ExpressJs res.json : Can't set headers after they are sent

Question

I'm using expressjs, I have a route like this:

router.route('/monitor')
.all(function (req, res, next) {
    next();
}).get(monitor.monitorServers);

And a controller (called monitor, that has monitorServers function called by the above router), here's the complete code for the controller:

    var argv = require('minimist')(process.argv.slice(2));
var path = require("path");
var config = require(path.resolve(argv.conf));
var bunyan = require('bunyan');
var log = require(path.resolve('./lib/logger')).createLogger;
// create the S3 object
var AWS = require('aws-sdk');

var commonFunctions = require(path.resolve('./lib/functions-common'));
var async = require('async');

/**
 * Monitor the different severs needed by the book server (check if they still running and connected to them)
 * @param req
 * @param res
 */
exports.monitorServers = function (req, res) {
    // Redis Clients
    var redisBs = commonFunctions.createRedisBS();
    var redisMapi = commonFunctions.createRedisMapi();
    var s3 = new AWS.S3();
    // default values for the response
    var status = 200;
    var message = "Ok";
    var errorMessage = "There's was an error in the book service, check the log to see which part";
    async.waterfall([
        /**
         * Check Redis server of the book service
         * @param callback
         */
        function (callback) {
            redisBs.on('ready', function (error) {
                if (error) {
                    log.fatal({part: 'Redis'}, error);
                }
                if (!redisBs.connected) {
                    status = 500;
                    message = errorMessage;
                    log.fatal({part: 'Redis'}, 'Book service failed to connect to its Redis Server');
                }
                callback(null);
            });
        },
        /**
         * Check the redis server of Mapi
         * @param callback
         */
        function (callback) {
            //REDIS MAPI
            redisMapi.on('ready', function (error) {
                if (error) {
                    log.fatal({part: 'Redis'}, error);
                }
                if (!redisBs.connected) {
                    status = 500;
                    message = errorMessage;
                    log.fatal({part: 'Redis Mapi'}, 'Book service failed to connect to Redis Mapi');
                }
                callback(null);
            });

        },
        /**
         * Check the S3 bucket connexion
         */
        function () {
            s3.headBucket({Bucket: config.bucketEnc}, function (error) {
                if (error) {
                    status = 500;
                    message = errorMessage;
                    log.fatal({part: 'AWS S3'}, 'Book service failed to connect to AWS S3 bucket ' + error);
                }
                res.status(status);
                res.json({book_service: message});
            });

        }

    ], function (err) {  // in case of unexpected error
        if (err) {
            log.error("There was an error on monitoring " + err);
            res.end();
        }
    });

};

The response is returned correctly from the controller, but from time to time, even when I don't call the route, I get error saying Can't set headers after they are sent The error is fired on line where I put res.json({book_service: message}); Do you have any idea where's the root of the error?

Answer 1

You should handle both success and failure case in the final callback of async.waterfall() rather than in the middle of waterwall process.

/**
 * Check the S3 bucket connexion
 */
function (callback) {
    s3.headBucket({Bucket: config.bucketEnc}, function (error) {
        if (error) {
            status = 500;
            message = errorMessage;
            log.fatal({part: 'AWS S3'}, 'Book service failed to connect to AWS S3 bucket ' + error);
            callback(error);
        }

        // VERY IMPORTANT TO ENSURE WATERFALL PROCESS CAN FINISH!
        callback(null, {book_service: message});
    });
}

// Final callback of async.waterfall()
function (err, data) {  // in case of unexpected error
    if (err) {
        log.error("There was an error on monitoring " + err);
        res.end();
    } else {
        res.json(data);
    }
}