An easy way to lock on a dynamic set of keys without having to worry about resource management. If the list of named locks is fixed then a simple Map could be used and each semaphore could be created on demand. If the list of locks is dynamic, possibly by userId, then you might want to cleanup unused locks to avoid running out of memory.

Here is an example where you might want to restrict the amount of parallel requests for each user down to 1. Imagine that there are thousands or millions of users in the system, so leaving the locks in memory is not feasible.

// Initialize the map of locks
var SemaphoreMap = require('semaphore-map');
var userLocks = new SemaphoreMap();

// For each request from a user
var userId = <some-user-id-lookup>
userLocks.take(userId, function() {
    // Do some work for the user

    userLocks.leave(userId);
});

The semaphore-map project will take care of cleaning up unused locks even if a wide range of users perform actions against the server.