Extending Pester for fun and profit

Of late, I’ve been working on a little side project to test a rather complex Akamai Property. We wanted to be confident, after making changes, that the important bits were still working as we expected them to, and for some reason there was no easy, automated solution to test this.

Obviously I decided I’d write my testing project in Pester, and so it was that I began writing a whole bunch of tests to see what URLs returned what status code, which ones redirected, which ones were cache hits and cache misses and what headers were coming back.

First up, I wrote a generic function called “Invoke-AkamaiRequest”. This function would know whether we were testing against Staging or production, and would catch and correct PowerShell’s error behaviour – which I found undesirable – and allow us to send optional Akamai pragma headers (I’ll share this function in a later post).

With that up and running, I could start writing my tests. Here’s a set of simple examples

Describe "An example test, to establish things" {
    Context "Hit up the homepage." {
        It "Should return 200" {
            (Invoke-AkamaiRequest -uristem /).StatusCode | Should Be 200

    Context "Hit up a non-existent page" {
        It "Should return 404" {
            (Invoke-AkamaiRequest -uristem /nonexistent.html).StatusCode | Should be 404

    Context "A redirect works" {
        It "Should gimme 301" {
            (Invoke-AkamaiRequest -uristem /redirectedfolder/nonexist).StatusCode | Should Be 301

Now, that last one, testing a 301, is interesting. Not only do you need to test that a 301 or 302 status code is coming back, you also need to test where the redirect is sending you. So I started to write tests like this

It "Should redirect /blog/ to /advice/" {
    $blog = Invoke-AkamaiRequest -path /blog/
    ($blog | select -expand statuscode | Should Be 301) -and 
    ($blog.headers.location | Should Be http://$tld/advice/)

And this worked fine. But it was a bit clunky. If only Pester had a RedirectTo assertion I could just throw in there, like so

It "Should redirect /blog/ to /advice/ " {
    Invoke-AkamaiRequest -path /blog/ | Should RedirectTo http://$tld/advice/ 

If. Only.

Oh, but it can!

Yes, you can write custom assertions for Pester. They’re pretty easy to do, too. What you need is a trio of functions describing the logic of the test, and what to return if it fails in some way. They are named PesterAssertion, PesterAssertionFailureMessage and NotPesterAssertionFailureMessage, where Assertion is the assertion name, in my case “RedirectTo”

For my particular case, the logic was to take in an HTTP response object, and check that the status was 301 (or 302), and match the Location: header to a specified value. Pretty simple really. Here’s the basic code:

function PesterRedirectTo($value, $expected)
    return [bool](($value.statuscode -eq 301 -or $value.statuscode -eq 302) -and 
                    $value.headers.location -eq $expected)

function PesterRedirectToFailureMessage($value,$expected)
    return "Expected to redirect to {$expected}"

function NotPesterRedirectToFailureMessage($value,$expected)
    return "Expected not to redirect to {$expected}"

I put these into my supporting module (not into the Pester module) and ran my tests. Happy happy days, it worked perfectly. Throwing different URLs at it resulted in exactly the behaviour I wanted.

All that remained was to make the failure messages a little smarter and make the Not assertion more useful, but I figured before I did that I should write this little post with the nice clean code before the extra logic goes in and makes everything unreadable.

You can probably think of several ways you could streamline your tests with assertions right now. I’ve also written RedirectPermanently and ReturnStatus assertions, and I’m looking into HaveHeaders and BeCompressed. I may even release these as an add-on module at some point soon.

You can probably think of things that should go right back into the Pester codebase, too. And there are a number of other ways you can customise and extend pester to fit your own use cases.

To summarise: Pester is not just a flexible and powerful BDD framework for PowerShell. It’s also easily extensible, adding even more power to your PowerShell toolbox.

Now get out there and test stuff.

One reply

  1. Ronald Rink says:

    Great article for starters! Ronald

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