api: Improve and optimize Random Exponential Backoff algorithm.

Fixes #537.

Author: orientor

zulip/zulip/__init__.py

@@ -56,12 +56,13 @@
 API_VERSTRING = "v1/"
 
 class CountingBackoff(object):
-    def __init__(self, maximum_retries=10, timeout_success_equivalent=None):
-        # type: (int, Optional[float]) -> None
+    def __init__(self, maximum_retries=10, timeout_success_equivalent=None, delay_cap=10.0):
+        # type: (int, Optional[float], float) -> None
         self.number_of_retries = 0
         self.maximum_retries = maximum_retries
         self.timeout_success_equivalent = timeout_success_equivalent
         self.last_attempt_time = 0.0
+        self.delay_cap = delay_cap
 
     def keep_going(self):
         # type: () -> bool
@@ -93,9 +94,9 @@ def fail(self):
         super(RandomExponentialBackoff, self).fail()
         # Exponential growth with ratio sqrt(2); compute random delay
         # between x and 2x where x is growing exponentially
-        delay_scale = int(2 ** (self.number_of_retries / 2.0 - 1)) + 1
-        delay = delay_scale + random.randint(1, delay_scale)
-        message = "Sleeping for %ss [max %s] before retrying." % (delay, delay_scale * 2)
+        delay_base = 0.5
+        delay = random.random() * min(self.delay_cap, delay_base * (2 ** self.number_of_retries))
+        message = "Sleeping for %ss before retrying." % (delay,)
         try:
             logger.warning(message)
         except NameError: