Sunday, July 28, 2013

Erlang Executor

Last time, I wrote a crude blocking queue in Erlang to better understand the actor model and the functional paradigm for managing state. This time, I will be building on the blocking queue to implement a simple executor service that emulates the Java concept using actors. To be clear, the goal of the exercise is not to understand how multithreading concepts from Java can be applied in Erlang, which probably goes against the entire point of using the actor model, but rather to develop a sense for how the familiar Java abstractions would manifest themselves in Erlang. That way, when confronted with a a problem for which the natural solution would be a blocking queue or an executor service, it is easier to see how the solution can be framed in terms of actors. Without further ado, the code:

Again, the public interface consists of just three simple functions: new(), submit(), and get(). These functions create a new executor with a specified number of threads, submit a task, and get the result of a task, respectively. When the executor is created, we initialize a blocking queue and spawn a number of processes corresponding to how many "threads" the client wants, each of which runs the worker() function. As in Java, worker() repeatedly removes tasks off the queue and runs them.

The interesting part of this is how the concept of futures is implemented. A future is a proxy to the result of a task that is submitted to an executor, and its primary feature is the ability to block until the result is available, which is the purpose of the get() function. In Java, this would typically implemented using a semaphore, but here the future is another Erlang process that is spawned when the task is submitted. This process runs the wait() function, which has two phases. First, it blocks until the result of the task is received (the "done" message is sent by the worker that executes the task), and once it has the result it will respond to an arbitrary number of "get" messages that are sent by the get() function, sending back the result. So while the result is not available, calls to get() will block on receiving the "done" message, giving us the equivalent of futures. And that is pretty much all there is to implementing the executor!

The relationship between shared, mutable state and Erlang actors is becoming quite clear from these examples. The restrictions of Erlang imply that, any time you want to deal with shared, mutable state, you must spawn a process which is responsible for managing that state. Anyone who wants to read or modify that state can only do so through messages sent to that process, naturally eliminating any possibility of race conditions and unsynchronized access to data. These are useful guarantees to have as a developer, so the benefits of the actor model and functional programming are clear. With that said, it has definitely been trickier for me to design these programs, having used imperative programming techniques for so many years. But after being plagued by synchronization issues, I'm open to the possibility that trading off the intuitive nature of imperative programming and threads is worthwhile for the superior concurrency model.

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