Generic IO playback for clojure
Clojure

README.md

vcr-clj

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vcr-clj is a general function recording/playback library for Clojure. It is intended to be used when testing code that does I/O, to achieve several goals:

  • Repeatable tests despite unreliable/unpredictable I/O sources
  • Tests that can be run without requiring the availability of I/O sources
  • Tests that involve realistic data

Any clojure function (er, var) can be recorded and played back.

Requirements

vcr-clj requires Clojure 1.4 or later.

Obtention

[com.gfredericks/vcr-clj "0.4.14"]

Usage

An example with clojure.test:

(ns my.project-test
  (:require [clojure.test :refer :all]
            [vcr-clj.core :refer [with-cassette]]))

(deftest here-is-my-test
  (with-cassette :foo [{:var #'my.io.ns/get-data, ...extra options...}]
    ... do some testy things ...
    ... that call my.io.ns/get-data ...))

There is also currently a separate namespace for recording clj-http requests in particular:

(ns my.project-test
  (:require [clojure.test :refer :all]
            [vcr-clj.clj-http :refer [with-cassette]]))

(deftest here-is-my-webby-test
  (with-cassette :foo
    ... do some testy things ...
    ... that call clj-http functions ...))

(deftest this-test-only-records-some-calls
  (with-cassette {:name :foo-2
                  :recordable? (fn [req] (worth-recording? req))}
    ... do some testy things ...
    ... that call clj-http functions ...))

The first time you run a with-cassette block, a cassette file is created in the /cassettes directory. Each subsequent time, playback is performed using the cassette in the directory. You can delete it to force a re-record.

Namespaced keywords can be used to group cassettes in the filesystem.

Customizing

Each var that is recorded can be customized with options:

  • :arg-transformer: A function with the same argument signature as the recorded function, which returns a vector of possibly transformed arguments. During recording/playback, the original arguments to the function call are passed through this transformer, and the transformed arguments are passed to arg-key-fn, recordable? and the recorded function. This can be useful for replacing an argument that would be destructively consumed (e.g. a mutable InputStream) with an indestructible substitute. The transformed arguments ought to be equivalent to the original arguments for the purpose of the code under test. The default is clojure.core/vector, which just passes along the original arguments.
  • :arg-key-fn: A function with the same argument signature as the recorded function, which returns a value for "fingerprinting" the arguments to each call. During recording, the value returned by this function will be saved along with the recorded call in the cassette. During playback, the value returned by this function will be used to look up a matching recorded call in the cassette. The default is clojure.core/vector, which just compares the arguments as given.
  • :recordable?: A function with the same argument signature as the recorded function, which returns truthy or falsy. If it returns falsy, the call will be passed through to the original function during both recording and playback. The default is clojure.core/identity.
  • :return-transformer: A single-argument function that takes a value returned by the recorded function and returns a transformed return value. During recording, the recorded function will be composed with this transformer function. This can be useful for ensuring serializability. The transformed return value ought to be equivalent to the original return value for the purpose of the code under test. The default is clojure.core/identity.

TODO

  • Add a better way to re-record than deleting cassette files. Maybe an environment variable?

License

Copyright (C) 2012 Gary Fredericks

Distributed under the Eclipse Public License, the same as Clojure.