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API Monitoring: Uptime, Response Time, and Correctness

By Andy Hawkes
Reading Time 7 Minutes

What is API Monitoring?

API monitoring is the practice of continuously checking that your API endpoints are available, responsive, and returning correct data. When something goes wrong, such as an endpoint returning errors, slowing down, or sending back malformed responses, the monitoring system alerts your team.

APIs are a natural fit for monitoring. They’re built to be called programmatically, they usually speak HTTP with JSON or XML, and their responses are easy to verify with code. An API monitor can check an endpoint every minute from several locations without anyone noticing the extra traffic.

This guide focuses on synthetic API monitoring, where scripted checks call your API on a schedule. It’s one part of the broader practice of synthetic monitoring. If you want to set up API monitors without building the infrastructure yourself, Loadster’s synthetic monitoring platform runs them from 8 locations around the world.

Synthetic API Monitoring vs Passive API Monitoring

There are two complementary ways to monitor an API.

Synthetic (active) API monitoring sends its own requests to your API on a schedule and checks the results. It works whether or not real clients are calling the API, which makes it good at catching outages quickly, including outages during quiet hours. It also tests the API from the outside, so it catches DNS, TLS, load balancer, and routing problems that internal metrics might miss.

Passive API monitoring watches the real traffic hitting your API, through access logs, an API gateway, or an application performance monitoring (APM) agent. It shows error rates and latency percentiles across all your real clients and endpoints, including ones you’d never think to script.

Most teams benefit from both. Synthetic monitors are usually the better alert source for availability, since their results are consistent and they don’t depend on traffic. Passive monitoring shows how many clients were affected and helps with endpoints too numerous to script individually. The same idea applies to websites, where it’s called synthetic monitoring vs real user monitoring.

What to Monitor in an API

A useful API monitor checks more than whether the server answered.

API Uptime and Availability

The most basic check: does the endpoint respond at all, and with the expected status code? This catches complete outages, crashed services, expired TLS certificates, and DNS problems. An uptime percentage over time is usually calculated from these checks.

API Response Time

How long does each request take? Track response time on every check and set a threshold for each endpoint based on its normal behavior. A gradual creep in API response time is often an early warning of a growing database table, a missing index, or a struggling dependency, long before it turns into an outage.

When you look at response time over many checks, watch the slow outliers as well as the average. A single synthetic monitor gives you a consistent baseline; percentiles across real traffic (p95, p99) come from passive monitoring.

Response Correctness

An endpoint can return a 200 with an empty list, an error message in the body, or a stale cached value. Good API monitors validate the response body as well:

  • The body parses as valid JSON or XML.
  • Required fields are present, like an id or a non-empty items array.
  • Values are sensible, like a price greater than zero or a timestamp from today.
  • Headers like Content-Type and caching headers are what you expect.

Authentication and Multi-Step API Flows

Many API problems only show up across several calls. A multi-step API monitor might request an OAuth token, use it to create a resource, read the resource back, and then delete it. This exercises authentication, writes, reads, and cleanup in one check, much like a synthetic transaction monitor does for a web application.

Multi-step monitors need to capture values from one response (a token, an ID) and pass them into the next request. Make sure your tool supports that without awkward workarounds.

Third-Party API Dependencies

If your application relies on external APIs, such as payment, shipping, email, or identity providers, consider monitoring the specific calls you depend on. You can’t fix their outage, but you can find out about it quickly, fail over if you have a fallback, and tell your own customers what’s happening.

REST API Monitoring Best Practices

A few habits make REST API monitoring more useful and less noisy.

Monitor the endpoints clients actually use. A dedicated /health endpoint is useful, but it often checks only that the process is running. Pair it with checks against real endpoints that hit the database and downstream services.

Use dedicated credentials. Create an API key or test account just for monitoring, with the minimum permissions it needs, so you can rotate it independently and tell monitoring traffic apart in your logs.

Watch out for rate limits. A monitor running every minute from several locations adds up. Make sure monitoring traffic won’t trip your own rate limiting or a partner’s.

Avoid side effects. Prefer read-only checks where possible. If a monitor has to write data, clean it up in the same check, or write to a sandbox account that nothing else reads.

Monitor from more than one location. Running checks from several regions helps tell a global outage apart from a regional network or CDN problem.

Alert on what matters. Requiring two consecutive failures before alerting can cut down on noise from brief network hiccups, while still catching real outages within a few minutes.

API Monitoring Tools

API monitoring tools range from simple uptime pingers that check a status code, to API testing tools with scheduled collections, to full synthetic monitoring platforms and observability suites.

When comparing API monitoring tools, consider whether they support multi-step checks with captured values, how flexibly you can validate response bodies, how many locations they run from, how alerts reach your team, whether you can monitor private APIs behind a firewall, and how pricing scales as you add more endpoints or run checks more often.

It’s also worth considering whether the same scripts can be used for API load testing. An API that passes every monitoring check at one request a minute might still fall over at a thousand requests per second.

API Monitoring with Loadster

Loadster runs API monitors with Protocol Bots, which make HTTP requests directly without the overhead of a browser. A monitoring script can be as simple as a single GET or as involved as a multi-step flow with authentication, captured tokens, and cleanup. Validation rules check response times, sizes, and content, including JavaScript validators that parse JSON or XML responses. Capturing rules carry tokens and IDs from one request to the next. If your API has an OpenAPI or Swagger spec, you can import it to build the script faster.

Monitors run from 8 dedicated locations across 5 continents, or from a self-hosted Loadster Engine if your API isn’t publicly reachable. When a check fails, Loadster opens an incident and alerts your team by email, SMS, phone call, Slack, PagerDuty, Opsgenie, or webhooks.

The same Protocol Bot script can also drive an API load test with thousands of concurrent bots, so you can find out how your API holds up under heavy traffic before your clients do. Our API performance testing guide covers that side.

To get started, visit Synthetic Monitoring or read the broader synthetic monitoring guide. A free account comes with 50 units of Loadster Fuel, enough to try a few API monitors with no credit card required.

Frequently Asked Questions

What is API monitoring?
API monitoring is the practice of checking your API endpoints on a schedule, or watching their production traffic, to make sure they’re available, fast, and returning correct responses. When a check fails or response times exceed a threshold, the monitoring tool alerts your team so you can fix the problem before API consumers are affected.
What is synthetic API monitoring?
Synthetic API monitoring sends scripted requests to your API from outside your infrastructure on a schedule, such as every minute, and verifies the responses. Because it generates its own traffic, it works even when no real clients are calling the API, and it tests the same requests the same way every time.
What should an API monitor check besides the status code?
A 200 status code only proves the server answered. Good API monitors also check response time, that the body is valid JSON or XML, that key fields are present with sensible values, and that headers like Content-Type are correct. For authenticated APIs, they should also verify that login or token exchange works.
What is a good API response time?
It depends on the endpoint. Simple reads from a cache or index often return in well under 100 milliseconds, while complex queries or writes may take several hundred. Rather than a universal number, set thresholds based on each endpoint’s normal response time with some headroom, and watch percentiles rather than just averages.
How often should I monitor my API?
Every minute is fairly common for critical public endpoints, and every five minutes is often enough for less critical ones. Multi-step API flows that exercise several endpoints can run a bit less often. The schedule should match how quickly you need to know about an outage.

Next Steps: Ready to monitor your API? Set up a Protocol Bot monitor with Synthetic Monitoring, then reuse the script for API Load Testing.