API is an Interface for Application Programming, or to put it as per the abbreviation, it is 'Application Programming Interface'.

We usually use an API as a means of communication to promote the connection between two different programs. We refer to APIs as "Web Services" whenever we use them for web networks.

API testing is becoming the software spine. Writing APIs to connect with a database or with another module has become a popular practice in an application.

API automation can speed up testing and improve performance as part of integration testing. And that is why we need to test the APIs for optimum test coverage as a tester.

Types of API tests
Via different perspectives, Software developers conduct API tests. They evaluate whether it delivers valid responses in the intended format, whether it provides answers within an expected period, and how well it correlates with software for the presentation layer.

Testers test an API's reactions to edge situations and possible security attacks (e.g. crashes, unintended or extreme inputs). So let's explore each of these elements.

1. Functional API Tests
The testing of a specific function within the code base is functional testing. It ensures that the API is running within the intended parameters, which means:

Returns the output desired for a given input;

Inspects errors when the outcomes are beyond the intended criteria.

Positive / Negative Tests
It is one of the types of functional testing. A negative test verifies how an API reacts to any conceivable incorrect input. A positive test shows that the API works correctly when the information is according to the norm.

It's a bit concerning if positive test cases crash since it means that the programme will not even function under optimal conditions.

2. API Performance Tests
To verify the API can manage the anticipated or stronger load, QA engineers check its capability and usability by artificially generating or simulating API calls.

We will outline the kinds of performance checking for the API.

Load Tests
The purpose of load testing is to calculate where, under high load, the limit of device output lies. That's why, when raising the number of calls, we calculate response times, latency, server conditions, etc.

Soak Tests
Load tests that run for an extended period can see device instabilities such as API memory leaks. So, keep the automated soak tests going when you have a weekend ahead. It will notify you on Monday if any inappropriate conduct has occurred.

Stress Tests
To identify the stage at which the API throws errors, loses momentum or stops answering, the idea is to gradually increase the number of virtual users.

Spike Tests
Parallel to stress testing, an API is subject to an unexpected user spike. Spike testing tests if the API will recover after that and resume normal functioning.

Scalability Tests
We conduct Scalability tests when we want to ensure that the efficiency of your system scales according to the load that evolves, we increase the amount of client data and track whether this triggers the response time to increase proportionately or not.

Peak Tests
Peak Tests Here, you expose your API to the heavier load while shortening the strike's time, close to soak testing.

3. Security API Tests
The security assessment process aims to check an API for vulnerabilities from external threats: security, penetration, and fuzz testing.

Security Tests
This type of test validates if the API test fulfils the safety criteria. It involves security, control to user access, and authorization.

It deals with certain questions like what it requires for authentication to use the API, how it encrypts the confidential data, how resource access is set for authorization, etc.

Penetration Tests
Moving security testing a bit further, some API features, services, procedures, or the whole API are under attack from the outside in penetration testing. It evaluates whether it is possible to hit the risk vector or not.

Fuzz Tests
Testers check The API at its extreme limits by the last step in the security audit. It checks whether the API can withstand it or end up with negative actions like a forced crash or overflow by forcefully entering large quantities of random data.

To help you understand the concept, we have discussed all the prominent API tests that even a beginner should know.