Copas - Coroutine Oriented Portable Asynchronous Services for Lua

Copas
Coroutine Oriented Portable Asynchronous Services for Lua

## Installing

You can install Copas using LuaRocks:

    luarocks install copas

## Introduction to Copas

Copas is a dispatcher that can help a lot in the creation of servers based on LuaSocket. Here we present a quick introduction to Copas and how to implement a server with it.

Assuming you know how to implement the desired server protocol, the first thing you have to do in order to create a Copas based server is create a server socket to receive the client connections. To do this you have to bind a host and a port using LuaSocket:

    server = socket.bind(host, port)

Then you have to create a handler function that implements the server protocol. The handler function will be called with a socket for each client connection and you can use `copas.send()` and `copas.receive()` on that socket to exchange data with the client.

For example, a simple echo handler would be:

    function echoHandler(skt)
      while true do
        local data = copas.receive(skt)
        if data == "quit" then
          break
        end
        copas.send(skt, data)
      end
    end

If all you will do with the socket is send and receive data, you may alternatively use `copas.wrap()` to let your code more close to a standard LuaSocket use:

    function echoHandler(skt)
      skt = copas.wrap(skt)
      while true do
        local data = skt:receive()
        if data == "quit" then
          break
        end
        skt:send(data)
      end
    end

To register the server socket with Copas and associate it with the corresponding handler we do:

    copas.addserver(server, echoHandler)

Finally, to start Copas and all the registered servers we just call:

    copas.loop()

As long as every handler uses Copas's `send` and `receive`, simultaneous connections will be handled transparently by Copas for every registered server.

Since Copas is coroutine based, using it within a Lua `pcall` or `xpcall` context does not work with Lua 5.1 yielding. If you need to use any of those functions in your handler we strongly suggest using coxpcall, a coroutine safe version of the Lua 5.1 protected calls. For an example of this usage please check Xavante.

## Why use Copas?

For those who already have a server implemented, here is an explanation of why and how to migrate to Copas. In a typical LuaSocket server usually there is a dispatcher loop like the one below:

    server = socket.bind(host, port)
    while true do
      skt = server:accept()
      handle(skt)
    end

Here `handle` is a function that implements the server protocol using LuaSocket's socket functions:

    function handle(skt)
      ...
      -- gets some data from the client - "the request"
      reqdata = skt:receive(pattern)
      ...
      -- sends some data to the client - "the response"
      skt:send(respdata)
      ...
    end

The problem with that approach is that the dispatcher loop is doing a busy wait and can handle just one connection at a time. To solve the busy waiting we can use LuaSocket's `socket.select()`, like in:

    server = socket.bind(host, port)
    reading = {server}
    while true do
      input = socket.select(reading)
      skt = input:accept()
      handle(skt)
    end

While this helps our CPU usage, the server is still accepting only one client connection at a time. To handle more than one client the server must be able to multitask, and the solution usually involves some kind of threads.

The dispatcher loop then becomes something like:

    server = socket.bind(host, port)
    reading = {server}
    while true do
      input = socket.select(reading)
      skt = input:accept()
      newthread(handle(skt))
    end

where `newthread` is able to create a new thread that executes independently the handler function.

The use of threads in the new loop solves the multitasking problem but may create another. Some platforms does not offer multithreading or maybe you don't want to use threads at all.

If that is the case, using Lua's coroutines may help a lot, and that's exactly what Copas does. Copas implements the dispatcher loop using coroutines so the handlers can multitask without the use of threads.

## Using Copas with an existing server

If you already have a running server using some dispatcher like the previous example, migrating to Copas is quite simple, usually consisting of just three steps.

First each server socket and its corresponding handler function have to be registered with Copas:

    server = socket.bind(host, port)
    copas.addserver(server, handle)

Secondly the server handler has to be adapted to use Copas. One solution is to use Copas `send` and `receive` functions to receive and send data to the client:

    function handle(skt)
      ...
      -- gets some data from the client - "the request"
      reqdata = copas.receive(skt, pattern)
      ...
      -- sends some data to the client - "the response"
      copas.send(skt, respdata)
       ...
    end

The other alternative is to wrap the socket in a Copas socket. This allows your handler code to remain basically the same:

    function handle(skt)
      -- this line may suffice for your handler to work with Copas
      skt = copas.wrap(skt)
      -- now skt behaves like a LuaSocket socket but uses Copas'
      ...
      -- gets some data from the client - "the request"
      reqdata = skt:receive(pattern)
      ...
      -- sends some data to the client - "the response"
      skt:send(respdata)
       ...
    end

Finally, to run the dispatcher infinite loop you just call:

    copas.loop()

During the loop Copas' dispatcher accepts connections from clients and automatically calls the corresponding handler functions.

## Controlling Copas

If you do not want copas to simply enter an infinite loop (maybe you have to respond to events from other sources, such as an user interface), you should have your own loop and just call `copas.step()` at each iteration of the loop:

    while condition do
      copas.step()
      -- processing for other events from your system here
    end

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