statvent

statvent gives you three things:

1. A developer API for recording metrics in your library or application.
2. A deployer API that writes stats to a named pipe.
3. A simple web service that will read from all named pipes on a host and serve the results as JSON.

The Developer API

It's really simple. There are three functions for recording data.

statvent.incr(name, value=1)

Call it with the name of a stat and it will increment it. If it doesn't exist yet, it will initialize it to the given value (defaults to 1).

It's very useful for keeping tabs on events that happen within your apps.

statvent.set(name, value)

Call it with the name of a stat and a value. The stat will be set to that value.

This function is useful for values that can fluctuate, like number of concurrent users, connections to a database, winning streaks, etc.

statvent.record(name, value, format_func=str.format)

Call it with the name of a stat and a value. By default the name must include a {0} format placeholder. Internally, the value is appended to a deque. When the pipe is read, the stats recorder will calculate some aggregate statistics from the contents of the deque (by default a few percentile breaks and the mean).

If your stat name scheme conflicts with the default ``str.format`` function you can provide your own function to format the stat name to include the calculated percentile labels.

The names of stats just need to be byte strings. You can format them however you want, include whatever punctuation makes you happy, etc. If you want percentiles or other calculated stats (using statvent.record), you'll need to take a bit of extra care when formatting your stat names.

Values can be integers or floats. Be aware that once you use a float, that stat will remain a float. It probably doesn't matter that much, but now you know.

The Deployer API

Notice that there is no mention of an API for reading stats. That's because the creation and consumption of your application's metrics should be decoupled. The decoupling is accomplished by writing the data out to a named pipe.

statvent.start_recorder()

Starts a thread that writes the application stats to a named pipe.

The thread blocks on the open system call won't spend any of your application's resources until another process opens the pipe for reading.

The Named Pipe

By default the pipes are located in /tmp/stats-pipe/ and are named <pid>.stats where <pid> is the UNIX process ID that is writing data into that named pipe. You can change the location where pipes are written/read by setting the statvent.stats.config['pipe_dir'] path. Make sure your processes have permission to write there though.

Each stat is written to its own line in the named pipe. The name of the stat and the current value are separated by a colon : followed by a space. The value follows and is either a floating point value or an integer value. Then there is a newline character. Here's an example:

a.b.c.d: 42
My Nice Stat: 123.45
another[one]: 0

You can peek at the current state of your app by running cat against the named pipe, or have a process that does that regularly and inserts the results into a database to track the data over time.

The Web Service

If you don't want to mess with consuming a weird plain-text format like you get from the named pipe, and you want to collect stats from more than one process on a host, the JSON web service might interest you.

It will sum the values of all stats and serve them up as JSON. Try running python -m statvent.web as a script to see it in action.