Types of APIs

COVID-19 Note: There is a virus shutting down the world right now. It’s destroying a lot of things including lives and livelihoods. I want to take some of the focus off the destruction this virus is causing, and focus instead on creating something. I’m doing a series of posts on API testing as my way of fighting back against the virus. We can still go on with life even in times like this.

In order to effectively test an API, you need to know what kind of API you are testing. Often in today’s world when we talk about APIs we are talking about REST APIs, but there are several other important API formats that you might need to test. Let’s look through them and see what we can learn about some of the major API types.


We’ll start with what is probably the most common type of API you’ll come across on the modern web; the RESTful API. REST stands for Representational State Transfer and refers to an architectural style that guides how you should create APIs. I won’t go into the details of the properties that a RESTful API should have (you can look them up on wikipedia here if you want), but there are a few clues that can let you know that you are probably testing a RESTful API.

RESTful APIs are based on a set of guidelines and so they do not all look the same. There is no official standard that defines the exact specifications that a response has to conform to. This means that the following are in general good clues and hints that you are dealing with a REST API, but they are not definitive. It is also worth noting that many APIs that are considered to be RESTful do not strictly follow all the REST guidelines. REST in general has more flexibility than a standards based protocol like SOAP, but this means that there can be a lot of diversity in the way REST APIs are defined and used.

How do I know if the API I’m looking at is RESTful?

So, what are some clues that you are looking at a REST API? Well in the first place, what kind of requests are typically defined? Most REST APIs will defined GET, POST, PUT, and DELETE calls with perhaps a few others. Depending on the needs of the API, it may not define all of these, but those are the common ones.

Another clue, is in the types of requests or responses that are allowed by the API. Often REST APIs will use JSON data in their responses (although the could use text or even xml). Generally speaking if the data in the responses and requests of the API is not xml, there is a good chance you are dealing with a REST based API of some sort.

REST API Example

There are many examples of REST APIs on the web, but let’s take a quick look at just one of them to help cement the idea of what RESTful API is in our minds. It’s the internet, so let’s look at cats, and since this is an article about API testing, let’s also look at http status codes (which are used by REST APIs). And of course, the internet knows that combining cats with http status codes needed to be done, so we can go to https://http.cat/ to see a cat image for each http status code.

This site provides an API and so we can call https://http.cat/200 to get the cat image for the 200 status code. For example, if we were to do this in Postman, we could create a request, set the method to GET and put in the url and click on send.

call cat API in Postman

We don’t have to specify any xml when we make the call and can just send the request directly in Postman (or another API tool) as a GET call, so we are most likely dealing with a REST API here.


Before REST there was SOAP. SOAP stands for Simple Object Access Protocol. The SOAP protocol has been around since long before Roy Fileding came up with the concept of REST APIs. It is not as widely used on the web now (especially for smaller applications), but for many years it was the default way to make APIs and so there are still many SOAP APIs around.

SOAP is an actual protocol with a W3 standards definition and so its usage is much more strictly defined than REST which is an architectural guideline as opposed to a strictly defined protocol.

If you want a little light reading, check out this document. It claims to be a:

non-normative document intended to provide an easily understandable tutorial on the features of SOAP Version 1.2

I’ll be honest, I’m not sure how well it delivers on the ‘easily understandable tutorial’ part of that statement. Looking at some of the examples in there may help you understand why REST APIs have become so popular. SOAP APIs require a highly structured xml message to be sent with the request. Being built in xml, these requests are not that easy to read for humans, and require a lot of complexity to build up. There are of course many tools like SoapUI that can help with this, but in general SOAP APIs tend to be a bit more complex to get started with. You need to know more information (like the envelope structure) in order to get started.

How do I know if the API I’m looking at is a SOAP API?

The most important rule of thumb here is: does it require you to specify structure xml in order to work? If it does, it’s a SOAP API. Since these kinds of APIs are required to follow the w3c specification, they must use xml and they must specify things like env:Envelope nodes inside of the xml. If the API you are looking at requires xml to be specified and that xml includes the Envelope node you are almost certainly dealing with a SOAP API.

SOAP API Example:

A SOAP API example is a little bit harder than just sending a GET request to an endpoint. We will use this webservice as an example SOAP API. Let’s look at the first action there, which gives us a list of continents. In order to call this API in Postman we will need to set up a few things. We of course need to create a request in Postman and then we will need to set the request method to POST and put in the url. However we can’t yet click send. Since this is a SOAP API, we need to send some xml information as well. In Postman, this means setting the body type to raw, and choosing XML from the dropdown and then putting in the xml envelope data as indicated by the documentation

SOAP API Information

For this particular API, we would also need to modify the Content-Type header (by adding a new one) at the bottom, so that is is set to application/soap+xml

Content-Type Header

As you can see, there is a lot more complexity to calling SOAP APIs. REST APIs can of course have complex bodies specified as well, but the requirement to do this in xml and the existence of the Envelope node in this, indicates that this API is indeed a SOAP API.

GraphQL APIs

SOAP came before REST and in many ways REST was designed to deal with some of the shortcomings of SOAP. Of course in software we are never done making things better and so along comes GraphQL. GraphQL is a query language and it was designed to deal with some of the situations where REST APIs have shortcomings. RESTful APIs don’t know what specific information you might be looking for and so when you call a REST API endpoint, it gives back all the information it has. This can mean that we are sending extra information that you don’t need, or it can mean that we aren’t sending all the information you need and you need to call multiple endpoints to get what you want. Either of these cases can slow things down and for big applications with many users that can become problematic. GraphQL was designed by Facebook to deal with these issues.

GraphQL is a query language for APIs, and so it requires you to specify in a query what you are looking for. With REST APIs you will usually need to know what the different endpoints are in order to find the information you are looking for, but with a GraphQL API a single ‘endpoint’ will contain most or all of the information you need and you will use queries to filter down that information to only the bits that you are interested in. This means that with GraphQL APIs you will need to know the schema or structure of the data so that you know how to properly query it, instead of needing to know what all the endpoints are.

How do I know if the API I’m looking at is a GraphQL API?

Well, if the documentation is telling you about what kinds of queries you need to write, you are almost certainly looking at a GraphQL API. In some ways a GraphQL API is similar to a SOAP API in that you need to tell the service some information about what you are interested in, but a SOAP API will always use xml and follow a strict definition in the calls, whereas GraphQL APIs will usually a bit simpler and not defined in xml. Also with GraphQL the way the schema is defined can vary from one API to another as it does not need to follow a strictly set standard.

GraphQL API Example

Let’s take a look at a real life example of calling a GraphQL API to understand it a bit better. This example will use the countries api as hosted here. There is information about the schema for it on github here and we can use that to create queries in the provided playground, but for this example let’s look at setting it up in Postman

Similar to calling a SOAP API, we will need to specify the service we want and do a POST request. We will also need to choose the GraphQL option on the body tab, and put in the query that we want

GraphQL query

As you can see in the example I’ve done, I’ve requested the name and languages of Canada. Once I have specified this information I can click Send and I get back some json with the country name and a list of the official languages. If I wanted additional information (Say the name of the capital city), I could just modify the query to include a request for that information and send it off again using the same endpoint.


We’ve looked at three different kinds of APIs. Each of them has it’s own strengths and weaknesses and is used in different applications. Some applications will even support more than one type of API, and as a tester you need to be ready to test any kind of API that you come across. Hopefully this article has at least helped you in that journey and given you some understanding of how the various APIs you might come across work and can be interacted with.

Photo by Markus Spiske on Unsplash

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