API Monitoring: How to Monitor REST Endpoints and Services Reliably

Why Standard Website Monitoring Is Not Enough for APIs

A standard uptime monitor typically only checks whether a URL is reachable and returns an HTTP 200 status code. For web pages, this is often sufficient. For APIs, however, this approach falls short:

• Different HTTP methods: APIs use GET, POST, PUT, PATCH and DELETE. A monitor that only sends GET requests will not detect problems with POST endpoints.
• Headers and authentication: Many APIs require specific headers (API keys, bearer tokens, Content-Type). A simple HTTP check without these headers merely receives a 401 or 403 error.
• Response body validation: An API endpoint can return HTTP 200 while delivering faulty data. Only response body verification detects such semantic errors.
• Multi-step workflows: Many API interactions consist of multiple sequential requests (e.g. authentication followed by data retrieval). A single request does not capture the entire chain.
• Different status codes: Depending on the endpoint, different status codes are correct: 200 for GET, 201 for POST, 204 for DELETE. The monitor must know which code to expect.

A study by Postman shows that 72% of all developers rate API outages as critical or very critical for their business (Postman State of the API 2025). Yet only 38% use dedicated API monitoring. This gap carries significant risks.

What API Monitoring Checks

Professional API monitoring checks various dimensions of each endpoint:

Availability and Status Codes

The most fundamental check: is the endpoint reachable and does it return the expected HTTP status code? The monitor must know the correct expected status code for each endpoint. A POST /api/orders should return 201 (Created), not 200 (OK). A DELETE /api/sessions/{id} should return 204 (No Content). Unexpected status codes like 500 (Internal Server Error) or 503 (Service Unavailable) immediately trigger an alert.

Response Times and Latency

The response time of an API endpoint is a critical performance indicator. According to a study by Akamai, API latency above 100 ms leads to noticeable user experience degradation in interactive applications (Akamai). API monitoring should not only measure the current response time but also track trends. An API that responds in 50 ms today and 200 ms next week may have a growing problem, such as expanding database tables, inefficient queries or memory leaks, before users are affected.

Response Body Validation

An endpoint can be technically functional (HTTP 200) while still delivering faulty data. Response body validation checks whether the response contains the expected structure and values. Typical checks include: Does the response contain a specific JSON field? Does an array have a minimum number of elements? Does a string field contain the expected value? These checks detect semantic errors that a pure status code check misses.

SSL/TLS and Security

API endpoints should be accessible exclusively via HTTPS. API monitoring additionally checks the TLS version, SSL certificate validity and cipher suites used. An API with an outdated TLS version or weak cipher suites poses a security risk. Integration with the security scan provides a complete security picture.

Header Validation

Certain response headers are particularly relevant for APIs: Content-Type, CORS headers (Access-Control-Allow-Origin), rate limit headers (X-RateLimit-Remaining) and Cache-Control. Changes to these headers can disrupt API clients even when the response body is correct. Professional monitoring also checks these headers for expected values.

Which Endpoints You Should Monitor

Not every API endpoint has the same priority. Focus your monitoring on business-critical endpoints:

• Authentication: Login, token refresh, OAuth flows. If authentication fails, users cannot sign in. Highest priority.
• Core functionality: The endpoints that power the main product. For an online shop: product search, shopping cart, checkout, payment processing.
• Third-party integrations: Payment providers (Stripe, PayPal), shipping services, CRM connections. These endpoints are not under your control and can fail at any time.
• Webhooks: If your system receives webhooks (e.g. payment confirmations), the receiving endpoint should be monitored.
• Health checks: A /health or /status endpoint that summarises the overall state of the API service is ideal for quick overview checks.
• Database-dependent endpoints: Endpoints that perform database queries are more susceptible to performance problems than static endpoints.

Setting Up API Monitoring: Step by Step

Setting up effective API monitoring requires a systematic approach:

1. Create an endpoint inventory: Document all API endpoints with HTTP method, URL, expected headers, request body (for POST/PUT) and expected status code.
2. Set priorities: Categorise endpoints by business criticality. Critical endpoints receive shorter check intervals and stricter thresholds.
3. Configure authentication: Store API keys, bearer tokens or OAuth credentials in the monitoring tool. Ensure credentials are created specifically for monitoring purposes and do not conflict with production accounts.
4. Define check intervals: Critical endpoints (authentication, checkout) should be checked every 30 to 60 seconds. Less critical endpoints every 5 minutes.
5. Configure response validation: Define for each endpoint which response body fields and headers should be checked.
6. Configure alerting: Determine who is notified about which endpoint problems. Distinguish between outage (immediate SMS/push) and performance degradation (email).

Common API Problems and How Monitoring Detects Them

API monitoring detects a wide range of typical problems that remain invisible with pure website monitoring:

Gradually Increasing Response Times

A classic pattern: an endpoint's response time rises slowly over weeks, from 50 ms to 100 ms, then to 200 ms, finally to 500 ms. Only then do users notice the problem. With trend-based monitoring, you detect the increase early and can address the cause, typically growing database tables, inefficient queries or memory leaks, before users are affected.

Intermittent Errors

Some API problems do not occur permanently but sporadically: an endpoint delivers correct results in 95% of cases but a 500 error in 5%. Without monitoring, such intermittent errors often remain undetected for weeks or months. Regular checks at short intervals increase the probability of catching such sporadic problems.

Third-Party Outages

If your online shop uses the Stripe API for payments and Stripe has a partial outage, you need to know immediately. By monitoring your third-party integrations, you detect such outages independently of whether the third party communicates them. This enables fast response, such as activating an alternative payment provider or displaying an informative error message.

Rate Limiting and Throttling

When an API reaches a rate limit, it typically responds with HTTP 429 (Too Many Requests). API monitoring detects this status code and alerts the team. This is particularly relevant when using third-party APIs and your traffic volume threatens to exceed the agreed limits.

Multi-Step API Monitoring

Many business-critical API workflows consist of multiple sequential requests. Example: an ordering process requires (1) authentication, (2) retrieve cart, (3) create order, (4) initiate payment. Each individual endpoint may work, but the overall flow can still fail, for instance because an endpoint expects a session ID that a previous endpoint should have provided.

Multi-step monitoring (also known as API chain monitoring or synthetic transaction monitoring) automatically executes these chains and tests the overall flow. XICTRAQ supports defining such multi-step checks where response values from one step are used as parameters for the next step.

API Monitoring and Microservices

In a microservice architecture, dozens of services communicate with each other via internal APIs. This makes API monitoring even more important, as an outage of a single service can have cascading effects on many other services. According to a study by Kong, 60% of companies with microservice architectures report outages caused by API problems between services (Kong).

For microservice environments, a multi-layered monitoring approach is recommended: health check endpoints for each service, targeted checking of critical inter-service communication and end-to-end tests that simulate the entire workflow from the user's perspective. XICTRAQ offers the flexibility to model all these scenarios.

Performance Benchmarks: What Are Good API Response Times?

The definition of a good response time depends on the use case. The following guidelines serve as orientation:

• Simple data retrieval (GET): Under 100 ms. Read-only endpoints delivering cached data should be particularly fast.
• Search endpoints: Under 200 ms for simple searches, under 500 ms for complex full-text searches with filtering and sorting.
• Write operations (POST, PUT): Under 200 ms for simple writes, under 500 ms for operations affecting multiple database tables.
• Authentication: Under 200 ms. Slow login endpoints frustrate users immediately.
• Payment endpoints: Under 2 seconds. Payment processing is naturally slower, but wait times over 2 seconds increase the abandonment rate.
• Health check endpoints: Under 50 ms. A health check should be as lightweight as possible.

These guidelines should serve as starting points for your own threshold configurations. Adjust the values to your system's specific requirements and use the historical data in XICTRAQ to determine realistic baselines.

API Monitoring and Documentation

An often overlooked aspect: API monitoring provides valuable data for API documentation and communication:

• SLA documentation: Monitoring data provides proof that your API meets the promised SLA values. XICTRAQ reports are suitable as a basis for SLA reviews.
• Performance baseline: Historical monitoring data documents the typical performance of each endpoint, which is helpful for API documentation and onboarding new developers.
• Incident documentation: Every API incident is automatically logged, including timestamp, duration and affected endpoints. This facilitates post-mortems and communication with API consumers.
• Trend reports: Monthly reports show performance trends over time periods that help with capacity planning and prioritisation of optimisations.

GDPR and API Monitoring

API monitoring captures technical data such as HTTP status codes, response times and response headers. If your API delivers personal data in the response body (e.g. usernames, email addresses), be aware that the monitoring tool potentially stores this data. XICTRAQ offers the ability to selectively check response body content without storing it completely. All data is processed and stored on servers in Germany, compliant with GDPR compliance requirements.

Start API Monitoring with XICTRAQ

XICTRAQ offers comprehensive API monitoring capabilities as an integrated part of the platform. Configure HTTP method, headers, request body and expected response values for each endpoint. Use configurable intervals starting from 30 seconds, multi-step chains and integration with alerting, reporting and security scans. Register for free, try the demo and experience API monitoring tailored to your requirements. All details on plans and features can be found on the pricing page.