API testing guide and tips for beginners (SOAP and REST) – in plain language

The API test, which is an abbreviation for the “Application Programming Interface”, is a type of software test in which the verification is done directly at the API level. This test is part of the integrity test by which the APIs meet the testing expectations of the functions, sustainability, performance and security. Unlike UI testing, the API test is conducted at the messenger level without a graphics interface (GUI).

In general, there are two categories of websites for the web API called SOAP and REST. SOAP, abbreviated for the term “Simple Object Access Protocol”, is a standard protocol used by W3C standards to send and receive web services requests and responses. Rest, which is an acronym for the term “Republic State Transfer”, is a web -based architecture for HTTP. But unlike SOAP-based websites, no formal standards have been provided for Restful Web APIs.

In this article we will discuss 10 tips that you need to know when testing API.

1. Understand the API requirements

Before testing APIs – you need to answer the following questions to fully understand the API requirements:

What is the API?

Knowing the purpose of the API makes it a strong foundation and you can prepare your test data on this basis for the input and output of the API. At this point you can define your approval approach. For example, in some APIs, the answers are confirmed by a database, and in others the answers should be approved based on other APIs.

What is the app’s workflow and where is the API?

In general, the APIs of an app are used to manipulate its resources. These APIs – are used to read, create, and update, and so knowing the purpose of the API allows you to properly design the input and output steps of the API. For example, the API output called “Create User” may be another API input called “Get User” for verification. The API output called “Get User” can also be used as an API input called “Update User” and so far.

API testing guide and tips for beginners (SOAP and REST) – in plain language

2. Determining API output states

The most common API output to verify when testing is the response status code.

Confirmation by checking the response code 200 to conclude that the test was successful and otherwise failed is considered an amateur work. Although this method is not incorrect; But it cannot show all API testing scenarios.

All status codes of API responses in a global standard can be divided into five categories. The first digit of the status code determines the response category. The last two digits have no role in the category or class of the code.

There are five values ​​for the prime digit:

  • 1xx (Notify): Request received and being processed.
  • 2xx (Success): The request has been successfully received, understood and accepted.
  • 3xx (Redirect): We need to do something else to complete the request.
  • 4xx (Client Error): The request contains an incorrect structure or cannot meet some requirements.
  • 5xx (Server Error): The server cannot respond to an apparently valid request.

However, the actual response status codes of an API are determined by the development team building it. A tester should therefore verify the following:

  • The code follows universal standard categories.
  • or code referenced in the API requirements.

3. Focus on small functional APIs

In a test project, there are always simple APIs such as the Login API (in the term Login API), token, health review, etc. with only one or two inputs. However, these APIs are essential and are regarded as a gate to enter other APIs. Focusing on these APIs before other APIs makes it sure that it works properly on APIs, the environment and authentication.

In each case of the test, you should avoid testing more than one API, because if there is an error, it will be difficult to discipline the circulation leading to consecutive APIs. So you should keep the API test steps as simple as possible. There are some things in which we need to test a series of APIs to get a scrubber. However, these tasks must be performed after all APIs are tested individually.

4. Organize the API end points

A test project can have a few or hundreds of APIs for testing. In any case, all of these APIs must be organized in categories in order to manage them correctly. This will add another step to the test steps; But it greatly contributes to the coating and coherence test scenarios.

The APIs within one category have some common information such as source type, pathway, etc. Organizing tests with the same structures makes your tests reusable and can be expanded depending on the integration circulation.

5. Using the automation feature to test the API

You should use automation capacity for API testing as much as possible. In the following, we have listed some of the advantages of automatically creating API tests:

  • Test data and execution history can be stored along with API endpoints. This makes it easier to re-run the tests later.
  • API tests are stable and change carefully. An API represents a business rule for a system. Any change in the API requires certain requirements. Hence, testers can always stay alert about any kind of changes in the API and adjust them in time.
  • Test execution becomes much faster compared to web UI testing.
  • API testing is typically a black-box process where users send input and get an output for verification. Automation with a data-driven approach, i.e. using different data sets in a single test scenario, can help increase API test coverage.
  • Data input and output follow specific formats and models, so that test scripts can be created only once and these test scripts can be reused many times throughout the test project.
  • API tests can be implemented early in the software development cycle. An automation approach by simulating the techniques used can be used to help validate the API and integrate it before the actual API is developed.
  • Therefore, the level of dependence within the team decreases.

6. Choosing the right tools for automation

Another step to take advantage of the automation capacity for API testing is to choose the most suitable tool or a set of the most suitable tools among the hundreds of options available in the market. Here are some criteria to keep in mind when choosing an automated API testing tool:

Does the tool support testing the types of API/Web services used by your AUT (application under test) or not? If the selected tool supports testing RESTful services, while your test application uses SOAP services, using this tool will be pointless.

Does the selected tool support the authentication methods required by the application service under your test? Here are some of the authentication methods that the API can use:

  • No Auth
  • Bearer Token
  • Basic auth
  • Digest Auth
  • NTLM Authentication
  • OAuth 1.0
  • OAuth 2.0
  • Hawk Authentication
  • AWS Signature

This is a necessary discussion, because without authentication you cannot start testing an API.

  • Does your tool support importing API/Web service endpoints with WSDL, Swagger, WADL and other service specifications? This is an optional feature, but it’s a great time-saver if you want to test hundreds of APIs.
  • Does the selected tool support data-driven methods? This feature is also optional; But the test coverage is greatly increased if the tool is supported.
  • As a last point, which is not a little important, you should see if you need to run other types of tests, such as WebUI or data sources, in addition to API testing. API testing takes place in the business layer between the data sources and the UI. Usually all these layers are tested together. Hence, a tool that can perform all these types of tests would be very convenient, as test objects and test scripts can be shared between all layers.

7. Choosing the right methods for verification

While the response status code specifies the current status of the response; But the textual content of the response is what the API returns based on user input. The response content of an API is quite variable in type and size. Responses can be plain text, JSON data structures, SML, documents, or more. The response can be a few simple words (or even blank) or a JSON or XML file with hundreds of pages of content. Hence, it is very important to choose the appropriate verification method for any given API. In general, there are some basic methods for validating the textual content of an API response:

Compare the entire textual content of the response with the expected information

This method is suitable for a simple response with static content. Dynamic information such as time-date, incremental ID, etc. cause problems in this case.

Compare each of the attribute values ​​in the answer

For those responses that are provided in JSON or XML format, it’s easy to get the value of an assumed key or attribute. Hence, this method is useful when we want to validate dynamic content or a single value instead of the entire content.

Comparison of matching with regular expressions (Regular Expression)

This method is used in conjunction with the verification of individual feature values ​​to verify data responses with a specific pattern to handle complex dynamic data.

Each verification method has its own advantages and disadvantages, and there is no one-size-fits-all option. You should choose the solution that best suits your test project.

8. Creating positive and negative tests

API testing requires positive and negative tests to ensure that the API is working correctly. Since API testing is considered a type of black box testing, both types of tests are based on input and output data. There are some general suggestions for generating test scenarios:

positive test

  • Verify that the API takes input and returns the expected output as specified in the requirements.
  • Verify that the status code is returned as specified in the API requirements, including returning 2xx success codes or error codes.
  • The entry with the least required fields and the most required fields should be specified.

negative test

Verify that the API returns appropriate responses when the expected output is missing.
Run the input verification test.
Verify that API behaviors are consistent across different levels of authentication.

9. Live testing process

It is highly recommended to schedule API test execution on a daily basis while the test process is live. Because API test execution is fast and small enough, it will be easy to add more tests to the current testing process with minimal risk. This mode will only be available for automated API testing tools that have the following features:

  • Test scheduling with built-in test commands
  • Integration with test management tools and defect tracking tools
  • Continuous integration with various CI tools
  • Create visual log reports

Once the testing process is complete, you can receive the results of these tests on a daily basis. If a test fails, you can review the outputs and find appropriate solutions to the problems.

10. Don’t underestimate automated API testing

API testing workflow is very simple and can be summarized in three main steps:

  1. Submit request with required input data
  2. Receive responses that contain output data.
  3. Verifying that the returned response matches the expected requirements.

The most difficult parts of API testing aren’t the parts of sending requests or receiving responses; Rather, data management sections are testing and verification. Usually, it is quite simple to test some basic APIs such as login, resource query, etc.; But as we move forward and reach more complex APIs, this task becomes increasingly difficult. Hence the task of API testing can easily be underestimated. Thus, there may come a time when you find yourself in the position of choosing the right approach to test data and validation methodology. The reason for this is that the returned data has similar structures; But this does not apply to a test project. Deciding whether to validate JSON/XML data key-by-key or use object mapping to leverage the power of the programming language can be difficult.

Using automated API testing is highly recommended for a real project under development. This process should be organized in such a way that it is extensible, reusable and maintainable.

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