Is it possible to build work queues in Erlang?

Question

I have seen lots of chat examples in Erlang but what about lists, like a work queue? If I want to build a work queue system, like a project management system, is it possible to re-order messages in a process mailbox or do I have to use message priorities? Are there examples of workflow systems built in Erlang?

Answer 1

You cannot reorder messages in process message queues in Erlang.

You can, however do selective receives in which you can receive the message you deem most important first. It's not entirely the same but works for most purposes.

Here's an example:

receive
    {important, Msg} ->
        handle(Msg)
after 0 ->
    ok
end,
receive
    OtherMsg ->
        handle(Msg)
end

It differs from:

receive
    {important, Msg} ->
        handle(Msg);
    OtherMsg ->
        handle(Msg)
end

In that it will always scan the whole message queue for {important, Msg} before continuing handling the rest of the messages. It means that those kinds of messages will always be handled before any others, if they exist. This of course comes at some performance cost (it takes more time scanning the whole queue twice).

Answer 2

Process mailboxes work quite well as-is for job queues.

Just have your messages include sufficient information so that selective receive patterns are easy to write, and you won't feel the need to re-order mailbox contents.

Answer 3

If you do need to reorder messages, you can follow the gatekeeper pattern: reify the mailbox as a separate process. When your original process is ready for another message, the gatekeeper can compute which message to forward, by any rule you choose.