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 – “the interface or contract” that specifies the operations, data formats and behaviors and the implementation that can change as needed.
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. 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.
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!
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.
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:
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