What is API Testing?

Introduction to API Testing 

An API — or Application Programming Interface — acts like a translator between two separate systems. It enables engineers to create their own applications and leverage the functionalities of different — and usually much larger — software systems or environments. For instance, a software developer creating a mobile app for home delivery services may use the functionality of Google maps. In that case, the app ‘talks’ to Google maps via the Application Programming Interface and retrieves the data it needs to display to its users.

With the plethora of applications being churned out every single day, it is extremely important for engineers to be 100% that the APIs that they use can be relied upon to deliver accurate data 365/24/7. The only way to ensure this is through API Testing.

So, what is API Testing? Generally speaking, API Testing involves passing input parameters to the system via the API and recording the outputs. API Testing checks whether the business logic of the software system functions the way it is expected to and whether the output of the system is in order. API Testing checks the reliability, functionality, security, and performance of the interface.

Types of API Testing  

Most of the time, API Testing is a part of Integration Testing as it involves validating the operation of two different systems working together in conjunction with each other. API Testing usually is conducted at the Business Logic layer of a system. Due to the absence of a GUI for APIs, this testing has to be performed by directly ‘messaging’ the system, i.e., via the message layer, often using customized subroutines or functions.

types of api testing

So, API Testing is typically implemented at the Application Logic or Business layer of the system being tested. API Tests can help detect errors or issues such as failure to handle errors gracefully, missing or duplicate functionality, reliability/security/performance issues, incorrect handling of parameters/arguments and so on.

Some of the common types of API Testing include:

Functional Testing 

This testing is a broad-level examination of the specific functions of the program. It evaluates the responses in terms of accuracy of output, whether it lies within expected parameters, and how errors are handled. Apart from regular operational scenarios, Functional Testing of APIs also considers edge cases for boundary conditions.

Security Testing 

Since the API provides access to all external applications to access the internals of the software product, it is usually considered the most exposed or vulnerable part of the system. Hence Security Testing is critical to ensure the safety of the system, as a single vulnerability or bug could jeopardize the entire operations of an enterprise. Penetration Testing and Fuzz/Noise Testing are subsets of Security Testing. While Penetration Testing, as the name suggests, attempts to breach an application, Fuzz/Noise Testing is a kind of legitimate DOS (Denial of Service) attempt to flood the system with digital noise, e.g., massive amounts of dummy data, and thereby check whether it leads to a system crash.

Load Testing 

Load Testing validates whether the API operates under massive and/or sustained loads, e.g., by progressively increasing user requests from 1k to 10k and 100k and so on. Loads are typically baseline or regular loads, theoretical maximum loads, and overloads which exceed the theoretical maximum by 15-20%. Load Testing of APIs focuses on how these progressive loads are handled and the failure rate is measured for each load level.

Runtime Error Detection 

While most tests are related to the implementation of the API and its functionalities, the Runtime Error Detection is concerned solely with the actual operation of the API. It examines the results of utilizing the API codebase and monitors the system for execution errors, memory leakage, and tests its error handling capabilities.

User Interface Testing 

As may be expected, UI Testing evaluates the User Interface of the application. It is an indirect test of the API in the sense that it does not test the API directly but rather tests the UI that is connected with the API. It however gives testers an overview of the performance, efficiency, and usability of the system.

Validation Testing

This is usually performed at the very end of Software Development Life Cycle but it is one of the important tests to be conducted. Validation Testing examines such issues as the appropriateness of the API for the application, code bloat, and API behavior as well as accuracy, efficiency, and optimization level of the program.

Web APIs broadly fall into two established classes of web services: SOAP or Simple Object Access Protocol, REST or Representational State Transfer, and now, GraphQL, which is a far more recent development in the web services arena. Unlike SOAP, which uses XML, REST APIs are URL-based (using HTTP) and can provide output data in a variety of formats JSON, CSV, or RSS, among others.

Due to its flexibility, REST APIs are the logical choice for web services. They comprise four main access methods, viz., GET, POST, PUT, and DELETE.

REST API Testing essentially checks the correctness of the HTTP status codes, verifies response headers and payload, examines the overall performance time, and occasionally also the application state. The common scenarios that are considered for REST API Testing include basic positive tests with and without optional parameters, negative tests with valid and invalid inputs, as well as security/authorization checks.

Benefits of API Testing   

API Testing can be used for three main purposes, viz., validation of an application, maintenance/upgrade of a system, or elimination of errors and defects in software.

benefits of api testing

The primary benefit of API Testing is that it enables developers and testers alike to gain access to an application, without a User Interface or getting into the internals of the system. It is extremely beneficial when dealing with disparate systems about which specific knowledge is lacking or unavailable.

Secondly, as API Testing occurs before the UI Testing phase, and occurs at the Application Logic Layer, it enables development teams to detect errors fairly early on in the Software Development Life Cycle, thus preempting them from escalating into full-blown bugs in the software. API Tests provide an early evaluation of build strength.

Thirdly, API Testing is much faster and less time- and resource-intensive than Functional UI Testing. Generally the scale of API Testing is 25–30x times faster than the corresponding GUI Testing for the same application.

Fourth, API Testing is language-independent, and exchanges data using XML or JSON, which enables you to use any programming language for conduction of the API Tests.

Last but not the least, by using Automation Tools, API Testing can be speeded up considerably, leading to significant cost savings as well.

Tips to know for API Testing 

To enable testers to gain the maximum mileage out of their API Testing, here are some useful tips that can be followed.

  1. Understand the requirements of the API, the purpose of the API, and the workflow of the application.
  2. Specify the output of API Testing — either Data such as strings, integers, etc., Binary such yes/no, true/false etc., a call to another program/function, or an response code in the case of web services. The HTTP response codes commonly fall into 5 categories, specified by the first digit of the 3-digit code. 1 stands for informational, 2 for success, 3 for redirection, 4 for client error, and 5 for server error.
  3. Divide the API Testing into smaller steps, each one focused on a specific functionality or section of the system. The lesser the number of APIs being tested at a time, the better.
  4. Categorize APIs into groups depending on the resource type, structure, path etc. to make the tests more manageable and scalable.
  5. Leverage Test Automation to increase speed and reduce costs. Using virtualization of APIs further enhances efficiency by verifying the API even prior to actual development.
  6. Test for both Positive and Negative cases, e.g., ensure that the API works correctly both when inputs/outputs are available and also when they are not available.


API Testing is an important part of the Testing Life Cycle and ensures that all components of a system work together successfully after integration. It provides access to disparate systems without the need for a User Interface. It is performed prior to UI Testing and is much faster and cost efficient, providing greater ROI. API Testing enables developers to preemptively detect and fix errors at an early stage in the development process.

Looking to improve your API’s functionality? Take a look at Zuci’s API Testing services and see how you can leverage Zuci for your business needs.