Concurrently route requests

README.md

@@ -123,6 +123,18 @@ Response with content only.
 }
 ```
 
+### Concurrency
+By default, `multifetch` will process each request sequentially, waiting until a request has been processed and fully piped out before it processes the next request.
+
+If the response to each of your requests is small but takes a long time to fetch (e.g. heavy database queries), `multifetch` supports concurrently processing requests.
+
+Passing `concurrency: N` as an option allows you to control the number of concurrent requests being processed at any one time:
+
+```javascript
+app.get('/api/multifetch', multifetch({concurrency: 5}));
+```
+In the above case, 5 requests would be routed through express concurrently, and the response of each is placed in a queue to be streamed out to the client sequentially.
+
 License
 -------
 

package.json

@@ -8,6 +8,7 @@
     "test": "mocha"
   },
   "dependencies": {
+    "async": "~0.9.0",
     "pump": "~1.0.0",
     "extend": "~2.0.0"
   },

source/index.js

@@ -2,6 +2,7 @@ var url = require('url');
 
 var pump = require('pump');
 var extend = require('extend');
+var async = require('async');
 
 var JsonStream = require('./json');
 var NullifyStream = require('./nullify');
@@ -68,20 +69,26 @@ var fetchBare = function(request, response) {
 	return nullify;
 };
 
-var create = function(options, callback) {
-	if(!callback && typeof options === 'function') {
-		callback = options;
+var endStream = function (jsonStream, error) {
+	jsonStream.writeObject('_error', error);
+	jsonStream.end();
+};
+
+var create = function(options, prefetch) {
+	if(!prefetch && typeof options === 'function') {
+		prefetch = options;
 		options = {};
 	}
 
-	options = options || {};
 
+	options = options || {};
 	var ignore = options.ignore || [];
 	var headers = options.headers !== undefined ? options.headers : true;
+	var concurrency = options.concurrency || 1; // Defaults to sequential fetching
 
 	var fetch = headers ? fetchWithHeaders : fetchBare;
 
-	callback = callback || noopCallback;
+	prefetch = prefetch || noopCallback;
 
 	return function(request, response, next) {
 		var app = request.app;
@@ -95,41 +102,65 @@ var create = function(options, callback) {
 
 		pump(json, response);
 
-		(function loop() {
-			var key = keys.pop();
-
-			if(!key) {
-				json.writeObject('_error', error);
-				return json.end();
-			}
-
-			var messages = createMessages(request, query[key]);
+		// Exit early if there is nothing to fetch.
+		if(keys.length === 0) {
+			return endStream(json, error);
+		}
 
-			var write = function(prevent) {
-				if(prevent) {
-					return loop();
+		// The resource queue processes resource streams sequentially.
+		var resourceQueue = async.queue(function worker(task, callback) {
+			pump(task.resource, json.createObjectStream(task.key), function(err) {
+				if(err) {
+					json.destroy();
+					return callback(err);
 				}
+				if(!(/2\d\d/).test(task.response.statusCode)) {
+					error = true;
+				}
+				callback();
+			});
+		}, 1);
 
-				var resource = fetch(messages.request, messages.response);
-
-				pump(resource, json.createObjectStream(key), function(err) {
-					if(err) {
-						return json.destroy();
-					}
-					if(!(/2\d\d/).test(messages.response.statusCode)) {
-						error = true;
-					}
-
-					loop();
-				});
-
-				app(messages.request, messages.response, function(err) {
-					json.destroy();
+		// Asynchronously fetch the resource for a key and push the resulting
+		// stream into the resource queue.
+		var fetchResource = function(key, callback) {
+			var messages = createMessages(request, query[key]);
+			process.nextTick(function() {
+				prefetch(request, messages.request, function(prevent) {
+					if (prevent) return callback();
+
+					var resource = fetch(messages.request, messages.response);
+					var task = {
+						resource: resource,
+						request: messages.request,
+						response: messages.response,
+						key: key
+					};
+
+					app(messages.request, messages.response, function() {
+						resourceQueue.kill();
+						json.destroy();
+					});
+
+					// Callback is called once the stream for this resource has
+					// been fully piped out to the client.
+					resourceQueue.push(task, callback);
 				});
-			};
-
-			callback(request, messages.request, write);
-		}());
+			});
+		};
+
+		// Fire off all requests and push the resulting streams into a queue to
+		// be processed
+		async.eachLimit(keys, concurrency, fetchResource, function(err) {
+			if(resourceQueue.idle()) {
+				endStream(json, error);
+			} else {
+				// Called once all streams have been fully pumped out to the client.
+				resourceQueue.drain = function() {
+					endStream(json, error);
+				};
+			}
+		});
 	};
 };
 

test/integration/multifetch.options.spec.js

@@ -238,4 +238,85 @@ describe('multifetch.options', function() {
 				});
 		});
 	});
+
+
+	describe('concurrent fetching', function() {
+
+		before(function(done) {
+			server = helper.server();
+
+			server.get('/api/multifetch', multifetch({ concurrency: 5 }));
+			server = server.listen(helper.port, done);
+		});
+
+		after(function(done) {
+			server.close(done);
+		});
+
+		describe('fetch multiple resources', function() {
+			before(function(done) {
+				request.get({
+					url: helper.url('/api/multifetch'),
+					qs: {
+						api: '/api',
+						user_1: '/api/users/user_1',
+						user_2: '/api/users/user_2',
+						user_3: '/api/users/user_3',
+						readme: '/README.md'
+					},
+					json: true
+				}, function(err, _, result) {
+					body = result;
+					done(err);
+				});
+			});
+
+			it('should be successful response', function() {
+				chai.expect(body).to.have.property('_error', false);
+			});
+
+			it('should fetch all resources', function() {
+				chai.expect(body)
+					.to.have.property('api')
+					.to.have.property('statusCode', 200);
+
+				chai.expect(body)
+					.to.have.property('user_1')
+					.to.have.property('statusCode', 200);
+
+				chai.expect(body)
+					.to.have.property('user_2')
+					.to.have.property('statusCode', 200);
+
+				chai.expect(body)
+					.to.have.property('user_3')
+					.to.have.property('statusCode', 200);
+
+				chai.expect(body)
+					.to.have.property('readme')
+					.to.have.property('statusCode', 200);
+			});
+		});
+
+		describe('hang up on bad request', function() {
+			var err;
+
+			before(function(done) {
+				request.get({
+					url: helper.url('/api/multifetch'),
+					qs: {
+						bad: '/api/not_found',
+					},
+					json: true
+				}, function(result) {
+					err = result;
+					done();
+				});
+			});
+
+			it('should emit an error', function() {
+				chai.expect(err).to.be.defined;
+			});
+		});
+	});
 });