Webhooks power modern integrations by enabling event-driven communication between systems. Instead of constantly polling an API asking "is my task done yet?" webhooks let the API notify you automatically when events occur. This makes integrations more efficient, responsive, and scalable.
However, implementing webhooks correctly requires understanding several important patterns and practices. In this guide, we'll cover everything you need to know to build robust webhook integrations that handle real-world conditions reliably.
What Are Webhooks
Webhooks are HTTP callbacks that deliver event notifications from one system to another. When something interesting happens in system A (like a document finishes processing), system A sends an HTTP POST request to a URL you provided. Your system receives this request, processes the event, and responds.
This pattern inverts the normal API flow. Instead of your code calling an API to get information, the API calls your code to deliver information. This is why webhooks are sometimes called reverse APIs or push notifications.
The main advantage of webhooks over polling is efficiency. Polling means making repeated API requests checking for changes. Most requests return "nothing new yet" and waste resources. Webhooks eliminate this waste by notifying you only when something actually happens.
Webhooks also enable real-time responses. With polling, there's always a delay between when an event occurs and when you discover it (the polling interval). Webhooks notify you within seconds of the event, enabling immediate action.
Common Webhook Use Cases
Document processing workflows benefit greatly from webhooks. When you submit a document for OCR or data extraction, processing might take several seconds or minutes. Rather than poll for completion, provide a webhook URL. When processing finishes, the API posts results to your webhook and your system proceeds immediately with the next steps.
The Scan Documents API uses webhooks for both file events (when files are created or deleted) and task events (when tasks are created, completed, fail, or are deleted). This lets you build responsive workflows that react instantly to processing completion.
Integration platforms like Zapier rely on webhooks extensively. When an event happens in one app, webhooks notify Zapier, which then triggers actions in other apps. This enables sophisticated cross-platform workflows without constant polling.
Notification systems use webhooks to deliver alerts. When something requires attention, a webhook notifies your system, which can send emails, SMS messages, push notifications, or updates to dashboards.
Setting Up Webhook Endpoints
Your webhook endpoint is the URL where you receive event notifications. It needs to be publicly accessible via HTTPS so the webhook sender can reach it.
The endpoint should accept POST requests with JSON bodies. Most webhooks deliver event data as JSON in the request body. Your endpoint extracts this data, processes it, and responds appropriately.
URL structure should be clear and versioned. A path like /webhooks/v1/scandocuments makes it obvious what the endpoint handles and allows future changes without breaking existing integrations. Some systems include security tokens in the URL itself, though headers are generally better for credentials.
Development and testing requires a publicly accessible endpoint even during development. Services like ngrok create temporary public URLs that tunnel to your local development machine. This lets you test webhook integrations without deploying to production servers.
Local testing tools can simulate webhook deliveries for development without needing real API calls. Store sample webhook payloads and replay them against your local endpoint to test handling logic.
Securing Webhook Endpoints
Security is critical because webhook endpoints are public URLs. Anyone who knows the URL could potentially send malicious requests. Several techniques protect against unauthorized webhook deliveries.
URL secrecy provides minimal security. If the webhook URL contains an unpredictable token (like /webhooks/abc123def456), attackers can't guess it easily. However, URLs often get logged in various systems, so this shouldn't be your only security measure.
Signature verification is the standard security approach. The webhook sender includes a signature header computed from the request body using a secret key. Your endpoint recomputes the signature using the same secret and compares. If they match, the request is authentic. If not, it's rejected as unauthorized.
The Scan Documents API implements signature verification using HMAC. Each webhook includes a signature header that you verify against your secret key. This ensures requests actually came from the API and weren't tampered with.
IP allowlisting restricts webhook delivery to known source IPs. If the API documents its webhook sender IPs, configure your firewall to only accept webhooks from those addresses. This adds an extra security layer but can be brittle if IPs change.
HTTPS encryption is mandatory for webhooks. Never use HTTP for webhook endpoints because request bodies contain sensitive data. HTTPS ensures data is encrypted in transit and prevents tampering.
Handling Webhook Requests
Your webhook endpoint code needs to follow several important patterns for reliability and correctness.
Respond quickly with a success status code (usually 200 or 204) before doing time-consuming processing. The webhook sender has timeouts. If your endpoint takes too long to respond, the sender might consider it failed and retry. Quick responses prevent this.
Queue the work rather than processing inline. When a webhook arrives, extract the event data, store it in a job queue, respond with success, and then process the queued job asynchronously. This keeps your endpoint fast and allows the webhook sender to move on immediately.
Validate the request thoroughly. Check the signature to ensure authenticity. Validate that the event type is expected. Verify the event data structure matches your expectations. Reject invalid requests with appropriate error codes.
Handle duplicate events idempotently. Network issues might cause the same webhook to be delivered more than once. Your processing should be idempotent, meaning processing the same event multiple times has the same effect as processing it once. Use event IDs to detect and skip duplicates.
Idempotency and Duplicate Handling
Webhooks can be delivered more than once due to retries, network issues, or bugs. Your system must handle this gracefully.
Event IDs uniquely identify each event. When processing a webhook, check if you've already processed this event ID. If yes, skip processing and respond with success. If no, process the event and record the ID as processed.
Store processed event IDs in a database or cache. A simple table with event ID and processed timestamp works well. Check this table before processing each webhook. Clean up old records periodically to prevent unbounded growth.
Idempotent operations naturally handle duplicates. Some operations are inherently idempotent. Creating a resource with a specific ID is idempotent (subsequent attempts either succeed or report that it already exists). Updating a resource to specific values is idempotent.
Non-idempotent operations like incrementing counters or sending notifications need special handling. Use the processed event IDs approach to ensure these operations happen exactly once.
Retry Logic and Failure Handling
Webhook senders typically retry failed deliveries. Understanding retry behavior helps you design reliable endpoints.
Temporary failures happen due to network issues, server restarts, or brief outages. Webhook senders retry failed deliveries with exponential backoff (waiting longer between each retry). If your endpoint was temporarily down, retries will eventually succeed once it's back up.
Respond with appropriate HTTP status codes to control retry behavior. 2xx codes indicate success and no retry is needed. 4xx codes (except 429) indicate permanent failures (like invalid data) where retries won't help. 5xx codes and 429 indicate temporary failures that should be retried.
Configure retry limits and delays according to your needs. Some systems let you control how many retries happen and how long to wait between them. Balance between giving your system time to recover and not delaying event processing excessively.
Dead letter queues capture events that fail repeatedly. After maximum retries, events go to a dead letter queue for manual investigation. This prevents losing events but flags them as needing attention.
Monitor webhook failures closely. Set up alerts when webhook delivery failure rates exceed thresholds. This helps you detect and fix endpoint problems quickly before too many events pile up in retry queues.
Testing Webhooks
Testing webhook integrations requires different approaches than testing normal API calls because the API calls you.
Manual testing with tools like curl or Postman involves crafting sample webhook payloads and POSTing them to your endpoint. This verifies your endpoint handles expected payloads correctly.
Webhook testing services provide tools for capturing and inspecting webhook deliveries. RequestBin and similar services give you temporary URLs that capture and display webhook requests. Send test events to these URLs to see exactly what the API sends.
Test accounts and sandbox environments let you trigger real events without affecting production data. Most APIs with webhooks offer test modes. Use these to generate authentic webhook deliveries during development.
Integration tests should cover your complete webhook handling flow. Trigger events in the API, verify your endpoint receives webhooks, check that processing happens correctly, and confirm your system's state updated appropriately.
Edge case testing matters because webhooks face various failure modes. Test how your endpoint handles malformed payloads, missing fields, invalid signatures, duplicate deliveries, and extremely large payloads.
Monitoring and Observability
Production webhook endpoints need comprehensive monitoring to ensure reliability.
Log every webhook received with key details like event ID, event type, timestamp, and processing status. These logs help debug issues and understand system behavior.
Metrics track webhook volumes and health. Monitor webhooks received per minute, processing success rate, average processing time, and queue depths if using async processing. Graph these metrics to spot trends and anomalies.
Alerting notifies you of problems. Alert on high failure rates (more than a small percentage of webhooks failing), processing delays (webhook queue depth growing), missing expected webhooks (you submitted tasks but never received completion webhooks), and signature verification failures (might indicate attacks or misconfiguration).
Distributed tracing connects webhook events to downstream processing. When a webhook triggers a complex workflow across multiple services, tracing shows the complete flow and helps identify bottlenecks or failures.
Webhook Payload Structure
Understanding webhook payload structure helps you process events correctly.
Event metadata typically includes an event ID (unique identifier for this specific event), event type (what happened, like task completed or file created), timestamp (when the event occurred), and potentially a resource ID (identifier of the resource the event relates to).
Resource data provides details about what changed. For a task completed event, this includes the task ID, status, results, and any error information. For a file created event, it includes file ID, filename, size, and content type.
The Scan Documents API webhook payloads follow a consistent structure. Each event has a type field indicating the event type. The data field contains event-specific information. File events include file details, and task events include task details and results.
Versioning prevents breaking changes. As APIs evolve, webhook payload structures might change. Version information in payloads (or separate webhook URLs for different versions) lets you handle changes gracefully.
Multiple Webhook Endpoints
Complex applications might need multiple webhook endpoints for different purposes or services.
Event filtering lets you subscribe to specific events. Rather than receiving all events and filtering in your code, configure which events get sent to each endpoint. This reduces noise and makes endpoints simpler.
Service separation routes events to appropriate services. If you have separate services for different workflows, each can have its own webhook endpoint. Task completion events go to the workflow service, file events go to the storage service, and so on.
The Scan Documents API lets you specify a callback URL when creating tasks. This provides event-specific webhooks rather than account-wide configuration. Different workflows can use different webhook endpoints naturally.
Webhook Registrations and Management
Some APIs require registering webhook endpoints before events are delivered.
Registration stores your webhook URL and configuration in the API. You provide the URL, select which events to receive, and potentially provide security credentials. The API then delivers matching events to your endpoint.
Multiple endpoints can be registered for redundancy or routing different events to different destinations.
Dynamic webhook URLs support per-user or per-resource webhooks. SaaS applications where each customer needs separate webhooks can register different URLs for different customers or resources.
The Scan Documents API uses callback URLs in task creation rather than global webhook registration. This provides flexibility (different tasks can use different callbacks) without requiring separate registration management.
Common Pitfalls and Solutions
Several common mistakes cause webhook integration problems.
Slow processing in the webhook handler causes timeouts and retries. Solution: respond immediately and queue the work for asynchronous processing.
Not handling duplicates idempotently causes double-processing. Solution: track event IDs and skip events you've already processed.
Forgetting to verify signatures exposes security vulnerabilities. Solution: always verify webhook signatures before processing.
Not handling schema changes breaks integrations when APIs evolve. Solution: use defensive parsing that handles missing or extra fields gracefully.
Insufficient error handling causes event loss. Solution: implement comprehensive error handling, logging, and dead letter queues.
Conclusion
Webhooks enable efficient, real-time integrations but require careful implementation. Follow these best practices for reliable webhook handling: respond quickly and process asynchronously, verify signatures for security, handle duplicates idempotently, implement proper retry logic, test thoroughly including edge cases, and monitor webhook health in production.
The Scan Documents API provides webhook support for file and task events, enabling responsive document processing workflows. Use callback URLs when creating tasks to receive immediate notification when processing completes.
Start simple with a basic webhook endpoint, add security verification, implement async processing, and build up monitoring and error handling. With proper implementation, webhooks will power reliable integrations that scale efficiently and respond instantly to events.