How to Do API Performance Testing

APIs must be functionally correct. They should also be available, secure, reliable, and fast. API testing is crucial for developers. It allows them to make promises to customers and keep them happy, but it also collects data to help improve their development. API testing is complex, particularly when it comes down to performance. It can be easy to get lost in all the terminology.

This article will help you understand the terminology and situations involved in API performance testing. Let’s get started.

What is API Performance Testing?

Performance tests are also a common term for API tests. They measure the system’s overall performance under certain circumstances. API performance tests can generally be divided into functional and load tests.

API Performance Types

API Functional Tests

Functional API testing is a method to verify that an API returns the expected output for a given input. These tests can be run on any environment: from the developer’s computer to staging environments to production systems. To ensure that the deployment does not cause any problems, it is recommended that they be run. If the deployment is successful, all tests should return the exact same results.

API Load Tests

Load API testing, on the contrary, is typically performed in production or an identical system. Non-functional constraints like reliability and responsiveness can look different under real-world conditions.

load test shows how APIs and clients form client-server systems. Multiple clients may be hitting the same server simultaneously, which can dramatically affect the API’s behavior. Virtual users (often abbreviated as VU) simulate clients and are used in load tests.

Multithreading can make it possible for a single machine to simulate multiple VUs. Cloud-based environments allow you to run multiple machines simultaneously to simulate a higher number of VUs and test more load.

BlazeMeter supports 50 VUs with the free plan, and up to tens of million with paid plans. There are thousands of machines that can generate load for your API server.

API performance testing is similar to website testing. However, website tests can include client-side behavior while API tests send only network requests.

Functional tests can be reused in load testing scripts. They can also be executed multiple times simultaneously. Load tests offer a wider range of KPIs that functional tests, but they also provide more information. Functional testers don’t usually care about response time as long as there isn’t a timeout. However, load testers record them each run to provide statistics like average, mean and maximum response times that API providers may wish to claim as part of their Service Level Agreement (SLA).

Planning Load Test Scenarios

Proper planning is essential for load tests. You need both a test program and a scenario to run it. It is important to ask yourself what your testing aims to achieve. Then, choose the right approach and parameters. You should also consider your API use cases and the likely load and changes in API request traffic that you will experience.

BlazeMeter allows you to create your test scenario using an open-source testing tool such as Apache JMeter(tm), Gatling and then configure the test attributes on the Load configuration screen.

Load tests

Load Testing is a standard performance test. The number of VUs, test script and time you want to run the test are all specified. All VUs that hit your API continuously for the specified time period are recorded by the testing tool. You can then compare the performance metrics to the SLA.

Stress tests

Stress testing starts with a low-to-medium number of VUs, and then gradually increases their count. This is known as ramping up. This testing method aims to increase the load continuously until the API is either too slow or non-responsive.

BlazeMeter’s load configuration allows you to select how many users you require, and set a long ramp up time so that you can monitor the system as it loads. The ramp-up begins with one user and increases linearly until it reaches the desired number.

Soak tests

Soak Testing is a method of testing a system to see if it performs as expected under certain loads and then suddenly stops responding without traffic changes. Leakage, not cleaning up logs, or running out of server memory and disk space are all reasons why servers can fail to function properly. It is common. You can take the soak test to discover these mistakes before they occur.

A soak test is a load-test that uses a large but not critical number of VUs over a prolonged period, from several hours up to several days.

Peak and Spike Testing

Peak Testing simulates your API during peak times. These are times when you can expect to experience a greater load. Peak tests are typically shorter than soak tests and involve increasing and decreasing pressure, but not as much as in a stress test. These tests can help you test the scalability and reliability of your API, particularly if it is run on serverless or autoscaling infrastructure.

Spike Testing allows you to test the system’s ability to handle sudden increases in load. BlazeMeter allows you to create peak tests by selecting a large number of users with short ramp-up times.

Endpoint vs. Scenario Testing

You can either call one URL or several URLs at once when performing performance testing. It can help to create multiple scenarios that simulate different API calls.

BlazeMeter allows you to set up functional endpoint tests. Click the URL/API Testing button below Create Test. Enter your API operation endpoints. BlazeMeter can be used to create scenario tests using open source JMeter and open source Taurus. The script will then be uploaded to BlazeMeter. is a feature that allows you to use a Swagger/OpenAPI file to build a test.

Other non-functional API tests

This post has been focused on API performance testing. However, I wanted to provide a wider overview of API testing.

One is User Acceptance testing (UAT), which involves actual users testing the API. This type of test is important because even an API with the best performance won’t solve the problem in the way that the customer expects.

UAT tests are usually manual tests. APIs are typically executed in clients such as Postman, or integrated in real-world applications. Some tools and approaches like Behavior-driven-development can be used for automation.

Security is another important non-functional restriction of an API. API security testing is essential, especially when APIs can pose a threat to personal privacy. API security testing can be included in your functional and performance tests. This involves adding API requests that have invalid inputs, missing authentication details, or to non-existent endpoints. The API must ensure proper error handling and does not break.

Beyond API Performance

Let’s not forget API Monitoring. This is closely related to testing. API monitoring usually includes simple uptime checks, as well as performance and functional tests. To build your monitoring system, you can use existing test definitions. Monitoring and testing are different. Testing is part of the development build and deploy process. Monitoring is done at regular intervals to spot potential problems in the system. Regular monitoring and analytics will provide you with KPIs that can be used to optimize your API.

BlazeMeter offers advanced KPIs that can be measured in real time. It also stores results over time, allowing for comparisons. BlazeMeter integrates well with APM tools so testers can drill down to the results and share reports.