An Application Programming Interface (API) at its core is a formal specification that acts as a guaranteed contract between two separate pieces of software
Modern computer systems are generally designed using the ‘layered architecture approach’:
This means that the core functionality of the system is contained within the “business logic” layer as a series of discrete but connected business components. They are responsible for taking information from the various user interfaces (UIs), performing calculations and transactions on the database layer and then presenting the results back to the user interface.
However, in addition to communicating with human beings via the UI layer, computers systems have to be able to communicate directly with each other. For example, your mobile ride sharing application will need to communicate with the mapping service, the traffic and weather services and other specialized applications used by the drivers providing the rides. In the modern, interconnected world, we take for granted that all these different systems can speak to each other seamlessly, in reality that would not be possible without APIs.
As mentioned above, an API at its core is a formal specification that acts as a guaranteed contract between two separate pieces of software. The API provider defines the set of operations, data formats and protocols that it expects, and the consumer of the API (called the client) will use those rules on the understanding that, as long as it follows the rules, the client will always be able to use the API without having to worry about the internals of the API itself:
The importance of APIs is that it lets different organizations create software applications that rely on other application and services without having to constantly update their application when the internals of the dependent applications or services change. As long as the API itself remains stable, the internal implementation can change. This is an important feature of APIs, they consist of a part that doesn’t change.
Important features of an API consist of:
So, what happens when you want to change an API and expose new functionality? You basically have two choices:
So now that we have established what an API is and why APIs are critical to modern interconnected, globally distributed applications and services, it is important to understand why API testing is critical.
API testing involves testing the application programming interfaces (APIs) directly and as part of integration testing to determine if they meet expectations for functionality, reliability, performance, and security.
The move to cloud computing has highlighted the importance of Application Programming Interfaces (APIs). With the rise in cloud applications and interconnect platforms, API testing is a necessity. Many of the services that we use every day rely on hundreds of different interconnected APIs, if any one of them fails then the service will not work!
API testing is a testing method of verifying that the web API responds to requests from the client applications in the correct way. GUI testing is a method of verifying that the user interface behaves as expected.
Since APIs lack a GUI, API testing is performed at the message layer. API testing is critical for automating testing because APIs now serve as the primary interface to application logic and because GUI tests are difficult to maintain with the short release cycles and frequent changes commonly used with Agile software development and DevOps.
When you release a new version of the system (e.g. changing some of the business components or internal data structures) you need to have a fast, easy to run set of API regression tests that verify that those internal changes did not break the API interfaces and therefore the client applications that rely on the APIs will continue to function as before.
API tests require less scripting efforts as compared to GUI tests. GUI test time is spent on loading and rendering web pages and UI elements that might not even be needed in the end, which can slow down the testing process. API testing reduces all these tasks to one and as a result, the testing process is faster with better coverage.
API testing can provide faster results with early defect diagnosis, which reduces overall testing costs.
API testing allows access to the application without a user interface or users interacting with the system. API tests can run without needing to experience the software application which gives an early insight into defects and errors. This allows for early issue resolution before the GUI is impacted.
API testing allows for highly integrable tests. This is especially beneficial if you plan to perform functional GUI tests following your API testing.
Testing the code-level functionality of an application provides an early evaluation of its overall build strength. This exposes small errors before they become larger problems during GUI testing. Core access enables testing in tandem with development, fostering communication and improved collaboration between both teams.
An API test exchanges data using XML or JSON. These transfer modes are completely language-independent, meaning that you can select any core programming language.
Functional tests validate the software system against functional specifications and requirements. To test each function of the API, a tester provides appropriate input parameters in XML or JSON payload, then verifies the output against the functional requirements.
API security testing reveals any risk, vulnerability, or threat within the API. Security testing identifies all potential loopholes and API weaknesses that can result in a loss of information, revenue, and reputation if misused by an unauthorized user.
UI testing is a test of the user interface for the API and other integral parts. UI testing focuses more on the interface which ties into the API rather than the API testing itself. Although UI testing is not a specific test of API in terms of the codebase, it still provides an overview of the health, usability, and efficiency of the frontend and backend.
Penetration testing emulates an external attacker or malicious insider specifically targeting a custom set of API endpoints and attempting to undermine the security in order to impact the confidentiality, integrity, or availability of an organization's resources.
Load testing emulates a constant steady stream of traffic at both normal and peak conditions. This ensures that the APIs do not contain memory leaks or other similar defects that might cause issues after running for a prolonged period of time.
Load testing often happens after a specific unit or the whole codebase has been completed.
Runtime error detection testing is related to the actual running of the API and the universal results of utilizing the API codebase. This test focuses on monitoring, execution errors, resource leaks, or error detection.
Fuzz testing or noise testing sets operation parameters to unexpected values in an effort to cause unexpected behavior and errors in the API backend. This helps discover bugs and potential security issues.
Validation testing helps ensures that the software meets the business requirements and is seen as an assurance of the correct development. Validation testing occurs among the final steps and plays an essential role in the development process. It verifies the aspects of product, behavior, and efficiency.
Web Services and Interoperability Compliance testing pertains to SOAP APIs and guarantees that they comply with Web Services Interoperability Profiles. The compliance is checked to see if the predetermined requirements are met.
API testing is performed by submitting requests to the software using the application’s programming interface of the application and then checking if it returns the expected data.
To be able to plan API tests, first, you need to determine testing boundaries and requirements. This includes:
Knowing the purpose of the API will set a firm foundation to prepare your test data for input and output.
Setting up an API test environment requires the configuration of the servers, databases, and every resource the API interacts with, depending on the software requirements.
Before diving into thorough testing, make an API call just to check that nothing is broken and the API is operational.
Plan all possible input combinations to use them in test cases, and to authenticate the results to determine whether the API performs as expected.
Execute test cases and compare actual results with the expected ones. A good practice is to group them by test category.
Examples of API test cases include:
The following factors should be considered when performing API testing:
There are five values for the first digit:
The first digit of the code defines the class of the response. Use these codes to determine the output of your API. Determine if the code follows global standard classes or if the code is specified in the requirement.
Analyze the validation response: during the API testing process, a request is raised to the API with the known data. This way you can analyze the validation response. While testing an API, you should consider:
Focus on functional APIs: focus on the simple APIs that have only one or two inputs. These are necessary access points to further APIs. By focusing on these before moving on to test the others you ensure that the initial points of access work.
Over the years, APIs have evolved from simple code libraries that applications could use to run code on the same computer, to remote APIs that can be used to allow code on one computer to call code hosted somewhere else.
Here is a quick list of the more common API technologies that exist in approximate chronological order:
When looking at an API testing tool, it is important to understand which API technologies you will be using and how best to test them. Nowadays most APIs you will come across will be of the Web Service variety (either REST or SOAP), but you may come across other technologies such as Java EJBs or Microsoft DCOM/ActiveX DLLs.
A Web service is a unit of managed code that can be remotely invoked using HTTP, that is, it can be activated using HTTP requests. So, Web Services allows you to expose the functionality of your existing code over the network. Once it is exposed on the network, other application can use the functionality of your program.
There are two broad classes of web service:
SOAP web services make use of the Web Service Definition Language (WSDL) and communicate using HTTP POST requests. They are essentially a serialization of RPC object calls into XML that can then be passed to the web service. The XML passed to the SOAP web services needs to match the format specified in the WSDL.
SOAP web services are fully self-descripting, so most clients do not directly work with the SOAP XML language, but instead use a client-side proxy generator that creates client object representations of the web service (e.g. Java, .NET objects). The web service consumers interact with these language-specific representations of the SOAP web service.
However, when you are testing SOAP services as well as having a nice interface for viewing the provide services and invoking test operations, you need to always have a way to verify the raw SOAP request and response packets being sent in XML:
This feature in particular distinguishes a true SOAP solution from merely a SOAP client library. The former helps you test the service and understand failures, whereas the latter is just a way of making SOAP web service calls more easily from different programming languages.
In addition, you ideally want to be able to generate your test scripts programmatically from the invoked endpoints and automatically include validation checks:
The following features should be looked for in a SOAP web service testing tool:
A RESTful web API (also called a RESTful web service) is a web API implemented using HTTP and REST principles. Unlike SOAP-based web services, there is no "official" standard for RESTful web APIs. This is because REST is an architectural style, unlike SOAP, which is a protocol.
Typically REST web services expose their operations as a series of unique "resources" which correspond to a specific URL. Each of the standard HTTP methods (POST, GET, PUT and DELETE) then maps into the four basic CRUD (Create, Read, Update and Delete) operations on each resource.
REST web services can use different data serialization methods (XML, JSON, RSS, etc.).
Traditionally the format used for REST Web Services was XML. This is partly because it was used extensively in SOAP web services and is therefore familiar, but also when bandwidth is not a limiting factor, it is self-describing, with the fields and data clearly described:
When choosing a tool to perform REST web service testing, you should look for:
Microsoft's Component Object Model (COM) also known as ActiveX is a standard for communication between separately engineered software components (source). Any object with a COM interface can be created and used remotely:
var doc = new ActiveXObject("Word.Application");
Using this approach, any API packaged as a COM or .NET accessible Dynamic Linked Library (DLLs) can be tested natively by testing tools such as Rapise.
Rapise from Inflectra provides support for testing the following different types of DLL API: