Node Patterns - Databases by Teixeira Pedro

Author:Teixeira, Pedro
Language: eng
Format: epub
Published: 2015-04-08T04:00:00+00:00

Good, all looks normal. Now let’s try replacing the event emitter by one of our distributed emitters:

distributed_event_emitter_race.js:

var DistributedEmitter = require('./distributed_emitter'); var emitter = DistributedEmitter(); emitter.on('some event', function() { console.log('some event happened'); }); emitter.emit('some event', 'some payload');

Let’s try to run this version then:

$ node distributed_event_emitter_race.js

The process just exits without outputting anything. This means that, when we call the on method, the distributed emitter will subscribe to the channel for the first time. But that envolves some I/O, so this is not done immediately: the command has to go to the network layer, has to be received, parsed and executed by Redis, and then a response comes back to Node. Before all this I/O even happened, though, we emit an event (the last line of the previous file). This event also involves I/O, which is also asynchronous. What happens is that both commands are racing to get to Redis using two different client connections. If the PUBLISH command reaches Redis before the SUBSCRIBE command, our client will never see that event.

To see exactly the sequence of events in Redis, we can use a little trick using the Redis command-line client bundled with Redis. With it you can monitor the Redis server to inspect which commands are being issued:

$ redis-cli monitor OK

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