Node.js and Redis

Question

I am trying to link up a redis database with a Node.js application I am building to be able to store comments about items. I'm using the node_redis library to handle the connection. When I attempt to retrieve the comments out of the database however only "[true]" is returned. For testing purposes I have stuffed everything into one method and I have hardcoded the values in, but I still receive "[true]".

exports.getComment = function (id){

var comments = new Array();

rc.hmset("hosts", "mjr", "1", "another", "23", "home", "1234");

comments.push(rc.hgetall("hosts", function (err, obj) {

    var comment = new Array();

    if(err){
        comment.push("Error");
    } else {
        comment.push(obj);
    }

    return comment;
}));

return comments;

}

Updated the code according to the tutorial and here is the result:

Retrieving the comment:

exports.getComment = function (id, callback){

  rc.hgetall(id, callback);

}

Adding the comment:

exports.addComment = function (id, area, content, author){

//add comment into the database
rc.hmset("comment", 
         "id", id, 
         "area", area, 
         "content", content,
         "author" , author,
         function(error, result) {
            if (error) res.send('Error: ' + error);
         });

//returns nothing

};

Code to render:

var a = [];
require('../lib/annotations').addComment("comment");
require('../lib/annotations').getComment("comment", function(comment){
    a.push(comment)
});
res.json(a);

Chris Maness: please update your question, not my answer :)

Florian Margaine
– Florian Margaine

2012-06-21 20:52:33 +00:00
Commented Jun 21, 2012 at 20:52 — Florian Margaine
– Florian Margaine, Commented Jun 21, 2012 at 20:52

Florian Margaine · Accepted Answer · 2012-06-21 13:53:36Z

2

Node.js is asynchronous. Which means it asynchronously does the redis stuff, and then gets the result back in the callback function.

I suggest you read this tutorial and fully understand it before getting further: http://howtonode.org/node-redis-fun

Basically, this way won't work:

function getComments( id ) {
    var comments = redis.some( action );
    return comments;
}

But it has to be this way:

function getComments( id, callback ) {
    redis.some( action, callback );
}

This way, you use the API like this:

getComments( '1', function( results ) {
    // results are available!
} );

edited Jun 21, 2012 at 13:53

answered Jun 21, 2012 at 13:27

Florian Margaine

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9 Comments

ControlAltDel Over a year ago

In other words, return comments is happening before return comment

Florian Margaine Over a year ago

And return happens before the datas are even set (I'm pretty sure hmset hasn't finished when the return is done).

Chris Maness Over a year ago

So how can I ensure that the hmset happens before the hmget

Florian Margaine Over a year ago

@ChrisManess Read the tutorial and understand it before getting further. You should understand how asynchronous works and easily find out what's wrong in your code.

Florian Margaine Over a year ago

@ChrisManess I edited my answer, hope it's not too hard to grasp the concept.

|

Chris Maness · Accepted Answer · 2012-06-26 14:19:35Z

The problem lays within the actual Redis-Node library when the call to addComment is made as it is below.

require('../lib/annotations').getComment("comment", function(comment){
    a.push(comment)
});

This call is missing an argument in the callback function. The first argument is the error report which should return null if everything is ok, the second is the actual data. So it should be structured like the call below.

require('../lib/annotations').getComment("comment", function(comment){
    a.push(err, comment)
});

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