or outage, our downstream systems may go stale.

• Example: Provider silently stops sending “user updated” events due to a release bug.

• Mitigation:

  • Implement watchdog processes that detect event silence and switch to polling temporarily.

9. No Built-In Replay or History (Unless Provided)

• What: Once a webhook is missed, it’s gone unless provider supports replay.

• Why It Matters:

  • Event loss can cause incomplete state.

• Example: Payment provider only sends each event once; if missed, we must manually reconcile from transaction logs.

• Mitigation:

  • Choose providers that support event replay APIs.

  • Maintain reconciliation jobs to ensure consistency.

10. Increased Operational Complexity

• What: Managing multiple webhooks across different providers adds monitoring, retries, authentication, and versioning overhead.

• Why It Matters:

  • More moving parts = more failure points.

• Example: A SaaS app integrating with 5 different webhooks needs a centralized handler, security checks, and alerting for each.

• Mitigation:

  • Use a central webhook gateway for all integrations.

  • Standardize payload validation and error handling.

Common Technologies & Protocols Used

Webhooks are built on top of standard web technologies, but effective implementation often requires additional tools, protocols, and patterns for security, reliability, and development convenience.

1. HTTP & HTTPS

• Core Protocol:

  • Webhooks are delivered over HTTP/1.1 or HTTPS.

  • HTTPS is strongly recommended (often mandatory) to protect against data interception and tampering.

• Common HTTP Methods:

  • POST – Most common, with JSON/XML payload in the body.

  • GET – Rare; usually for basic “ping” type webhooks.

  • PUT/PATCH – Used by some APIs for idempotent updates.

• HTTP Status Codes:

  • 2xx – Success (usually 200 OK or 202 Accepted).

  • 4xx – Consumer-side issue (invalid request, unauthorized).

  • 5xx – Consumer server error (triggers retries in many systems).

2. Data Formats

• JSON – Most common due to simplicity and universal parsing support.

• XML – Older or enterprise APIs (e.g., some SOAP-compatible systems).

• Form-Encoded – Sometimes used in legacy integrations.

• Protobuf / Avro – Rare for public webhooks, but used in high-performance internal systems.

3. Security Mechanisms

Webhooks require strong authentication since they’re open endpoints.

Method	Description	Example Use Case
HMAC Signatures	Provider sends a cryptographic hash of payload + secret, consumer verifies it.	Stripe, GitHub
Basic Auth	Username/password in request headers.	Internal services
Bearer Tokens (OAuth 2.0)	Token in Authorization header.	Modern APIs
Mutual TLS (mTLS)	Both sides exchange certificates.	High-security enterprise
IP Whitelisting	Only allow known provider IPs.	Internal or static-IP providers

4. Retry & Delivery Guarantees

• Retry Policies:

  • Exponential Backoff: Delay between retries grows after each failure.

  • Fixed Interval: Retry every n seconds.

• Maximum Retry Attempts: To avoid infinite loops, providers limit attempts (e.g., 24 hours with exponential backoff).

• Dead Letter Queues: Failed deliveries are sent to a holding queue for later review.

5. Development & Testing Tools

Since webhooks require public endpoints, developers use tunneling or simulation tools:

• ngrok – Creates a public HTTPS tunnel to local machine.

• Localtunnel – Similar to ngrok, free and open source.

• Webhook.site – Public endpoint to inspect incoming webhook payloads.

• PostBin / RequestBin – Store and display incoming requests for debugging.

• Provider Simulators – Stripe CLI, GitHub webhook tester, Twilio’s webhook replay tool.

6. Message Queues & Buffers

To make webhook handling more reliable:

• RabbitMQ – Queue incoming events for async processing.

• Kafka – For high-throughput event streaming.

• AWS SQS – Managed queue to decouple provider from consumer processing.

• Redis Streams – In-memory event buffering.

7. Event Standards

While many providers have custom payloads, some follow common eventing standards:

• CloudEvents (CNCF) – Vendor-neutral event specification.

• Activity Streams – JSON format for social and activity-related data.

• JSON:API – API format that can be adapted for webhook events.

8. Logging & Monitoring Tools

For operational visibility:

• ELK Stack (Elasticsearch, Logstash, Kibana) – Centralized logging.

• Prometheus + Grafana – Metrics & alerting.

• Sentry / New Relic – Error monitoring.

9. Common Provider Implementations

Some popular services and how they deliver webhooks:

• GitHub – JSON payload + HMAC signature.

• Stripe – JSON payload, versioned events, signature verification.

• Slack – JSON payload, signed secret.

• Twilio – Form-encoded or JSON, with request validation.

• Shopify – JSON payload with HMAC verification.

10. Example Webhook Request

POST /webhook/payment HTTP/1.1
Host: api.example.com
Content-Type: application/json
X-Signature: sha256=2c5f0f0e88a9d...

{
  "event": "payment.success",
  "id": "evt_123456",
  "timestamp": "2025-08-13T08:15:30Z",
  "data": {
    "amount": 1500,
    "currency": "USD",
    "transaction_id": "txn_98765"
  }
}

• Security: HMAC in X-Signature header.

• Versioning: Often included in event name or headers.

• Timestamp: Used for ordering and replay protection.