The Exakat.io blog has a post with an interesting method for preventing the execution of multiple instances of a script at once - locking execution with an external indicator (like files, semaphores and streams/sockets).
Like everything, it all started from a simple problem : how to prevent multiple PHP scripts at the same time. And turned into an odyssey of learning, full of evil traps and inglorious victories. In the end, it works, that’s the most satisfying and it possibly matters to no one except me. But "the way is the goal", as said Confucius, so, I decided to share the various findings.
Exakat runs in command line, and it uses a graph database. The database is central to the processing, and it is crucial to avoid running several scripts at the same time : they will write over each other. So, the problem is simple : preventing several instances to run at the same time, on the database. In commandline, there is no web server that may serve as common place between scripts, sharing some memory and implementing a locking system. It requires to use another common ground : the system.
He shares some of the methods he tried to help prevent the simultaneous execution of the Exakat process including:
- file locking using flock
- creating a "lock" file
- making it "crash proof"
- using semaphores
- using a socket for the lock
He describes some of the issues he found when running the tool using locking inside of a Docker container and, finally, the use of sockets and streams to place a "hold" until the script closes (also preventing issues on a crash). He ends the post talking about the "final boss" in his battle for locking support - the handing off of the socket connection to another process between parent and child. The final list in the post is a list of each method he tried, their benefits and downsides (but only in certain situations).