count number of sessions in connect-redis - node.js

Question

I have been struggling to count the number of users or session ids, when using connect-redis for sessions. I've referred to this: http://expressjs.com/guide.html#users-online
var express = require('express');
//var redis = require('redis');
//var db = redis.createClient();
var app = express();
var RedisStore = require('connect-redis')(express);
app.use(express.cookieParser());
app.use(express.session({
store: new RedisStore({
host: 'localhost',
port: 6379,
db: 2,
pass: 'RedisPASS'
}),
secret: '1234567890QWERTY'
}));
app.use(function(req, res, next){
var ua = req.headers['user-agent'];
db.zadd('online', Date.now(), ua, next);
});
app.use(function(req, res, next){
var min = 60 * 1000;
var ago = Date.now() - min;
db.zrevrangebyscore('online', '+inf', ago, function(err, users){
if (err) return next(err);
req.online = users;
next();
});
});
app.get('/', function(req, res) {
db.zadd('online', Date.now(), req.sessionID, next);
});
app.get('/', function(req, res){
res.send(req.online.length + ' users online');
});
app.listen(3000);
Obviously, the above code does not understand the variable db as there is no explicit redis.createClient(); ... Does connect-redis do the redis.createClient internally ?
Please help on how to count the number of session ids with connect-redis.

Answer 1

Yes, connect-redis does use redis.createClient() internally if you only provide it with credentials like you have above. You can also pass it a client directly.
var redisClient = redis.createClient(6379, "127.0.0.1", { auth_pass: "RedisPASS" });
redisClient.select(2);
app.use(express.session({
store: new RedisStore({
client: redisClient
}),
secret: '1234567890QWERTY'
}));
To get the number of sessions you'll want to query redis for the set of keys matching that key prefix. For connect-redis this is "sess:".
redisClient.keys("sess:*", function(error, keys){
console.log("Number of active sessions: ", keys.length);
});
As they state in the redis documentation, the keys command takes linear time so in large systems it's not a good idea to query this frequently. You'd be better off keeping a separate counter that you increment when a user signs in and decrement when a user signs out.