repoze.retry implements a WSGI middleware filter which intercepts “retryable” exceptions and retries the WSGI request a configurable number of times. Such exceptions are normally raised when a conflict is detected in an optimistic concurrency scheme  is configured for a database such Postgres  or ZODB  (in ZODB, optimistic concurrency is always enabled).
If the request cannot be satisfied via retries, the filter re-raises the exception.
If your WSGI pipeline includes the transaction filter provided by repoze.tm or repoze.tm2, the retry filter should come before it (to the “left”), so that retried requests are first aborted and then restarted in a new transaction
Install using setuptools, e.g. (within a virtualenv):
$ easy_install repoze.retry
Wire up the middleware in your application:
from repoze.retry import Retry mw = Retry(app, tries=3, retryable=(ValueError, IndexError))
By default, the retryable exception is repoze.retry.ConflictError.
tries is an integer count, defaulting to 3 times.
To use the default configuration, you can just include the filter in your application’s pipeline.
[pipeline:main] pipeline = egg:Paste#cgitb egg:Paste#httpexceptions egg:repoze.retry#retry egg:repoze.tm#tm egg:repoze.vhm#vhm_xheaders zope2
If you want to override the defaults, e.g. to change the number of retries, or the exceptions which will be retried, configure the filter in a separate section:
[filter:retry] use = egg:repoze.retry tries = 2 retryable = mypackage.exceptions:SomeRetryableException
and then use it in your pipeline:
[pipeline:main] pipeline = egg:Paste#cgitb egg:Paste#httpexceptions retry myapp
The repoze developers hang out in the repoze IRC channel.
Email discussion of the filter’s development takes place on the repoze-dev mailing list.
Visit http://bugs.repoze.org/ to report bugs.
Visit http://svn.repoze.org/ to check out development or tagged versions.