Introducing Aspec: A black box API testing DSL

Caltrak is the service that stores Songkick users’ tracked artists and cities. It has no other service dependencies. You put data into the Caltrak box, then you get it back out.

For instance, you might make two POST requests to store artist trackings, and then want to retrieve them, which would look like this:

# create and retrieve artist trackings
POST /users/7/artists/1    204
POST /users/7/artists/2    204
 GET /users/7/artists      200    application/json   [1, 2]

Did you understand basically what that was saying? I hope so, because that’s an executable spec from the Caltrak tests.

It’s pretty simple. Every line is both a request and an assertion. Every line says “If I make this request then I expect to get this back”.

This works because the behaviour of this service can be entirely described through the REST API. There are no “side affects” that are not visible through the API itself.

Here is a longer portion from the aspec file.

# no users have pending notifications
   GET /users/with-pending-notifications                200  application/json  []

# users with events on their calendar have pending notifications
  POST /users/764/metro-areas/999                       204
  POST /users/764/artists/123                           204
  POST /events/5?artist_ids=123&metro_area_id=999       204
  POST /events/5/enqueue-notifications                  204
   GET /users/with-pending-notifications                200  application/json  [[764, "ep"]]

# users are unique in the response
  POST /users/764/artists/123                           204
  POST /users/764/artists/456                           204
  POST /users/764/metro-areas/999                       204
  POST /events/5?artist_ids=123,456&metro_area_id=999   204
  POST /events/5/enqueue-notifications                  204
   GET /users/with-pending-notifications                200  application/json  [[764, "ep"]]

Some aspects:

  • Each line has the format Verb, Url (with Params), Status, Content Type, Body separated by whitespace. These are the only things that can be asserted about the service responses.
  • Each “paragraph” is a separate test. The database is cleared in-between.
  • Lines beginning with # are comments.
  • Aspec stubs time, so that the first line of the test occurs precisely on the epoch and each subsequent line occurs 2s after that. This allows us to test responses with creation timestamps in them.


When we began developing Caltrak, I wasn’t happy with the process of writing tests for this service.

I wanted the test framework to expose the simple nature of the API. You could make something almost as simple in RSpec or Cucumber with judicious use of helpers and so on, but you would end up with a DSL that obscured the underlying REST API.

In an Aspec file, there is no syntax that does not express real data either sent or received from the service. You’re basically writing down the actual HTTP requests and responses with lots of parts omitted. It is technical, but it is very readable. I think it is better documentation than most service tests.

Also, there is no context that is not immediately visible, as there might be with nested RSpec contexts, for example, where in a large test file the setup may be very distant from the test and assertion.


NB This project is very immature. Use at your own risk.

Aspec assumes your project uses Rack, and uses Rack/Test to talk to it. The code is published on GitHub and there is a tiny example API project.

It is very similar to rspec in operation. You write a .aspec file, and put an aspec_helper.rb next to it.

Then run

aspec aspec/my_service.aspec

I’d be interested in hearing your thoughts on this testing style.