From Redis Keyspace Notifications

Keyspace notifications allow clients to subscribe to Pub/Sub channels in order to receive events affecting the Redis data set in some way.

Examples of events that can be received are:

All the commands affecting a given key.
All the keys receiving an LPUSH operation.
All the keys expiring in the database 0.

Events are delivered using the normal Pub/Sub layer of Redis, so clients implementing Pub/Sub are able to use this feature without modifications.

So, if you need just pub/sub, there is no need of extra configuration regarding Keyspace Notifications

You forgot to subscribe the subscriber client to a specific channel. Furthermore, if you want to watch for all events, you need to use pattern-based subscription.

You may want to do something like this (untested):

subscriber.on("pmessage", function (pattern, channel, message) {
    console.log("("+  pattern +")" + " client received message on " + channel + ": " + message);
    switch (channel) {
        // blah blah blah
        // ...
    }
});
subscriber.psubscribe(''__key*__:*')

See more information in the Redis documentation, and in node_redis examples.

Update:

Here is an example to illustrate the difference between channel subcription and pattern subscription. Proper error handling is omitted for brevity sake.

var redis = require("redis");

var client = redis.createClient('6379','127.0.0.1');
var subscriber1 = redis.createClient('6379','127.0.0.1');
var subscriber2 = redis.createClient('6379','127.0.0.1');

// First subscriber listens only to events occurring for key mykey
function S1(next) {
    subscriber1.on('message', function(channel, msg) {
        console.log( "S1: received on "+channel+" event "+msg )
    });
    subscriber1.subscribe( "__keyspace@0__:mykey", function (err) {
        next();
    });
}

// Second subscriber listens to events occuring for ALL keys
function S2(next) {
    subscriber2.on('pmessage', function(pattern,channel, msg) {
        console.log( "S2: received on "+channel+" event "+msg )
    });
    subscriber2.psubscribe( "__keyspace@0__:*", function (err) {
        next();
    });
}

// Do something with keys mykey and anotherkey
function do_something() {
    client.set("mykey","example", function( err ) {
        client.set("mykey", "another example", function( err ) {
            client.del("mykey", function( err ) {
                client.set("anotherkey","example", function( err ) {
                    client.del("anotherkey");
                });
            });
        });
    });
}

// Here we go
S1( function () {
    S2( function () {
        do_something();
    });
});

The result of this script is:

S1: received on __keyspace@0__:mykey event set
S2: received on __keyspace@0__:mykey event set
S2: received on __keyspace@0__:mykey event set
S1: received on __keyspace@0__:mykey event set
S1: received on __keyspace@0__:mykey event del
S2: received on __keyspace@0__:mykey event del
S2: received on __keyspace@0__:anotherkey event set
S2: received on __keyspace@0__:anotherkey event del

You can see that first subscriber received only events for mykey, while second subscriber receives events for all keys.

Yes, it is possible. You need to use Redis Keyspace Notifications.

See especially the Events generated by different commands part. You probably want to be notified when the SET command is used :

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

// enable notify-keyspace-events for all kind of events (can be refined)
client_redis.config('set','notify-keyspace-events','KEA');

client_redis.subscribe('__keyevent@0__:set');
// you can target a specific key with a second parameter
// example, client_redis.subscribe('__keyevent@0__:set', 'mykey')

client_redis.on('message', function(channel, key) {
  // do what you want when a value is updated
});

I've finally figured it out, it works with client.psubscribe and client.on("pmessage",...).

You can use RedisGears to process keyspace notification and get both key and value.

You can write your processing code in python and register it in Redis. e.g. Capture each keyspace event and store to a Stream

GearsBuilder() \
.foreach(lambda x: execute('XADD', "notifications-stream", '*', *sum([[k,v] for k,v in x.items()],[]))) \
.register(prefix="person:*", eventTypes=['hset', 'hmset'], mode='sync')

You can read more about this example here: https://oss.redis.com/redisgears/examples.html#reliable-keyspace-notification