Testing REST APIs with Postman

Due to my recent work at FinancialForce.com‘s Product Innovation Lab, I have been creating REST APIs, and those APIs needed testing. Initially, I tested them manually, using tools like SOAP UI for POST requests and the browser for GET requests. But soon I needed a way of performing multiple tests, with different sets of data, in short periods of time. And for that, Postman became the ideal tool. To illustrate this, I have created a simple server with an even simpler REST API in Node.js. It is available in this GitHub repo.

 

The example app

The provided example app implements a server that can be started from a terminal with NPM and Node.js installed. After cloning it, simply go to its folder and call this from the command line:

 

> npm install
> PORT=8080 npm start

 

It exposes the following REST API methods:

  • GET /api/v1.0/math/add/<value1>/<value2>
  • GET /api/v1.0/math/subtract/<value1>/<value2>
  • GET /api/v1.0/math/square/<value>
  • POST /api/v1.0/math/accumulate
    • It expects a JSON body with this format:
{
	"data": <number to accumulate>
}

All of them return a JSON object with this format:

{
  "input": <input data>,
  "result": <result value>;
}

The REST API methods are defined in src/server.js. If you want more information about Node.js, NPM and how to create servers in Node, these links can be useful:

 

Now let’s start talking about Postman.

 

A powerful UI for testing APIs

Postman screenshot

There is a native app for all major desktop operation systems. And it performs really well, at least on Mac OS X. This app allows to:

  • Define collections of requests
  • Organise them in folders
  • Export collections to JSON files
  • Parameterise requests
  • Define tests for each request
  • Run collections of tests along with their tests in sequence

The last three points are what make Postman so interesting for our purposes. Almost everything in a request can be parameterised, allowing us to reuse requests with variable data. For example, if we want to take the port from a variable (let’s call it LocalhostPort), we just need to put {{LocalHostPort}} in the URL instead of the port. The actual value will be taken from one of these places:

  1. Global Variables. A set of pairs <key, value> defining values for variables, that are available to all requests at every time.
  2. Environment. A ser of pairs <key, value> defining values for variables. One environment, or none, can be selected each time. If a variable with the same name is defined on both the Global Variables and the selected Environment, the latter takes precedence.
  3. Iteration Data. Apart from the Global Variables and the Environment, it is possible to specify Iteration Data when running requests and tests in Postman. Iteration Data contains a list of sets of pairs <key, value>, defining multiple sets of values for the same variables. It is meant to be used in different runs, passing different data for each run. If a variable with the same name is defined on either the Global Variables or the selected Environment, the value from the Iteration Data takes precedence.

All of them can be exported to JSON files. More information about variables can be found here: Setting up an environment with variables.

A full Postman project (if you want to call it that) has been included in the example app, under the postman folder. Simply import the files by clicking Import at the top left => Choose File => select the files and click Open. You can import the collection (with all the requests and their associated tests), the environment (which defines a default value for some parameters to be used in the requests, in case of calling them independently), and the global variables (which defines the port as 8080). The inputData.json file includes data to iterate through when running the requests from the Runner.

 

The Runner

PostmanRunner Screenshot

By clicking the Runner button at the top left of the app, a separate window focused on running collections is opened. There, we can choose one of the collections (or folders inside a collection), an Environment, and optionally, iteration data, to run all of the requests and their tests in sequence. If you have imported everything after the previous section, then you should be able to select the Nodejs-Postman collection and the Nodejs-Postman environment. The global variables will be implicitly applied. Choose the inputData.json file from Data – Select File and click Start Run. If the server is running (if not, you can start it by following the explanation above), the tests should work fine.

 

The tests

A test in Postman is coded in Javascript and can look like this:

var resultObject;

try
{
    resultObject = JSON.parse(responseBody);
}
catch (ex)
{
    console.log("Unexpected response body: " + ex);
    console.log(responseBody);
    
    return;
}

function defined(value) { return value !== null && value !== undefined; }

var input1 = Number(defined(data.Input1) ? data.Input1 : environment.Input1),
    input2 = Number(defined(data.Input2) ? data.Input2 : environment.Input2);

tests["Request succeeded"] = responseCode.code === 200;
tests["Input included"] = !!resultObject.input;
tests["Input is an object"] = resultObject.input.constructor === Object;
tests["Input 1 is correct"] = resultObject.input.input1 === input1;
tests["Input 2 is correct"] = resultObject.input.input2 === input2;
tests["Correct result"] = resultObject.result === (input1 + input2);

This code goes in the Tests section under a request in Postman. Postman provides a simple API that allows us, among other things, to:

  • Get the request’s response data
  • Get Global, Environment and Iteration Data variables
  • Update Environment variables
  • Define tests

All actual tests are defined by adding an entry to the tests object. The key is the test description, and the value is a boolean variable (true if the test succeeded, false otherwise). If nothing crashes, the whole code snippet is run; Postman will raise error information for each test that failed after the script finishes.

More info about Postman tests here: Testing Examples. Other useful links:

 

Newman: a CLI for Postman

It would be great if we could leverage Postman’s testing features in an automated testing workflow and integrate it in a continuous release system. It is perfectly possible thanks to the availability of Newman, a command-line interface for Postman. You can install it through NPM.

 

Newman is compatible with all files Postman can export, and provides nicely formatted results, which ultimately allows us to integrate these tests into any automated workflow or script. An example of a command line running exactly the same tests as above could be:

> newman run postman/Nodejs-Postman.postman_collection.json --iteration-data postman/inputData.json --environment postman/Nodejs-Postman.postman_environment.json --globals postman/globals.postman_globals.json

 

The line above has been included in the postman.sh script, which can be run with:

> sh postman.sh

… which, in turn, thanks to the scripts property in package.json, can be run with:

> npm test

… providing a very straightforward and standard way of running the tests for this app.

 

Authentication

The provided example is very simple and does not require any authentication. A more realistic case will probably need some kind of authentication, typically OAuth. Postman does provide functionality for authentication, but it is also possible to provide the session token, or access token, depending on the authentication system, retrieved by other means. This token can be provided via header. The way of getting it can differ depending on the platform the API we are testing is hosted on. To get the most of this, an automatic way of getting the token and putting it into the header must be sorted out.

 

Conclusions

Postman, and its CLI Newman, have been extremely useful for me while implementing APIs, thanks to their usability, power and performance. I strongly recommend give them a go in case of working with APIs.

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