A circuit breaker according to the logic outline in Michael T. Nygard's great book Release It!.
The circuit breaker monitors communication with a remote peer and in the case of a high error rate may break the circuit and not allow further communication for a short period. After a while the breaker will let through a single request to probe to see if the peer feels better. If it does, it will close the circuit and allow requests once again. If not, it will open the circuit again.
CircuitBreakerSet can handle the state for multiple peers at the
same time. Use the
context method to pick which peer to track. The
first argument is used to identify the peer. Make it a string of some
kind, since it will be used to identify the peer in logs.
Below is a small example of how the circuit breaker can be used:
import logging import time from circuit import CircuitBreakerSet circuit_breaker = CircuitBreakerSet(time.time, logging.getLogger( 'circuit-breaker')) circuit_breaker.handle_error(ValueError) def fn(circuit_breaker): try: with circuit_breaker.context('my-remote-peer'): raise ValueError('oh no') except CircuitOpenError: # the circuit was open so we did not even try to communicate # with the remote service. raise
If you call
fn often enough the circuit breaker will open and
CircuitOpenError will be raised.
CircuitBreakerSet class takes a few keyword arguments:
time_unit(default 60) -- Number of seconds to sample seconds over.
maxfail(default 3) -- Number of seconds that is allowed over a time unit.
reset_timeout(default 10) -- Seconds that the circuit is open before going into half-open mode.
It is also possible to create a single instance of a circuit breaker. The
circuit.CircuitBreaker class takes the following arguments:
clock-- A callable that returns the time in seconds.
logging.Loggerobject used for logging.
error_types-- A list of error types that are treated as errors.
maxfail-- Number of seconds that is allowed over a time unit.
reset_timeout-- Seconds that the circuit is open before going into half-open mode.
time_unit-- Number of seconds to sample seconds over.
There's also support for using the circuit breaker with Twisted. Note that the circuit breaker still use pythons standard logging framework. Example:
import logger from circuit import TwistedCircuitBreakerSet circuit_breaker = TwistedCircuitBreakerSet(reactor, logging.getLogger( 'circuit-breaker'))
TwistedCircuitBreakerSet adds support for
which uses exceptions internally.)
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