Clojure programs often run services that are required to be unique. For example, anything that listens on a network port cannot be duplicated, since only one service can accept connections on a given port.

It is common during development to compile and subsequently recompile namespaces, many times over. However, when re-initialising state that represents services, it is necessary to stop services that might get restarted. This is often done in an ad-hoc manner, and this small library presents a pattern that can take away much of the manual service teardown work that a developer might need to do.

By providing shutdown logic in defemeral declarations, you can recompile the namespace safely, and if the service that the defemeral represents will be started (after stopping any existing services if necessary).

A call to defemeral causes an object to be registered that satisfies the defemeral.defemeral/Ephemeral protocol, which contains begin and end functions. The result of the begin function is given to the end function - this is usually a value that represents the service and gives the end function the means to shutdown the service cleanly, closing any resources involved.

(defemeral my-service
  (begin [_]
    (println "Starting my service")
    :something)
  (end [_ something]
    (println "Stopped my service")))

Copyright © 2013 Malcolm Sparks

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