RateLimitingFilter

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The RateLimitingFilter is a filter for the Python logging system that allows you to restrict the rate at which messages can pass through your logging handlers.

The filter can be useful if you're using a handler such as Python's logging.handlers.SMTPHandler to send error notification emails. Error notification emails provide a useful means of keeping an eye on the health of a running system, but these emails have the potential to overload a mailbox if they start arriving in quick succession due to some kind of critical failure.

The RateLimitingFilter can help prevent mailbox overload by throttling messages based on a configurable rate, whilst allowing for periodic bursts of messages which can be a useful indicator that something somewhere has broken.

Compatibility

• Python 2.7, 3.4+

Installing

::

$ pip install ratelimitingfilter

or

::

$ git clone https://github.com/wkeeling/ratelimitingfilter.git
$ cd ratelimitingfilter
$ python setup.py install

Usage

You can rate-limit a logging handler simply by creating a new instance of the RateLimitingFilter and adding it to the handler:

.. code:: python

from ratelimitingfilter import RateLimitingFilter

...

ratelimit = RateLimitingFilter()
handler.addFilter(ratelimit)

Creating an instance of the RateLimitingFilter without any arguments like in the example above will restrict the flow of messages to 1 every 30 seconds.

You can customize the flow rate by supplying your own values for the rate, per and burst attributes. For example, to allow a rate of 1 message every 2 minutes with a periodic burst of up to 5 messages:

.. code:: python

ratelimit = RateLimitingFilter(rate=1, per=120, burst=5)
handler.addFilter(ratelimit)

SMTPHandler Example

A typical use case may be to throttle error notification emails sent by
the ``logging.handlers.SMTPHandler``.

Here's an example of how you might set that up:

.. code:: python

    import logging.handlers
    import time

    from ratelimitingfilter import RateLimitingFilter

    logger = logging.getLogger('throttled_smtp_example')

    # Create an SMTPHandler
    smtp = logging.handlers.SMTPHandler(
        mailhost='smtp.example.com',
        fromaddr='[email protected]',
        toaddrs='[email protected]',
        subject='An error has occurred'
    )
    smtp.setLevel(logging.ERROR)

    # Create a formatter and set it on the handler
    formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
    smtp.setFormatter(formatter)

    # Create an instance of the RateLimitingFilter, and add it to the handler
    ratelimit = RateLimitingFilter()
    smtp.addFilter(ratelimit)

    # Add the handler to the logger
    logger.addHandler(smtp)

    # Logged errors will now be restricted to 1 every 30 seconds
    while True:
        logger.error('An error message')
        time.sleep(2)

Advanced Usage
--------------

It is possible to pass some additional configuration options to the
``RateLimitingFilter`` initializer for further control over message
throttling.

Perhaps you want to selectively throttle particular error messages
whilst allowing other messages to pass through freely. This might be the
case if there is part of the application which you know can generate
large volumes of errors, whilst the rest of the application is unlikely
to.

One way to achieve this might be to use separate loggers, one configured
with rate limiting, one without, for the different parts of the
application. Alternatively, you can use a single logger and configure
the ``RateLimitingFilter`` to match only those messages that you want to
throttle.

Applying selective rate limiting allows for constant visbility of lower
volume errors whilst keeping the higher volume errors in check.

The ``RateLimitingFilter`` supports two ways to selectively throttle
messages:

Substring based message throttling

You can pass a list of substrings to the RateLimitingFilter which it will use to match messages to apply to.

.. code:: python

config = {'match': ['some error', 'a different error']}

ratelimit = RateLimitingFilter(rate=1, per=60, burst=1, **config)
smtp.addFilter(ratelimit)

# Can be rate limited
logger.error('some error occurred')

# Can be rate limited
logger.error('a different error occurred')

# Will not be rate limited
logger.error('something completely different happened')

Automatic message throttling

*This is an experimental feature*.

You can let the ``RateLimitingFilter`` automatically throttle messages
by setting the ``match`` option to ``auto``.

.. code:: python

    config = {'match': 'auto'}
    ratelimit = RateLimitingFilter(rate=1, per=60, burst=1, **config)

The filter will then attempt to identify messages based on their content
in order to figure out whether to throttle them or not. It will tolerate
slight differences in content when identifying messages. So for example,
if error messages are being rapidly logged that are the same apart from
a timestamp, or perhaps an incrementing id, then these messages will be
treated as the same as far as rate limiting is concerned.

License
-------

MIT

Contributing
------------

Feedback and improvements are more than welcome. Please submit a pull
request!

https://github.com/wkeeling/ratelimitingfilter