Hi Dima, Thanks for your time and explanations :-) However, I have the intuition that it will take me more time to implement your idea compare to the builtin feature of PostgreSQL. Nevertheless, I keep your idea in mind in case of I have problems with PostgreSQL. Have a nice day. -- Ludovic Gasc (GMLudo) 2018-04-17 14:17 GMT+02:00 Dima Tisnek <dimaqq@gmail.com>:
Hi Ludovic,
I believe it's relatively straightforward to implement the core functionality, if you can at first reduce it to: * allow only one coro to wait on lock at a given time (i.e. one user per process / event loop) * decide explicitly if you want other coros to continue (I assume so, as blocking entire process would be trivial) * don't care about performance too much :)
Once that's done, you can allow multiple users per event loop by wrapping your inter-process lock in a regular async lock.
Wrt. performance, you can start with a simple client-server implementation, for example where: * single-threaded server listens on some port, accepts 1 connection at a time, writes something on the connection and waits for connection to be closed * each client connects (not informative due to listen backlog) and waits for data, when client gets the data, it has the lock * when client wants to release the lock, it closes the connection, which unblocks the server * socket communication is relatively easy to marry to the event loop :)
If you want high performance (i.e. low latency), you'd probably want to go with futex, but that may prove hard to marry to asyncio internals. I guess locking can always be proxied through a thread, at some cost to performance.
If performance is important, I'd suggest starting with a thread proxy from the start. It could go like this: Each named lock gets own thread (in each process / event loop), a sync lock and condition variable. When a coro want to take the lock, it creates an empty Future, ephemerally takes the sync lock, adds this future to waiters, and signals on the condition variable and awaits this Future. Thread wakes up, validates there's someone in the queue under sync lock, tries to take classical inter-process lock (sysv or file or whatever), and when that succeeds, resolves the future using loop.call_soon_threadsafe(). I'm omitting implementation details, like what if Future is leaked (discarded before it's resolved), how release is orchestrated, etc. The key point is that offloading locking to a dedicated thread allows to reduce original problem to synchronous interprocess locking problem.
Cheers!
On 17 April 2018 at 06:05, Ludovic Gasc <gmludo@gmail.com> wrote:
Hi,
I'm looking for a equivalent of asyncio.Lock (https://docs.python.org/3/library/asyncio-sync.html#asyncio.Lock) but shared between several processes on the same server, because I'm migrating a daemon from mono-worker to multi-worker pattern.
For now, the closest solution in term of API seems aioredlock: https://github.com/joanvila/aioredlock#aioredlock But I'm not a big fan to use polling nor with a timeout because the lock I need is very critical, I prefer to block the code than unlock with timeout.
Do I miss a new awesome library or do you have an easier approach ?
Thanks for your responses. -- Ludovic Gasc (GMLudo)
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