Add multi-process support

[brian-brazil]

This works by having a shelve per process that are continously updated
with metrics, and a collector that reads from them.

Histogram buckets need accumulation and special handling for _count,
and for gauges we need to offer a number of options.

Fixes #30

[dmclain]

Rather than hard coding the choice of _ValueClass here, could the class be defined by the users and loaded from anywhere on the path? #70 could be implemented in a separate package entirely and be developed/updated on it's own schedule. The shelf implementation can certainly ship with client_python, but users of other multi-process python servers could be free to experiment without needing to land code here.

Something like the following, shamelessly stolen from django/utils/module_loading.py with error handling stripped dangerously:

module_path, class_name = os.environ.get(PROMETHEUS_VALUE_CLASS, 'prometheus_client.core._MutexValue').rsplit('.', 1)
module = import_module(module_path)
_ValueClass = getattr(module, class_name)

Then if #70 was in it's own package prometheus_uwsgi, you might start the server with PROMETHEUS_VALUE_CLASS=prometheus_uwsgi.UwsgiCacheValue or PROMETHEUS_VALUE_CLASS=prometheus_client.core.MultiProcessValueandprometheus_multiproc_dir=/path/to/multiproc_dir` to use the shelf implementation described in this PR (removing the _ since it would become part of the public API)

Those _ValueClass implementations would be responsible for configuring themselves from the environment or wherever, which feels non-pythonic. Somehow configuring the system explicitly at process startup seems like the best path, and the developers could pass in whatever class they want, but I haven't dug in enough to the rest of the implementation to suggest what I would want that API to look like.

[brian-brazil]

This seems like a reasonable idea.

Somehow configuring the system explicitly at process startup seems like the best path

That won't work, as by the time your code is running all this code has likely already been imported and many metrics already exist. That's why I'm using environment variables, as they're set at exec() time before any python code has a chance to run.

[bendemaree]

That won't work, as by the time your code is running all this code has likely already been imported and many metrics already exist.

That's true assuming you want to set the _ValueClass once and only once and never have it specified in code. You could circumvent this by using a client pattern or parameterizing metric instantiation with a _ValueClass as well. I think it would make the interface nicer to use the environment variable by default, but also allow:

REQUEST_TIME = Summary('request_processing_seconds', 'Time spent processing request', value_class=prometheus_client.ShelveValue)

or something of the sort.

[tlc]

What is the status of this? Is it going to be pulled?

I need the functionality and will have to dl the branch if not.

Thanks!

[brian-brazil]

The core functionality is fine, however the performance isn't good enough to be used in production. I haven't had the time to spare to find a better backing store for that.

[jonashaag]

FWIW, I started to work on performance improvement of your branch last week and simply went with JSON instead of shelve/BSDdb, which also gave an improvement of a few orders of magnitude. (I don't know the exact number right now but it was somewhere around a 2x slowdown compared to non-multiprocessing.) Maybe that's an alternative that involves less custom code.

[brian-brazil]

That's about the same slowdown as this code, which is presumably dominated by the 2 mutexes in use (why does using a mutex take most of a microsecond?). Hopefully we'll get that faster some time, but I doubt you got too much better than the 80 lines of code I managed it in.

[jonashaag]

I doubt you got too much better than the 80 lines of code I managed it in.

Sorry, I did not mean to say that your code was bad or something. Actually, the binary format is very simple and the code is easy to understand. My point is that if it does not make a difference in terms of performance, maybe it's more future-proof and pragmatic to simply go with JSON -- if only so that future readers of the code don't have to worry about learning about a new serialisation format. It's not a big deal at this point, but it might become one if features are added to serialisation format; that's what I'm saying.

[brian-brazil]

Let's see where it goes. It's all self-contained, and we don't need to worry about backwards compatibility. The tricky stuff is all in the filenames anyway.