A distributed lock server built on persistent socket connections, with an emphasis on preventing false positives
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"What is pkLockServer?" pkLockServer is a distributed lock server for PHP applications. pkLockServer is intended to replace the use of flock() with a mechanism that is equally able to detect clients that have died, yet also suitable for use where a filesystem that supports flock() is not shared by all servers concerned. "Why not use [x] instead?" pkLockServer relies on persistent socket connections. The use of persistent connections allows us to detect dead clients rather than using a timeout to break locks as is commonly done with Redis, memcached, etc. We like to use locks for mission-critical stuff and long-lived stuff. In both cases a false positive is infinitely worse than a false negative. So timeouts just aren't a great choice for us. YMMV. "How well does it work?" pkLockServer is implemented as an efficient single-threaded server using select(). As such, the performance should be about as good as is possible in a pure PHP implementation. But in our case, we need locks in relatively few cases, but when we need 'em we really need 'em to work. So performance is not the biggest constraint. "How do I test it? And how do I use it?" pkLockServer's pkLockClient and pkLockServer classes have good phpdoc comments, and they are demonstrated by the server.php and testClient.php programs in the bin/ folder. Configure server.php by copying config/settings-example.php to config/settings.php and reviewing the settings there. Then run the programs like this: php server.php [Leave that running; in another terminal window:] php testClient.php Fire up several copies of testClient.php to see how each holds the lock for 10 seconds before yielding to the next waiting client, then lingers 10 additional seconds before actually exiting. Note that, out of the box, server.php ignores connections that are not from 127.0.0.1 (the same machine). You can edit your copy of settings.php to limit connections to any suitable list of IP addresses. You want a limit to avoid DOS attacks. It's a pretty good idea to block the port at your firewall for outsiders too. "What are those pkMessageServer and pkMessageClient classes about?" These are pretty cool actually. pkMessageServer and pkMessageClient implement message-based communication over TCP sockets, so you can conveniently send and receive complete messages - anything that JSON can encode and decode can be sent as a message. pkMessageServer looks for a 'command' key in the message. If the command is 'lock', and a method called 'commandLock' exists, it is invoked (much like writing actions in Symfony). These classes also support RPC (Remote Procedure Calls). pkMessageClient::rpc sends a message and waits for a response. Handy when you need to know if you got that lock or not before proceeding. There are phpdoc comments explaining how to leverage all this if you get excited about writing your own subclasses to implement other applications that call for persistent socket connections between PHP processes. It's pretty neat what you can accomplish when you don't have HTTP tied around your neck. "How do I set this up as a Symfony plugin?" Check it out via svn:externals to plugins/pkLockServerPlugin. The lib folder will automatically be picked up by the Symfony autoloader. "What are the license terms?" Copyright 2012, P'unk Avenue LLC. Released under the MIT License. "How stable is this?" This is an early and exciting release! The test programs sure work though. Put it through its paces for yourself and see.