qlimit


A Q promises library helper to restrict the concurrency of q promises

npm install qlimit

Reasoning

Sometimes you need to perform an operation operation over a large number of items. The usual way to handle this when using Q promises is to do something like this:

return Q.all(items.map(function(item) { 
  return performOperationOnItem(item);
});

Each item will then be processed in parallel.

If each execution of performOperationOnItem issues an HTTP request or database operation, you may find that this code quickly overwhelms the backend server, or the HTTP client times out before issuing the operation (by default the nodejs's global HTTP.Agent will issue 5 concurrent requests to any single host, the rest will be queued). Too many concurrent connections may result in timeout errors though.

Using qlimit

It's best explained with a code example:

var qlimit = require('qlimit');
var limit = qlimit(2); // 2 being the maximum concurrency

// Using the same example as above
return Q.all(items.map(limit(function(item) { 
  return performOperationOnItem(item);
}));

qlimit can also be used to limit access to a specific resource, like this

var qlimit = require('qlimit');
var limit = qlimit(2); // 2 being the maximum concurrency

var fetchSomethingFromEasilyOverwhelmedBackendServer = limit(function (id) {
  // Emulating the backend service
  return Q.delay(1000)
    .thenResolve({ hello: 'world' });
});

var promise1 = fetchSomethingFromEasilyOverwhelmedBackendServer(1);
var promise2 = fetchSomethingFromEasilyOverwhelmedBackendServer(2);

In this example, calls can then be made to fetchSomethingFromEasilyOverwhelmedBackendServer with a maximum concurrency of two. Callers do not need to worry about co-ordinating their calls.

Licence

License The MIT License (MIT)

Copyright (c) 2014, Andrew Newdigate

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.