Http client e2e testing

D4no0

I have a project where we do http requests to public domains.

I am currently using Mox for testing and following their practices by having a callback for the http client and a implementation:

defmodule HttpClient do
  @callback request(atom(), binary(), binary(), keyword(), keyword()) ::
              {:ok, HttpClient.Response.t() | HttpClient.MaybeRedirect.t()}
              | {:error, any()}
end

While it works great for tests with mocked data, I still find that these tests are double edged, as they are fully dependent on the correctness of data and format of it, we already had cases where the tests would pass and the application would fail at runtime.

I was wondering if maybe replacing this kind of mocked tests with a real local http server (similarly to how ecto uses sandbox) that exhibits real properties would be better and make for cleaner and easier to manage tests.

Any thoughts on this or tools you could recommend?

10 comments

#http_client #test

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2023-11-27 20:25:31 UTC

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hst337

Bypass

I would recommend Patch for everybody. It is universal and more friendly solution.

This is caused by different data in tests and in reality. You can just copy-paste data from real responses and use it on testing. Or you can even use solutions like ExVCR

Anyway, final decision depends on amount of time you have and what quality of tests you want

Post #1

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hst337

Cassettes can contain outdated data: some services have TTLs for their responses. Or something like your ID in the third-party service can change and responses in cassettes can become stale. A lot of other stuff too
They’re not easy to rerecord correctly. Because manual changes to the cassettes have to be reapplied every time you rerecord them

Post #5

dimitarvp

I get it that it might not be scalable to manually collect all of the API responses but I have used ExVCR in the past and ended up… manually collecting all the API responses that I needed for tests. The API had slight changes – 3 times in a single year – but it was enough to piss us off because we shipped non-working features in production due to outdated cassettes.

So we rolled up our sleeves and what do you know, something like 70 API responses took 3 people 5-6 work days (and we were not doing only that, it was just an ongoing effort). Not a huge deal, though granted it’s annoying to do.

So I am with @hst337 here – take control of this.

Of course nobody is stopping you scripting this somewhat. I was able to devise a small text file format a while ago (next time I needed something like this) and just have a bash/zsh script loop over the lines inside the file and do curl requests and record the responses. Took me 80% the way there (though it was super specific and was not generalizable).

Post #7

hst337

Beware: VCR solutions are hard to maintain

Post #3

Last Post!

pdgonzalez872

Good question. Here are my 2c:

I’ve seen that VCR and the likes “feel” great but as folks here pointed out, they get outdated. It’s a picture/snapshot in time saying that things may have worked given a certain set up you had. A green VCR test doesn’t give me confidence, unfortunately. Much like a test that relies only on Mox/unit tests.

Bypass works well, but I personally feel like the tests are too complex. That’s just my preference, I’m a simple person Even when the tests are done correctly, I don’t have good confidence from a successful test run.

I’ve seen projects that leverage their own http server and I personally dislike those. It’s a layer of complexity that I haven’t seen yield good returns in practice, but that may be biased to my experience (as all of the points in this answer by the way).

What gives me the most confidence is having integration tests that exercise the live implementation and you need to load the correct keys and such, just like Dashbit discusses here:

set boundaries in your code so you can leverage Mox (do what the readme says)
create a test where you load real envs and the live impl is chosen.
set things up so these are excluded from ci runs, as they are meant to be run locally with the correct env vars/keys. You can use @moduletag or multiple @tags. So, ci runs will just exercise the boundary and args (your Mox unit tests).
every once in a while run those from your local machine. You can set up mix aliases to help here. Depending on the service/whatever you are testing maybe you pay for calls/only have a certain number. In certain projects, before merging a PR (after reviews, dev, etc), I’d run these locally to gain confidence.

This setup has given me (and a few teams) the most confidence when working with things we don’t fully control (when we should use Mox). I’ve personally seen:

the feedback loop be drastically decreased
caught errors in docs → docs say something, service does something else. You can now prove it and not be seen as a maniac on your team, questioning your sanity.
added documentation → since you can assert on results, this is very helpful to see outputs and help onboard folks into projects
caught staging/prod discrepancies an api → Things worked in the tests, but cried in prod. Now you can repro and discuss, you have proof. It’s a little surprise when it happens, but a lot of times these would happen and go undocumented/discussed. Once you can repro (by running the tests) you are playing a much better game imo :).

Downsides: Usually these tests hit test environments/sandboxes, which in turn are NOT prod. Even though these have given me the most confidence, you can still get little surprises in prod. As usual, make sure you have log and metrics to give yourself a better shot of handling the surprises, because they will come

I’ve also seen a successful mix of Bypass and integration tests: avalanche/test/integration_test.exs at main · HGInsights/avalanche · GitHub. Even though there is no switch there (there is no live impl for Avalanche) but we achieve the integration/live tests by not intercepting the request. It works well in this case.

PS: Oh, I mention integration tests and I realized that this is an overloaded term. Much like a mock → in some contexts/communities means something to some people and others it’s different. So maybe a better name for these tests would be live unit tests, but naming things is hard.

Post #10