Advanced EventMachine

We use EventMachine heavily in production. It is handling uploads to S3, managing thousands of messages a second or distributing agent workload. This taught us a load about EventMachine and some weird> corner-cases. I want to about such advanced EventMachine topics and shared some use-cases and experiences from the trenches.

• Chunk up IO to keep from blocking the main loop!

• EM.next_tick &block schedules your block (see also EM.defer)

• Deferrables (EM::Deferrable)

  • Good for allowing EM style concurrency in your classes (callbacks, errbacks)
  • benjaminoakes: I'm not sure I understand how Deferrables are different from Promises or Futures. #EventMachine #RubyConf http://t.co/83bxGvJQ

• Queues (EM::Queue)

  • push on, pop off in worker

• Channels (EM::Channel)

  • Pub/Sub
  • Push messages on. Can subscribe or unsubscribe

• Iterator (EM::Iterator)

  • Close to the sequential stuff in Ruby stdlib
  • Kinda MapReducey.
    • "map": what you do in individual steps
    • "reduce": what you do when you get all the results (e.g. which is the biggest?)

• Fibers & EM-Synchrony

  • Helps with "callback hell" (lots of nesting)
  • Ruby 1.9+: "lightweight cooperative concurrency"
  • Let's you write synchronous-looking code that actually runs asynchronoulsy
  • See also: Goliath (lets you do async Rack)

• Testing

  • What do you do without the event loop?
  • Test domain logic
  • Then make sure the right stuff is fired

• Exception Handling

  • Error handling is hard
  • Deeply nested
  • Don't want to kill the main loop