A fair semaphore

With this module, you can block work from multiple incoming sources and the work will be unblocked in a fair order. One source cannot cause starvation of the other sources.

A fair queue could be used to schedule incoming requests from users, to make sure that no user reduces the performance of other users on the same server. Or, the queue could be used to balance outgoing requests to a downstream server, making sure that one user does not consume all of the connections in a pool.

A simple example

In this example, source 1 will request two items of work before source 2 can try to request anything. But, when the work is unblocked, the work from source 2 will be unlocked first so that source 1 does not starve source 2.

const FairSemaphore = require('fair-semaphore');
const semaphore = new FairSemaphore();

semaphore.take('source1', function () {
  console.log('Inside first request from source 1');
  semaphore.leave();
});

semaphore.take('source1', function () {
  console.log('Inside second request from source 1');
  semaphore.leave();
});

semaphore.take('source2', function () {
  console.log('Inside first request from source 2');
  semaphore.leave();
});

When the above code is executed, the output will be

Inside first request from source 1
Inside first request from source 2
Inside second request from source 1

Notice that the request for source 2 was allowed before the second request from source 1, so that source 1's double requests did not starve source 2.