Python implementation of the Circuit Breaker pattern
PyBreaker is a Python implementation of the Circuit Breaker pattern, described in Michael T. Nygard’s book Release It!.
In Nygard’s words, “circuit breakers exists to allow one subsystem to fail without destroying the entire system. This is done by wrapping dangerous operations (typically integration points) with a component that can circumvent calls when the system is not healthy”.
Run the following command line to download the latest stable version of PyBreaker from PyPI:
$ easy_install -U pybreaker
If you are a Git user, you might want to download the current development version:
$ git clone git://github.com/danielfm/pybreaker.git $ cd pybreaker $ python setup.py test $ python setup.py install
The first step is to create an instance of CircuitBreaker for each integration point you want to protect against:
import pybreaker # Used in database integration points db_breaker = pybreaker.CircuitBreaker(fail_max=5, reset_timeout=60)
CircuitBreaker instances should live globally inside the application scope, e.g., live across requests.
Integration points to external services (i.e. databases, queues, etc) are more likely to fail, so make sure to always use timeouts when accessing such services if there’s support at the API level.
There’s no need to subclass CircuitBreaker if you just want to take action when certain events occur. In that case, it’s better to subclass CircuitBreakerListener instead:
class DBListener(pybreaker.CircuitBreakerListener): "Listener used by circuit breakers that execute database operations." def before_call(self, cb, func, *args, **kwargs): "Called before the circuit breaker `cb` calls `func`." pass def state_change(self, cb, old_state, new_state): "Called when the circuit breaker `cb` state changes." pass def failure(self, cb, exc): "Called when a function invocation raises a system error." pass def success(self, cb): "Called when a function invocation succeeds." pass class LogListener(pybreaker.CircuitBreakerListener): "Listener used to log circuit breaker events." pass
To add listeners to a circuit breaker:
# At creation time... db_breaker = pybreaker.CircuitBreaker(listeners=[DBListener(), LogListener()]) # ...or later db_breaker.add_listeners(OneListener(), AnotherListener())
Let’s say you want to use a circuit breaker on a function that updates a row in the customer database table:
@db_breaker def update_customer(cust): # Do stuff here... pass # Will trigger the circuit breaker updated_customer = update_customer(my_customer)
Or if you don’t want to use the decorator syntax:
def update_customer(cust): # Do stuff here... pass # Will trigger the circuit breaker updated_customer = db_breaker.call(update_customer, my_customer)
According to the default parameters, the circuit breaker db_breaker will automatically open the circuit after 5 consecutive failures in update_customer.
When the circuit is open, all calls to update_customer will fail immediately (raising CircuitBreakerError) without any attempt to execute the real operation.
After 60 seconds, the circuit breaker will allow the next call to update_customer pass through. If that call succeeds, the circuit is closed; if it fails, however, the circuit is opened again until another timeout elapses.
By default, a failed call is any call that raises an exception. However, it’s common to raise exceptions to also indicate business exceptions, and those exceptions should be ignored by the circuit breaker as they don’t indicate system errors:
# At creation time... db_breaker = CircuitBreaker(exclude=[CustomerValidationError]) # ...or later db_breaker.add_excluded_exception(CustomerValidationError)
In that case, when any function guarded by that circuit breaker raises CustomerValidationError (or any exception derived from CustomerValidationError), that call won’t be considered a system failure.
A circuit breaker provides properties and functions you can use to monitor and change its current state:
# Get the current number of consecutive failures print db_breaker.fail_counter # Get/set the maximum number of consecutive failures print db_breaker.fail_max db_breaker.fail_max = 10 # Get/set the current reset timeout period (in seconds) print db_breaker.reset_timeout db_breaker.reset_timeout = 60 # Get the current state, i.e., 'open', 'half-open', 'closed' print db_breaker.current_state # Closes the circuit db_breaker.close() # Half-opens the circuit db_breaker.half_open() # Opens the circuit db_breaker.open()
These properties and functions might and should be exposed to the operations staff somehow as they help them to detect problems in the system.