SystemEvent

Introduction

SystemEvent provides a simple synchronization primitive for use across multiple processes. The SystemEvent object emulates the threading.Event API exactly. In addition, simple scripts (evt_set, evt_wait, and evt_clear) are installed for easy usage from shell scripts.

The main reason to use SystemEvent is in situations when you want processes to wait on other processes without the need for polling.

Installation

pip install SystemEvent

Usage

SystemEvent uses named posix semaphores under the hood, so you need to choose event names that are unique to your application. Any event references will use this unique name.

From Python, use it exactly like you would use a threading.Event instances, with the main difference being that you need to give your event a name so that other processes can reference it.

For example, in as many consoles as you like, set up an event and have it wait (the last line will block on each ```wait()`` call):

>>> import SystemEvent
>>> evt = SystemEvent.SystemEvent("my_event")
>>> evt.wait()

Alternatively, you can just run evt_wait my_event from your favorite shell (this is just a small script that does the above almost exactly).

In another console, set the event and note that the first event releases:

>>> import SystemEvent
>>> evt = SystemEvent.SystemEvent("my_event")
>>> evt.set()

All events blocking on "my_event" will be immediately released by this set() call. Subsequent calls to evt.wait() from any process will not block, since the event is now globally latched.

To clear the event (so that calls to evt.wait() will block again), call evt.clear().

As with threading.Event (and multiprocessing.Event) there is also an isSet() method which tells you the current state (but watch out for race conditions when checking it).

Shell scripts

Three shell scripts are provided, with the following usage:

evt_wait <event_name> [timeout_s]

evt_set <event_name>

evt_clear <event_name>

These scripts are thin shells over SystemEvent usage. The timeout_s option on evt_wait is optional, and defaults to infinity.

All scripts have an exit code of 0, unless evt_wait times out, in which case it returns 1.

How does it work?

SystemEvent currently uses a posix semaphore internally. To integrate with other non-python applications, you can just access the same named semaphore. Just be careful that you increment and decrement correctly. Check out the code for details... it is ridiculously small.

License

MIT. See LICENSE file.

TODO

1. Add tests
2. Make it work in Windows, too
3. Remove the posix_ipc requirement