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How does one do proper concurrency in Python? As in separate "processes" running with encapsulation, fault tolerance, etc. For example, how does one bundle up a

• andai 3y ago
  Something something libuv?
• da39a3ee 3y ago
  Can you give an example of what you consider a good solution in a different language? That will help us see exactly what you're looking for and how/whether it might be achieved in Python.
  • bmitc 3y ago
    Erlang and Elixir. :)
    • staticautomatic 3y ago
      Well RabbitMQ is built in Erlang so there you go :)
      • bmitc 3y ago
        But RabbitMQ is just a messaging bus, isn't it? That doesn't handle the processes (operating system processes or virtual processes) actually processing the messages and those processes handling fault-tolerance for the things they're connected to.
        • staticautomatic 3y ago
          That is correct. Honestly I’m not sure this is a good use case for Python but it’s definitely possible. I’ve used the RabbitMQ + gevent + multiprocessing pattern in the past and it works but I find the code extremely hard to reason about. If I were doing it again from scratch I’d probably choose another language with better concurrency primitives.
          • bmitc 3y ago
            > Honestly I’m not sure this is a good use case for Python but it’s definitely possible It definitely isn't, but this is a new role with a rushed timeline in a place dominated by Python. > and it works but I find the code extremely hard to reason about This is also a major concern of mine.
            • staticautomatic 3y ago
              Rather than handling the multiprocessing and message passing yourself it would be much easier to use celery + gevent and let celery do the work of spinning up processes for executing the tasks. It may feel limiting but my advice would be to keep all the celery job queue stuff isolated from your server, especially if you are using an async web framework. Have your web server just put all the jobs in the celery queue and let it handle executing them, regardless of whether they’re cpu or io-bound. If you try to optimize too much by doing something like leaning on celery for cpu-bound tasks but letting your web server handle the io-bound ones you’re going to be in for a world of hurt when it comes to both debugging and enforcing the order of execution. Celery has its warts but you’ll at least know where in the system your problem is and have reasonably good control over the pipeline.
              • bmitc 3y ago
                Can Celery help handle state internal to the workers? What about using Pkykka alongside Pyro such that each Pyro remote object is actually a Pykka actor? Such that Pyro allows splitting workers across separate Python OS processes and the "messaging" while Pykka handles the internal state.
    • da39a3ee 3y ago
      That was what I was worried you'd say. So it doesn't seem like your question is about Python at all, and all your criticism of Python seems entirely unjustified. What you're actually saying is "I have to work in a language other than Erlang/Elixir and I don't like it" But Erlang/Elixir are defined by the sort of concurrency model that you say you want, so as far as I can see your question would be just the same if you were asking about Java or Go. If you have a question like "I can do this in Java or Go. How do I do it in Python?" Then people might be able to help. But please stop with the criticism of Python for not being Erlang/Elixir, since Java and Go also are not that.
      • bmitc 3y ago
        Do you have any particularly helpful advice? This is honestly pretty frustrating trying to build a system like this in Python. If you think asyncio, coroutines, tasks, multiprocessing, threads, subprocesses, etc. are simple and understandable in Python, then I would love for someone to explain them to me in a way that is manageable. And no, the question would not be the same. Java and Go have proper concurrency substrates. Python does not.
• Siecje 3y ago
  Use asyncio. Bind to a socket, set blocking False, then asyncio.run(on_new_connection(sock)). Inside that coroutine get the loop and await loop.sock_accept(sock) And then asyncio.create_task(on_connection_data(connection)). The only gotcha is you need to keep a reference to that task so it doesn't get garbage collected.
  • bmitc 3y ago
    Thank you. Do you have more information regarding this? Why do I await the socket acceptance? How do I handle the disconnect? How do I send messages into and out of the coroutine?
  • staticautomatic 3y ago
    Get garbage collected by what? Doesn’t Python use reference counting?
    • d0mine 3y ago
      asyncio doesn't store strong references to tasks. It is your responsibility to keep the ref: https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task https://docs.python.org/3/library/asyncio-task.html#asyncio.... Consider organizing the code using TaskGroup.
    • jph00 3y ago
      Python has a GC.
    • foresto 3y ago
      The reference implementation (CPython) does use reference counting, but that is not its only approach to garbage collection. "The default build implementation is a generational collector. The free-threaded build is non-generational; each collection scans the entire heap." https://devguide.python.org/internals/garbage-collector/ https://devguide.python.org/internals/garbage-collector/ https://docs.python.org/3/library/gc.html https://docs.python.org/3/library/gc.html (And as someone else pointed out, asyncio's event loop keeps only weak references to tasks, so the GC implementation doesn't really matter here.)
• jononor 3y ago
  If you are I/O bound then asyncio or good old fashioned gevent will do great. If you are CPU bound, then use multiprocessing. If you need to accept "jobs" from elsewhere, use RabbitMQ (with or without Celery). If you have mixed CPU/IO workload that fits the worker pattern, then you would do all 3. At the top level you have a RabbitMQ consumer, fetching jobs from a remote queue and then putting these into a multiprocessing queue processed by N=~cpucount processes. And each of these use asyncio/gevent to do their work.
  • bmitc 3y ago
    So do you recommend a single Python process running asyncio or multiprocessing or both? Or do people normally split these things amongst several Python processes? My understanding is that multiprocessing creates multiple interpreters but that it still comes across some GIL issues if all under the same Python process. I am in general quite comfortable with the actor model, and I would ideally use Erlang/Elixir here, but I can't for various reasons.
    • jononor 3y ago
      Single process with async if I/O bound. Multiprocessing if CPU bound. Both if the workload is mixed.
  • varispeed 3y ago
    Why some people are extremely averse to RabbitMQ? I saw that at one company having RabbitMQ / Celery setup - every time a new software engineer comes in, they complain about RabbitMQ and ask why would company use it. The infrastructure was running like this without hiccups for years. At one point company has let go of many experienced engineers and this time one developer found some issue with the code and blamed it on rabbit as it was locking the queue. There were no more senior developers to contest it, so he convinced manager to swap it out for Redis. He took about two months to rewrite it. Surprise, the same issue existed on Redis. The Redis solution works fine, but has its own limitations...
• sn9 3y ago
  This talk on concurrency in Python by Ray Hettinger is quite good: https://www.youtube.com/watch?v=9zinZmE3Ogk https://www.youtube.com/watch?v=9zinZmE3Ogk