Consumer-driven contracts, both ways
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Latest commit 7141dae Nov 28, 2013

Janus: Consumer-driven Contracts and Mocks

While on a recent mobile project with a client, it was obvious that consumer-driven contracts would have been very valuable for the service implementors. The other thing I noticed was that the service clients had no way to verify that the out-of-process mocks they were building and testing against actually had any relationship to the data that was returned by the services.

When talking to the service team, they were really keen on the idea of using consumer-driven contracts. They desperately wanted more client developer involvement, but just couldn't get it. However, I've previously had a lot of trouble with getting client teams to actually write the contracts. It is more work for them, for a promise of eventually easier integration.

This is where Janus comes in. The idea was to write the contracts in such a way that as well as using the contract to verify the behaviour of the service, use that very same contract to provide mocks to the consuming applications. Executing the contract in one mode would test the service; executing it in another mode would create an out-of-process mock server. As soon as the client team writes the contracts, they would get immediate value out of it.

Getting Started

Janus currently only verifies services against contracts, it can't yet use those contracts to create mock services. To verify a service against a contract, create a contract like so:

 "Service name"
 (contract "contract name"
           (method <one of :get, :post, :put or :delete>)
           (url "full, absolute URL to the service")
           (header "header name" "header value")
           (body <:xml, :json or :string>
                 <Clojure data structure that will be serialized as above>)

           (should-have :path "json path" :matching <regex>)
           (should-have :path "json path" :of-type <:string, :array, :object or :number>)
           (should-have :path "json path" :equal-to <value>)))

Save that in a file with the suffix .jns and then run it with janus.

java -jar janus-0.0.2.jar --verify file.jns

A janus contract file is a Clojure data structure, and it is not interpreted as code. This means you can't include Clojure code in there. If you need any code, then create a directory called support in the same directory as you run janus, any file with a .clj suffix in that directory will be loaded and executed.

You can use that file to start and stop any services that you may need.

Json Path

janus will automatically deserialize JSON responses received from services into Clojure data structures. Hence, to verify the structure of the document, Json Path is used to identify parts of the document. It's actually a pretty powerful language, though not quite as powerful as XML.

For more info, have a look at the Json Path implementation that janus uses.


  • janus only support Json response documents.
  • No ability to mock services based on a contract.