From b4efa7e45b827c332403a44c3d597f352508b108 Mon Sep 17 00:00:00 2001
From: Jimmy Zelinskie <jimmy.zelinskie+git@gmail.com>
Date: Wed, 1 Feb 2017 19:46:04 -0500
Subject: [PATCH] util.failover: init
---
util/failover.py | 46 ++++++++++++++++++++++++++++++++++++++
util/test/test_failover.py | 44 ++++++++++++++++++++++++++++++++++++
2 files changed, 90 insertions(+)
create mode 100644 util/failover.py
create mode 100644 util/test/test_failover.py
diff --git a/util/failover.py b/util/failover.py
new file mode 100644
index 000000000..c1490b772
--- /dev/null
+++ b/util/failover.py
@@ -0,0 +1,46 @@
+import logging
+
+from functools import wraps
+
+
+logger = logging.getLogger(__name__)
+
+
+class FailoverException(Exception):
+ """ Exception raised when an operation should be retried by the failover decorator. """
+ def __init__(self, message):
+ super(FailoverException, self).__init__()
+ self.message = message
+
+def failover(func):
+ """ Wraps a function such that it can be retried on specified failures.
+ Raises FailoverException when all failovers are exhausted.
+ Example:
+
+ @failover
+ def get_google(scheme, use_www=False):
+ www = 'www.' if use_www else ''
+ r = requests.get(scheme + '://' + www + 'google.com')
+ if r.status_code != 200:
+ raise FailoverException('non 200 response from Google' )
+ return r
+
+ def GooglePingTest():
+ r = get_google(
+ (('http'), {'use_www': False}),
+ (('http'), {'use_www': True}),
+ (('https'), {'use_www': False}),
+ (('https'), {'use_www': True}),
+ )
+ print('Successfully contacted ' + r.url)
+ """
+ @wraps(func)
+ def wrapper(*args_sets):
+ for arg_set in args_sets:
+ try:
+ return func(*arg_set[0], **arg_set[1])
+ except FailoverException as ex:
+ logger.debug('failing over: %s', ex.message)
+ continue
+ raise FailoverException('exhausted all possible failovers')
+ return wrapper
diff --git a/util/test/test_failover.py b/util/test/test_failover.py
new file mode 100644
index 000000000..340092179
--- /dev/null
+++ b/util/test/test_failover.py
@@ -0,0 +1,44 @@
+import pytest
+
+from util.failover import failover, FailoverException
+
+
+class Counter(object):
+ """ Wraps a counter in an object so that it'll be passed by reference. """
+ def __init__(self):
+ self.calls = 0
+
+ def increment(self):
+ self.calls += 1
+
+
+@failover
+def my_failover_func(i, should_raise=None):
+ """ Increments a counter and raises an exception when told. """
+ i.increment()
+ if should_raise is not None:
+ raise should_raise()
+ raise FailoverException('incrementing')
+
+
+@pytest.mark.parametrize('stop_on,exception', [
+ (10, None),
+ (5, IndexError),
+])
+def test_readonly_failover(stop_on, exception):
+ """ Generates failover arguments and checks against a counter to ensure that
+ the failover function has been called the proper amount of times and stops
+ at unhandled exceptions.
+ """
+ counter = Counter()
+ arg_sets = []
+ for i in xrange(stop_on):
+ should_raise = exception if exception is not None and i == stop_on-1 else None
+ arg_sets.append(((counter,), {'should_raise': should_raise}))
+
+ if exception is not None:
+ with pytest.raises(exception):
+ my_failover_func(*arg_sets)
+ else:
+ my_failover_func(*arg_sets)
+ assert counter.calls == stop_on