How does I/O work in Akka?

Question

How does the actor model (in Akka) work when you need to perform I/O (ie. a database operation)?

It is my understanding that a blocking operation will throw an exception (and essentially ruin all concurrency due to the evented nature of Netty, which Akka uses). Hence I would have to use a Future or something similar - however I don't understand the concurrency model.

1. Can 1 actor be processing multiple message simultaneously?
2. If an actor makes a blocking call in a future (ie. future.get()) does that block only the current actor's execution; or will it prevent execution on all actors until the blocking call has completed?
3. If it blocks all execution, how does using a future assist concurrency (ie. wouldn't invoking blocking calls in a future still amount to creating an actor and executing the blocking call)?
4. What is the best way to deal with a multi-staged process (ie. read from the database; call a blocking webservice; read from the database; write to the database) where each step is dependent on the last?

The basic context is this:

• I'm using a Websocket server which will maintain thousands of sessions.
• Each session has some state (ie. authentication details, etc);
• The Javascript client will send a JSON-RPC message to the server, which will pass it to the appropriate session actor, which will execute it and return a result.
• Execution of the RPC call will involve some I/O and blocking calls.
• There will be a large number of concurrent requests (each user will be making a significant amount of requests over the WebSocket connection and there will be a lot of users).

Is there a better way to achieve this?

Answer 1

What Raymond and paradigmatic said, but also, if you want to avoid starving the thread pool, you should wrap any blocking operations in scala.concurrent.blocking.

It's of course best to avoid blocking operations, but sometimes you need to use a library that blocks. If you wrap said code in blocking, it will let the execution context know you may be blocking this thread so it can allocate another one if needed.

The problem is worse than paradigmatic describes since if you have several blocking operations you may end up blocking all threads in the thread pool and have no free threads. You could end up with deadlock if all your threads are blocked on something that won't happen until another actor/future gets scheduled.

Here's an example:

import scala.concurrent.blocking
...

Future {
  val image = blocking { load_image_from_potentially_slow_media() }
  val enhanced = image.enhance()
  blocking {
    if (oracle.queryBetter(image, enhanced)) {
      write_new_image(enhanced)
    }
  }
  enhanced
}

Documentation is here.

Answer 2

Blocking operations will generally not throw exceptions, but waiting on a future (for example by using !! or !!! send methods) can throw a time out exception. That's why you should stick with fire-and-forget as much as possible, use a meaningful time-out value and prefer callbacks when possible.

1. An akka actor cannot explicitly process several messages in a row, but you can play with the throughput value via the config file. The actor will then process several message (i.e. its receive method will be called several times sequentially) if its message queue it's not empty: http://akka.io/docs/akka/1.1.3/scala/dispatchers.html#id5

2. Blocking operations inside an actor will not "block" all actors, but if you share threads among actors (recommended usage), one of the threads of the dispatcher will be blocked until operations resume. So try composing futures as much as possible and beware of the time-out value).

3 and 4. I agree with Raymond answers.

Answer 3

Blocking operations do not throw exceptions in Akka. You can do blocking calls from an Actor (which you probably want to minimize, but thats another story).

1. no, 1 actor instance cannot.
2. It will not block any other actors. You can influence this by using a specific Dispatcher. Futures use the default dispatcher (the global event driven one normally) so it runs on a thread in a pool. You can choose which dispatcher you want to use for your actors (per actor, or for all). I guess if you really wanted to create a problem you might be able to pass exactly the same (thread based) dispatcher to futures and actors, but that would take some intent from your part. I guess if you have a huge number of futures blocking indefinitely and the executorservice has been configured to a fixed amount of threads, you could blow up the executorservice. So a lot of 'ifs'. a f.get blocks only if the Future has not completed yet. It will block the 'current thread' of the Actor from which you call it (if you call it from an Actor, which is not necessary by the way)
3. you do not necessarily have to block. you can use a callback instead of f.get. You can even compose Futures without blocking. check out talk by Viktor on 'the promising future of akka' for more details: http://skillsmatter.com/podcast/scala/talk-by-viktor-klang
4. I would use async communication between the steps (if the steps are meaningful processes on their own), so use an actor for every step, where every actor sends a oneway message to the next, possibly also oneway messages to some other actor that will not block which can supervise the process. This way you could create chains of actors, of which you could make many, in front of it you could put a load balancing actor, so that if one actor blocks in one chain another of the same type might not in the other chain. That would also work for your 'context' question, pass of workload to local actors, chain them up behind a load balancing actor.

As for netty (and I assume you mean Remote Actors, because this is the only thing that netty is used for in Akka), pass of your work as soon as possible to a local actor or a future (with callback) if you are worried about timing or preventing netty to do it's job in some way.