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.
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Chunk up IO to keep from blocking the main loop!
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EM.next_tick &blockschedules 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
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Queues (
EM::Queue)- push on, pop off in worker
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Channels (
EM::Channel)- Pub/Sub
- Push messages on. Can subscribe or unsubscribe
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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?)
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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)
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Testing
- What do you do without the event loop?
- Test domain logic
- Then make sure the right stuff is fired
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Exception Handling
- Error handling is hard
- Deeply nested
- Don't want to kill the main loop