Python "Circuit Breaker" implementation
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This is a Python implementation of the "Circuit Breaker" Pattern (http://martinfowler.com/bliki/CircuitBreaker.html). Inspired by Michael T. Nygard's highly recommendable book Release It! (https://pragprog.com/book/mnee/release-it).
The project is available on PyPI. Simply run::
$ pip install circuitbreaker
This is the simplest example. Just decorate a function with the
from circuitbreaker import circuit
@circuit def external_call(): ...
This decorator sets up a circuit breaker with the default settings. The circuit breaker:
Exception) as an expected failure
A failure is a raised exception, which was not caught during the function call. By default, the circuit breaker listens for all exceptions based on the class
Exception. That means, that all exceptions raised during the function call are considered as an "expected failure" and will increase the failure count.
It is important, to be as specific as possible, when defining the expected exception. The main purpose of a circuit breaker is to protect your distributed system from a cascading failure. That means, you probably want to open the circuit breaker only, if the integration point on the other end is unavailable. So e.g. if there is an
ConnectionErroror a request
If you are e.g. using the requests library (http://docs.python-requests.org/) for making HTTP calls, its
RequestExceptionclass would be a great choice for the
All recognized exceptions will be re-raised anyway, but the goal is, to let the circuit breaker only recognize those exceptions which are related to the communication to your integration point.
When it comes to monitoring (see Monitoring_), it may lead to falsy conclusions, if a circuit breaker opened, due to a local
The following configuration options can be adjusted via decorator parameters. For example::
from circuitbreaker import circuit
@circuit(failure_threshold=10, expected_exception=ConnectionError) def external_call(): ...
By default, the circuit breaker opens after 5 subsequent failures. You can adjust this value with the
By default, the circuit breaker stays open for 30 seconds to allow the integration point to recover. You can adjust this value with the
By default, the circuit breaker listens for all exceptions which are based on the
Exceptionclass. You can adjust this with the
expected_exceptionparameter. It can be either an exception class or a tuple of exception classes.
By default, the circuit breaker name is the name of the function it decorates. You can adjust the name with parameter
By default, the circuit breaker will raise a
CircuitBreakerexception when the circuit is opened. You can instead specify a function to be called when the circuit is opened. This function can be specified with the
fallback_functionparameter and will be called with the same parameters as the decorated function would be.
If you apply circuit breakers to a couple of functions and you always set specific options other than the default values, you can extend the
CircuitBreakerclass and create your own circuit breaker subclass instead::
from circuitbreaker import CircuitBreaker
class MyCircuitBreaker(CircuitBreaker): FAILURE_THRESHOLD = 10 RECOVERY_TIMEOUT = 60 EXPECTED_EXCEPTION = RequestException
Now you have two options to apply your circuit breaker to a function. As an Object directly::
@MyCircuitBreaker() def external_call(): ...
Please note, that the circuit breaker class has to be initialized, you have to use a class instance as decorator (
@MyCircuitBreaker()), not the class itself (
Or via the decorator proxy::
@circuit(cls=MyCircuitBreaker) def external_call(): ...
To keep track of the health of your application and the state of your circuit breakers, every circuit breaker registers itself at the
CircuitBreakerMonitor. You can receive all registered circuit breakers via
To get an aggregated health status, you can ask the Monitor via
CircuitBreakerMonitor.all_closed(). Or you can retrieve the currently open circuits via
CircuitBreakerMonitor.get_open()and the closed circuits via