conc

Concurrency utilities for Javascript.

asyncqueue

asyncqueue is an asynchronous FIFO queue brokered by Promises. Feed the queue with offer(item), and retrieve it with poll(). Because poll may be called on an empty queue, the returned Promise provides a way to suspend the poll until a rendezvous with the next call to offer.

To illustrate its power and simplicity, below is an implementation of a resource pool. We use the pool by passing it a closure, to which the resource is applied, afterwards the resource goes back into the pool, e.g.:

// Initialize the pool with a capacity and an action that creates
// a new resource.
var withResource = createPool(10, () => new DbConnection);
// Pass `withResource` into some other part of the application
withResource(conn => conn.sql('SELECT ...')).then(result => {
  // ...Read the result...
});

The implementation of createPool.

var asyncqueue = require('conc').asyncqueue;
function createPool(cap, newResource) {
  var queue = asyncqueue();
  for (var i = 0; i < cap; i++) queue.offer(newResource());
  return function(perform) {
    // Retrieve a resource from the pool.
    return queue.poll().then(resource =>
      // Perform an action with the resource.
      perform(resource).finally(() =>
        // Finally, return the resource to the pool.
        queue.offer(resource)));
  };
}