Jake McCrary

Preventing duplicate long-running invocations in Clojure

A couple months ago I was looking into a problem and noticed that there was a situation where an expensive operation could be running simultaneously multiple times. This was wasteful.

This operation happened on a timer and could also be triggered by a power user through the UI. A power user could accidentally (or purposefully) mash on a UI button and cause the instance they’re interacting with to grind to a halt1.

It was pretty easy to prevent. All I needed to introduce was an atom and lean on compare-and-set!. compare-and-set! is a pretty neat function (and concept found in many languages). Here is the docstring:

Atomically sets the value of atom to newval if and only if the current value of the atom is identical to oldval. Returns true if set happened, else false

Basically, compare-and-set! changes the value of an atom only if it starts from a specified value and returns a boolean letting you know if it did.

To prevent an operation from running multiple times, introduce an atom and wrap calling the operation in a conditional using compare-and-set!. After doing the work, be sure to reset! your atom back to the starting value.

Below is the code.

(defonce running? (atom false))

(defn- expensive-operation!' []
  ;; do work

(defn expensive-operation! []
  (when (compare-and-set! running? false true)
        (reset! running? false)))))

  1. OK, not really grind to a halt, but consume unnecessary resources.

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