luaproc - A concurrent programming library for Lua
luaproc is a Lua extension library for concurrent programming. This text provides some background information and also serves as s reference manual for the library. The library is available under the same terms and conditions as the Lua language, the MIT license. The idea is that if you can use Lua in a project, you should also be able to use luaproc.
Lua natively supports cooperative multithreading by means of coroutines. However, coroutines in Lua cannot be executed in parallel. luaproc overcomes that restriction by building on the proposal and sample implementation presented in Programming in Lua (chapter 30). It uses coroutines and multiple independent states in Lua to implement Lua processes, which are user threads comprised of Lua code that have no shared data. Lua processes are executed by workers, which are system threads implemented with POSIX threads (pthreads), and thus can run in parallel.
Communication between Lua processes relies exclusively on message passing. Each message can carry a tuple of atomic Lua values (strings, numbers, booleans and nil). More complex types must be encoded somehow -- for instance by using strings of Lua code that when executed return such a type. Message addressing is based on communication channels, which are decoupled from Lua processes and must be explicitly created.
Sending a message is always a synchronous operation, i.e., the send operation only returns after a message has been received by another Lua process or if an error occurs (such as trying to send a message to a non-existent channel). Receiving a message, on the other hand, can be a synchronous or asynchronous operation. In synchronous mode, a call to the receive operation only returns after a message has been received or if an error occurs. In asynchronous mode, a call to the receive operation returns immediately and indicates if a message was received or not.
If a Lua process tries to send a message to a channel where there are no Lua processes waiting to receive a message, its execution is suspended until a matching receive occurs or the channel is destroyed. The same happens if a Lua process tries to synchronously receive a message from a channel where there are no Lua processes waiting to send a message.
luaproc also offers an optional facility to recycle Lua processes. Recycling consists of reusing states from finished Lua processes, instead of destroying them. When recycling is enabled, a new Lua process can be created by loading its code in a previously used state from a finished Lua process, instead of creating a new state.
luaproc is compatible with Lua 5.1, 5.2 and 5.3.
luaproc.newproc( string lua_code )
luaproc.newproc( function f )
Creates a new Lua process to run the specified string of Lua code or the
specified Lua function. Returns true if successful or nil and an error message
if failed. The only libraries loaded in new Lua processes are luaproc itself and
the standard Lua base and package libraries. The remaining standard Lua
libraries (io, os, table, string, math, debug, coroutine and utf8) are
pre-registered and can be loaded with a call to the standard Lua function
luaproc.setnumworkers( int number_of_workers )
Sets the number of active workers (pthreads) to n (default = 1, minimum = 1). Creates and destroys workers as needed, depending on the current number of active workers. No return, raises error if worker could not be created.
Returns the number of active workers (pthreads).
Waits until all Lua processes have finished, then continues program execution. It only makes sense to call this function from the main Lua script. Moreover, this function is implicitly called when the main Lua script finishes executing. No return.
luaproc.recycle( int maxrecycle )
Sets the maximum number of Lua processes to recycle. Returns true if successful or nil and an error message if failed. The default number is zero, i.e., no Lua processes are recycled.
luaproc.send( string channel_name, msg1, [msg2], [msg3], [...] )
Sends a message (tuple of boolean, nil, number or string values) to a channel. Returns true if successful or nil and an error message if failed. Suspends execution of the calling Lua process if there is no matching receive.
luaproc.receive( string channel_name, [boolean asynchronous] )
Receives a message (tuple of boolean, nil, number or string values) from a channel. Returns received values if successful or nil and an error message if failed. Suspends execution of the calling Lua process if there is no matching receive and the async (boolean) flag is not set. The async flag, by default, is not set.
luaproc.newchannel( string channel_name )
Creates a new channel identified by string name. Returns true if successful or nil and an error message if failed.
luaproc.delchannel( string channel_name )
Destroys a channel identified by string name. Returns true if successful or nil and an error message if failed. Lua processes waiting to send or receive messages on destroyed channels have their execution resumed and receive an error message indicating the channel was destroyed.
A paper about luaproc -- Exploring Lua for Concurrent Programming -- was published in the Journal of Universal Computer Science and is available here and here. Some information in the paper is already outdated, but it still provides a good overview of the library and some of its design choices.
A tech report about concurrency in Lua, which uses luaproc as part of a case study, is also available here.
Finally, a paper about an experiment to port luaproc to use Transactional Memory instead of the standard POSIX Threads synchronization constructs, published as a part of the 8th ACM SIGPLAN Workshop on Transactional Computing, can be found here.
GitHub source repository: https://github.com/askyrme/luaproc
Copyright © 2008-2015 Alexandre Skyrme, Noemi Rodriguez, Roberto Ierusalimschy. All rights reserved.
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