Question

I am looking at different ways of implementing concurrency in clojure and these seem to be two competing ways of doing the same thing, so I was wondering where I should use each technique.

Answer 1

Watches and promises are both very useful for concurrency, but are suited to slightly different uses. You may well find that you want to use both in different places in the same application.

Use a watch if you want notification of change in a reference. For example, if one thread is handling events and updates a ref in response to some of these events, you could use add-watch to enable other parts of your system to receive notification of the update. A single watch can handle many updates over time.

Use a promise if you want to pass another thread a handle to access a value that is not yet computed. If the other thread tries to dereference the promise, they will block until the computation of the promise has finished (i.e. the original thread places a value in the promise via “deliver”). A single promise is only intended to be used once – after that it is just a fixed value.