How can I use arr.forEach to call async JavaScript redis calls?

Question

I'm working with node.js and redis. I've got a redis database with a bunch of keys. Something like this:

user/chris/potion user/pete/potion user/chris/race user/pete/race user/chris/weapon user/pete/weapon

I want to do a redis call which retrieves all user stats, puts the stats into a JS object, then passes it to the client for displaying character stats in the browser. Using javascript I inject the username chris at u into the redis call like this:

KEYS user/u/*

which returns:

1) "user/chris/weapon"
2) "user/chris/race"
3) "user/chris/potion"

Now I can iterate through those results, get the value of each key with GET, and make a javascript object. Seems super simple so I write the code. I quickly run into problems using forEach:

var redis = require('redis');
var client = redis.createClient();

    exports.getUserObject = function(requesteduser, callback) {

        var userstats = {}; // the object to hold the user stats once retrieved from the db
        client.KEYS('user/' + requesteduser + '/*', function(err, replies) {

             replies.forEach(function (reply, i) {
                 client.GET(reply, function(err, value) {
                      // get the key name so we can populate a js object
                      var n = reply.lastIndexOf('/');
                      var key = reply.substring(n + 1, reply.length);
                      userstats[key] = value;

                      console.dir(userstats);

                      callback(null, userstats); // parent expects (err, userstats)
                 });

             });
        });
}

When ran, output is like this:

{ weapon: 'twosword' }
{ weapon: 'twosword', race: 'elf' }
{ weapon: 'twosword', race: 'elf', potion: 'mana' }

callback(null, userstats) is called three times. Calling callback more than once will be problematic, since eventually callback will trigger data being sent data to the client.

I think I know what is happening. client.GET is ran three times because I asked it to. The replies array has three elements, so each time the result comes in for client.GET, the callback is called.

What I need to happen is for the forEach loop to finish iterating through all the redis keys, and once that's done, call the callback once. I tried solving this problem first with promises, then with async, because async has async.each. I got stuck solving the problem with both. I'm back to square one now, I'm convinced I have to do something different with forEach to make progress.

How can I accomplish this?

Would moving callback(null, userstats); outside of the forEach not solve the problem? — Greg
– Greg, Commented Mar 22, 2014 at 2:58
@Greg, That was one of the first things I tried but because client.GET(?) is an asynchronous function,the callback gets called before client.GET returns the first value. I end up with an empty userstats object. — Chris Grimmett
– Chris Grimmett, Commented Mar 22, 2014 at 3:05

Greg · Accepted Answer · 2014-03-22 03:12:47Z

2

Since you're iterating over replies, you can check when you've reached the last element and only call callback in that instance.

client.KEYS('user/' + requesteduser + '/*', function(err, replies) {

         replies.forEach(function (reply, i) {
             client.GET(reply, function(err, value) {
                  // get the key name so we can populate a js object
                  var n = reply.lastIndexOf('/');
                  var key = reply.substring(n + 1, reply.length);
                  userstats[key] = value;

                  console.dir(userstats);

                  if (i == replies.length-1) callback(null, userstats); // parent expects (err, userstats)
             });

         });
    });

answered Mar 22, 2014 at 3:12

Greg

2,1731 gold badge22 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rodrigo Salinas · Accepted Answer · 2015-04-08 13:40:57Z

I know it's a little late to help @Grimtech but I'll leave my opinion for other people that arrives here eventually:

First of all I rather to model @Grimtech's problem differently. Something like this:

client.hmset("user:chris", "weapon", "some weapon", "race", "some race", "potion", "some potion");
client.hmset("user:pete", "weapon", "same weapon", "race", "other race", "potion", "other potion");

Then I would have a method in Node returning a json like the following, assuming that I can have more than one key starting with "user:chris":

router.get('/users/:user_id', function(req, res, next) {
    var response = [];
    var client = redis.createClient();
    client.keys("user:" + req.params.user_id + "*", function (err, users) {
        users.forEach(function (key, pos) {
            client.hgetall(key, function (err, user) {
                response.push(user);
                if (pos === users.length - 1) {
                    res.json(response);
                    client.quit();
                }
            });
        });
    });
});

The if (pos === users.length - 1) solves the async issue.

The json returned will have all the "user:chris" attributes so yo can do whatever you want in the client browser.

Collectives™ on Stack Overflow

How can I use arr.forEach to call async JavaScript redis calls?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related