API Testing Tutorial: A Complete Guide to Beginners

Definition: 

API testing measures the response based on the request made to the application. During this process, the tester runs several test cases to check a few parameters like performances, integrations, functionality, security, and extent of reliability to meet the expectation. 

Before we get into the details of API testing, let's understand what an API is. It is nothing but a set of functions that perform specific procedures to interact between the client's application and server source.

For example, Zomato has integrated Google Map APIs to predict the food delivery time. When an order is placed, Zomato sends a request to Google Map API to get the estimated delivery time. 

Types of API testing: 

Unit testing: testing a set of codes or a specific unit to test a particular functionality. Usually, it is carried out with specific modules in isolation, with the slightest chance of affecting other modules.  

Functional testing: testing is carried out to ensure the functionality meets expectations, i.e., responses are according to the requests, and errors are tackled outside parameters. 

UI testing is done for the user interface, ensuring UI functionalities are in place. 

Security testing: this testing is done to check the API is protected from potential threats. A few aspects of security testing are managing user rights in applications, validating access controls of resources, and encryptions.  

Reliability testing: is done to measure the consistency of expected outputs or responses from the server. It also measures the stability of the connection between two endpoints. 

Load testing is performed to measure the results under load, i.e., the number of API calls handled. Often the testing is done by providing a specific load and measuring the behavior. 

Fuzz testing: it is carried out to analyze the behavior of the application when fuzzy data as input is fed to it. Testers push random data checks on how the API handles such scenarios. 

Steps to perform API testing 

Step 1: Set up a testing environment best suited to use cases and integrate database and server configuration. 

Step 2: Outline all possible test cases and input parameters. To perform this step efficiently, consider below factors:

i. The problem statement upon which the test revolves. 

ii. Define target users and user stories to base the testing on the user's perspective. 

iii. What are the anticipated results in the best-case scenario?

iv. Set a fallback mechanism that would come into play when the responses are beyond regularities. 

Step 3: Choose an appropriate testing method(s) – once the test cases and environment are finalized, go for the testing method. 

i. Automation testing: it reduces the manual effort to the maximum by calling the API at regular intervals. 

ii. Usability testing: it is performed to check the extent of user-friendliness of the functionalities. It also checks the validations and verifies integration with other API endpoints.  

iii. Discovery testing: is performed to keep track of API calls when manually tested are shaped or deleted based on requirements. 

iv. Security testing: it is carried out to check authentications and ensure the encryptions and security of data. 

Step 4: After the testing is done, now let's know about probable errors.

i. API requests not handling valid requests properly 

ii. Faulty or unstable connections lead to unusual responses.

iii. Improper encryption of data, results in security concerns. 

iv. Certain functionalities are not functioning as per the expected output leading to performance complications. 

v. Responses are not structured. 

Step 5: knowing probable challenges of API testing:  

i. Setting up an automated API to run the testing process. It is one of the highly labor-intensive tasks.  

ii. Reporting an error in Blackbox and measuring the coverage during testing, thereby affecting the requests & responses. 

iii. Securing the API by covering a large number of combinations of parameters. Incorrect combinations sometimes lead to exposure to attacks; It is suggested to test all possible combinations to prevent this. 

iv. Data formatting should be maintained to handle newly added parameters to the schema. 

Basic HTTP commands used in API testing: 

 1. GET: used to fetch data from the  source

 2. POST: used to direct the data to the server. 

 3. PUT: used for data creation 

4. DELETE: used to delete the current state 

API testing tool: 

API testing is currently made easy and time-saving by ready-to-use testing tools. These tools provide the capability to design frameworks based on personalized test cases. Testers get access to unlimited plugins adding to the testing capability.  

Below are a few API testing tips for beginners:

i. The initial approach should be to define the scope and document it clearly.

ii. Requests and responses to measure the success of the implementation.

iii. Before moving to test, define the variables, write down all possible test cases, and set up a testing environment. 

iv. Probable errors and respective error messages for unsuccessful execution.

v. Ensuring the security of the API against attacks. 

vi. Checking the adaptability across browsers & devices. 

Best practices for beginners to perform API testing 

i. Aim to have the entire length of test coverage by writing all possible test cases or inputs for API.

ii. Documentation should be of higher priority, and it should be easy to understand and interpret. 

iii. To get the best testing performance, use practical automation testing tools. It saves time & effort. 

iv. Maintain a higher level of reusability of test cases.

v. Reducing dependencies among parameters gives a better testing performance. 

vi. Synchronize API calls to maintain a sequence. 

vii. Initially aimed to accommodate maximum failures, it clarifies issues that sometimes remain unnoticed. 

viii. Notice the behavior of APIs, i.e., responses as per requests.