From 073a43715ff285731923019b437a9bcbd6f1329d Mon Sep 17 00:00:00 2001
From: Ben Adida <ben@BigDaddy.local>
Date: Fri, 29 Jan 2010 17:23:03 -0800
Subject: [PATCH] added autoretrying to datastore

---
 autoretry.py | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 manage.py    |  4 +++
 2 files changed, 90 insertions(+)
 create mode 100644 autoretry.py

diff --git a/autoretry.py b/autoretry.py
new file mode 100644
index 000000000..503684323
--- /dev/null
+++ b/autoretry.py
@@ -0,0 +1,86 @@
+def autoretry_datastore_timeouts(attempts=5.0, interval=0.1, exponent=2.0):
+    """
+    Copyright (C)  2009  twitter.com/rcb
+    
+    Permission is hereby granted, free of charge, to any person
+    obtaining a copy of this software and associated documentation
+    files (the "Software"), to deal in the Software without
+    restriction, including without limitation the rights to use,
+    copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following
+    conditions:
+
+    The above copyright notice and this permission notice shall be
+    included in all copies or substantial portions of the Software.
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+    EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+    OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+    NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+    HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+    WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+    OTHER DEALINGS IN THE SOFTWARE.
+    
+    ======================================================================
+    
+    This function wraps the AppEngine Datastore API to autoretry 
+    datastore timeouts at the lowest accessible level.  
+
+    The benefits of this approach are:
+
+    1. Small Footprint:  Does not monkey with Model internals 
+                         which may break in future releases.
+    2. Max Performance:  Retrying at this lowest level means 
+                         serialization and key formatting is not 
+                         needlessly repeated on each retry.
+    At initialization time, execute this:
+    
+    >>> autoretry_datastore_timeouts()
+    
+    Should only be called once, subsequent calls have no effect.
+    
+    >>> autoretry_datastore_timeouts() # no effect
+    
+    Default (5) attempts: .1, .2, .4, .8, 1.6 seconds
+    
+    Parameters can each be specified as floats.
+    
+    :param attempts: maximum number of times to retry.
+    :param interval: base seconds to sleep between retries.
+    :param exponent: rate of exponential back-off.
+    """
+    
+    import time, logging
+    from google.appengine.api import apiproxy_stub_map
+    from google.appengine.runtime import apiproxy_errors
+    from google.appengine.datastore import datastore_pb
+    
+    attempts = float(attempts)
+    interval = float(interval)
+    exponent = float(exponent)
+    wrapped = apiproxy_stub_map.MakeSyncCall
+    errors = {datastore_pb.Error.TIMEOUT:'Timeout',
+        datastore_pb.Error.CONCURRENT_TRANSACTION:'TransactionFailedError'}
+    
+    def wrapper(*args, **kwargs):
+        count = 0.0
+        while True:
+            try:
+                return wrapped(*args, **kwargs)
+            except apiproxy_errors.ApplicationError, err:
+                errno = err.application_error
+                if errno not in errors: raise
+                sleep = (exponent ** count) * interval
+                count += 1.0
+                if count > attempts: raise
+                msg = "Datastore %s: retry #%d in %s seconds.\n%s"
+                vals = ''
+                if count == 1.0:
+                    vals = '\n'.join([str(a) for a in args])
+                logging.warning(msg % (errors[errno], count, sleep, vals))
+                time.sleep(sleep)
+
+    setattr(wrapper, '_autoretry_datastore_timeouts', False)
+    if getattr(wrapped, '_autoretry_datastore_timeouts', True):
+        apiproxy_stub_map.MakeSyncCall = wrapper
\ No newline at end of file
diff --git a/manage.py b/manage.py
index 57352cc9e..4d600bcd2 100644
--- a/manage.py
+++ b/manage.py
@@ -17,6 +17,10 @@
 from appengine_django import InstallAppengineHelperForDjango
 InstallAppengineHelperForDjango()
 
+# install auto-retrying for database
+import autoretry
+autoretry.autoretry_datastore_timeouts(attempts=5.0, interval=0.2, exponent=2.0)
+
 from django.core.management import execute_manager
 try:
     import settings # Assumed to be in the same directory.