Contract testing for Clojure
Integration tests are brittle, slow and error prone. Conventionally, when integrating with external side effects, like a 3rd party API or a networked database, users are presented with a choice: either all tests run against the real database or API which is slow or unreliable, or using mocks. Mocks are fast and deterministic, but there's no guarantee that the mock behavior corresponds to reality.
If you have a large number of tests which run against a slow external service, odds are the vast majority of the tests aren't specifically about the external service, but they depend on it for behavior.
Contracts are the solution. A contract is a generative test suite against the caller and implementation of protocol, covering inputs, outputs and state mutations over time. All implementations may be tested against the contract, and mocks can be automatically built from the contract.
Use contracts to unify the behavior of integration tests and mocks, while decoupling tests. Behavior for both sides is specified in one place. Write integration tests against the real service in one place, and test that thoroughly. The rest of the system then uses a mock which is guaranteed to uphold the guarantees which were specified.
- significantly reduce the number of integration tests in a test suite
- fast, reliable, deterministic unit and integration tests
- ability to deterministically introduce faults into the system while testing
A model describes the protocol under test. It describes the methods that can be called, the arguments to use for those methods, and the state transitions that result from calling the function.
The model is used to generate a sequence of calls against the protocol. The calls are run against the implementation under test, and specs returned by the model are used to validate the implementation return values.
(defprotocol RemoteAPI
:extend-via-metadata true
(create-file [this file]))Define a protocol as normal. Because of how mocks are constructed, :extend-via-metadata must be true.
In our example, let's say it's an error to attempt to create a file twice.
Define a model of the protocol:
(def model
(c/model
{:protocols #{RemoteAPI}
:methods [(c/method
#'create-file
(fn [state [file]]
(if (not (get-in state [:files file]))
(c/return (gs/constant :ok)
:next-state (update state :files conj file))
(c/return (gs/constant :error/file-exists)
:next-state state)))
:args (fn [_state] (gen/tuple gen/string)))]
:initial-state (fn [] {:files #{}})})):methods is a coll-of method definitions. The first argument is a the method var, and the second is (Fn [state args] -> contract/Return). contract/return specifies the spec/predicate used to test implementations, and optionally also mutates state and defines the generator for mock return values. :args is used to return a generator for arguments to the method given the current state.
verify takes the model, and a no-arg constructor for the IUT and runs a generative test
(deftest remote-api-contracts
(c/verify model ->RealRemoteAPIClient))
=>
{:result true,
:pass? true,
:num-tests 100,
:time-elapsed-ms 115823,
:seed 1664457195393}Run it in your test suite for each implementation.
(c/mock model)Returns an instance of the protocol based on the model definition. Return values will be generated from the correponding method return :gen or :spec.
For all other tests in your suite which require an instance of the protocol, prefer using the mock.
(c/test-proxy model impl)Returns a new instance of the protocol that passes calls to the 'real' implementation, and compares the implementation's return value against the model. Throws when the implementation and model disagree.
Prefer using it in integration tests to identify discrepancies between the model and the implementation.
A significant part of the contracts API deals with preconditions and whether it is "valid" to make a call. Clearly of a users can call any protocol method at any time, so this is concern is mostly about interestingness and test coverage, rather than correctness.
Consider a hypothetical API that takes IDs as an argument:
(create 1)
(create 2)
(delete 2)By default, if a property doesn't hold, test.check shrinks the inputs randomly, so one possible shrunk call sequence is:
(create 1)
(delete 2)This is not an interesting case, but it is possible, in the sense that a user could make that sequence, and something will happen. In a stateful system, the search space is exponentially larger than a stateless system (every new bit of implicit state multiplies the possible states by 2). We use preconditions to prune the search space to cases that are interesting.
To make shrinking work, the model state must be a single immutable value, passed from init-state to next-state. Consider a similar example as above
(create 1)
(create 2)
(delete 1)which this time, due to precondition checks, shrinks to
(create 1)
(delete 1)When we shrink, we are effectively "rewinding" state. The call to (create 2) must no longer be present in state during the shrunk call sequence. Therefore, the state must be an immutable value, and we pass it in to the next generated call.
The corollary here is that the model must not close over any state which mutates during a test.
Coming Soon
If the model supplies :cleanup, (fn [state impl calls]), it will be called at the end of verify. Use it to clean up e.g. real resources created during the test.
Test.contract is not intended to completely replace all other testing strategies, nor be the sole source of testing for a protocol implementation. It is intended to provide confidence that callers and implementers of a protocol agree on what the specific behavior is. A good heuristic for asking whether a behavior should be included in the contract is to ask "will tests that consume the protocol break if this detail is not specified?". If not, leave it out.
Significant inspiration from: