Black Sheep Code

Using Postman as a contract/blackbox tests - Part 2 - Setting up some tests

In this scenario, here's what I want to do

  1. Run a real server (ideally with an in memory database... which is what we already have)
  2. Use the Postman tooling to define queries to run against this server
  3. Ideally reset the server between tests
  4. We want to be able to run these tests from a command line - we want these tests running in a build pipeline

Two example tests

We're going to start small, nice and easy tests, and we can work them up to be more complex.

Test 1 - Create a pet, get all pets, and expect to see our pet there.

Test 2 - Create a pet, create a pet with the same id, expect to get a HTTP 409.


My google of 'run postman tests from command line' points us to this article from postman suggests using a tool called Newman. Let's give that a go.

This commit implements basic usage of Newman.

This works fine, but it really isn't want we want.

This will run the whole collection, including any tests on them, (which we currently don't have any).


  1. It doesn't look like we have an ability to do some imperative logic (eg. clear down the server) between runs. I've looked at the docs and there appears to be no ('before each') type functionality.

  2. Newman itself isn't for defining tests, it just runs your collection.

Tests on Collections

Let's explore the basic test functionality of the tests feature of collections.

To be honest, I'm struggling a bit to see how these tests are useful.

Basically you can write assertions after each request, so if we know that GET /widget/123 is always going to return the same object (perhaps because we've seeded our server with test data as such), then sure we can make assertions on it.

But against a live server, I struggle to find this useful.

Let's try something anyway.

Ok, so what we can do is use folders within the collection, to the requests will run in order.

We can then use the technique of randomising values to ensure uniqueness.

This commit here adding the two tests.

I can't say I enjoy the developer experience of this.

I don't want to write significant amounts of code in the browser, as good as Postman's browser experience is.

Also, this technique necessarily involves copy pasting requests. Were we to update requests later... that would be a huge pain. I want my queries to be reusable, and just configure them with variables.


Flows appear to be a new feature by Postman. Let's give it a go.

Ok, so flow appear to be a way to do a series of requests, including forking logic.

However, it's not suitable for our needs.

  1. Firstly, Flows can't access the collection that is our version controlled API. We have to copy the API to the main collections area, and then we lose the synchronisation.

  2. Secondly, it appears that it doesn't have test functionality. You only do control and make more requests with it.

  3. I definitely don't want to be generating significant amounts of logic with a drag and drop tool.


So far I'm not that impressed with Postman as far as a testing tool goes.

  1. I don't want to write large amounts of logic in a browser.

    • I do like viewing data/results in a browser.
    • It is nice to be able to make a quick edit in the browser.
    • But for the most part, I prefer to be doing my writing in a text editor like VSCode.
    • Technically I could be editing the Postman files in my VSCode. But they are ugly JSON files, and I would lose syntax highlighting, autocomplete etc.
  2. Postman, as far as I can tell, doesn't have a mechanism for reusing queries, say I wanted to compose a collection that executes queries in a certain order (for example, GET then POST then GET), the only way to achieve would be to copy post the queries.

  3. Generally I think where a lot of people get a lot use from Postman, is for making adhoc queries, and so it's always good to still have a tool to do that. Any other way can be a bit fiddly.

Feels like I might need to go for something that I'm a bit more familiar with, Jest or Cypress or Playwright perhaps.

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