What is Extend-protocol in a Jepsen context?

Question

I'm new to clojure and I am trying to figure out what Jepsen does, a software used to check consistency of distributed system.

• My questions are:
  1. What does extend-protocol do?
  2. To be more specific, In Jepsen.Generator, what is op in the Protocol.
  3. What does mix mean in the context

Kind Regards

Answer 1

What does extend-protocol do?

A protocol is an abstract thing that looks like an interface in Java. It does nothing, but some other entities may implement it. In Java, you declare a class that implements an interface. In Clojure, you extend a particular protocol with a custom type declared either with deftype or defrecord calling extend-protocol on it.

When extending a protocol with a type, you need to specify implementation for signatures mentioned in that protocol.

A good example might be JSON serialization. Say, the protocol would look like as follows:

(defprotocol JSON
  (to-json [obj]))

If you call (to-json ...) on any value, you'll have an error saying that there is no to-json implementation for that type. You need to extend it:

(extend-protocol JSON

  Integer
  (to-json [obj] (str obj))

  Boolean
  (to-json [obj]
    (if obj "true" "false")))

Now that, calling (to-json 42) and (to-json false) will work. You may extend that protocol for the rest of types: floats, array, maps and so on.

what is op in the Protocol

Protocols do not have implementations, only signatures. os is a signature of some function that just takes three arguments: [gen test process]. It's up to what should it return. As an example, you may refer the line #46 where its behavior is implemented for the clojure.lang.AFunction type. Since I'm not familiar with jepsen, I cannot say more on that.

What does mix mean in the context

I think its docstring is pretty clear as well as the code is. I takes a collection of gens. If it's empty, the result would be a special Generator instance named void. It's an anonymous type that extends Generator protocol returning just nil when calling op without any computations.

It the gens are not empty, the code returns an instance of Generator type with such op implementation that takes a random gen when executing op.

Hope that will help.