airbnb/synapse — pull request #106 by jnb (2014-12-11 02:09:30+00:00)
Title: Add 'use_previous_backends' option.

* This option defaults to true: if there are no default servers and a watcher
reports no backends, then use the previous backends that we already know
about.
* Factor out code that updates `@backends` into the watcher base class.

This code is running in production with no issues.