Erlang：什么时候产生一个新进程是合乎逻辑的？ 什么时候不？

Question

If we have really heavy-processes system where process spawning is made for some kind of distribution of load - that's clear.

If we are talking about web-server : it's a good idea to spawn a new proccess for each connection, because then can be distributed. But what else? A single process for Model, View and Controller? Sounds strange, because they all run in a "liner" way, so it can not be good paralleled and we only get overhead on swapping. Also, those "Model, View and Controller" are so light, so they can stay in a single process, isn't it?

So, where is it good to spawn a new process excepting "new connection" situation.

Thank you in advice.

Answer 1

In general, it's anywhere you have a shared resource to manage. It may be a socket, or a database connection, but it may also be some shared in-memory data, or a state machine of some kind.

You may also want to do parallel processing of a list of values (see pmap).

To your "swapping" point you should know that Erlang processes do no use op-sys facilities for scheduling, and scheduling is all but free.

In the specific case of a web-application server, I understand your question. If you are writing a conventional web application with very little share state. Your web framework probably already handles caching and session state and such (these facilities will spawn process).

We are all highly indoctrinated into this stateless web application model. We have all been told since we were pups the stateful systems are hard to develop and they don't scale. I think you will find that there are those that are challenging that. As browser support for WebSockets improve, and with server-side language like Erlang and Clojure providing scalable platforms with safe state management, there will be those who are able to make more interactive web-applications. As an extreme example, could you image WoW as a web application?

Answer 2

One reason to spawn a new process for each connection is that it makes programming the connections much simpler. As a process only handles one connection doing things like having blocking access to data-bases, long polling or streaming becomes much easier. That this process blocks will not affect any other connections.

In Erlang the general "rule" is that you use processes to model concurrent activity and to manage shared resources. Processes are the fundamental way for structuring your system.