Check if redis variable changed externally in nodejs

Question

Check if redis variable changed externally in nodejs

I need to check whether a redis value has been changed by another process and return it's value if it has been changed and false if it hasn't.

I believe that the watch function may be designed for this but I am struggling to understand how to implement it as the watch always returns OK irrespective of whether the variable was changed or not.

Here's what I have now, but this always returns the variable even if not changed.

function changed(){
    client.watch("tally", function (err,reply) {
        if(reply=="OK") client.get("tally", function(err, reply) { console.log(reply);});
        else console.log('nochange');
    });
    var x = setTimeout(function(){ changed(); },1000);  
}

changed();

node.js
redis

Answer 1

Watch always returns OK; see http://redis.io/commands/watch – it is used only in combination with MULTI/EXEC (transactions) to check EXEC return value.

You can monitor/poll a Redis key value like this:

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

var keytracker;
function changed() {

    client.get("tally", function(err, reply) {
        if (err)
            console.log('A Problem with Redis: ' + err);

        if (reply == keytracker) {
            console.log('nochange');
        } else {
            keytracker = reply;
            console.log(reply);
        }
    });

    setTimeout(function() { changed(); }, 1000);
}

changed();