Problem:
during a rolling bounce of the members of a replset, the following was seen in the mongos logs

Sun Oct 10 22:05:40 [WriteBackListener] MessagingPort say send() errno:32 Broken pipe 10.121.14.5:27018
Sun Oct 10 22:05:40 [WriteBackListener] WriteBackListener exception : St9exception
Sun Oct 10 22:05:46 [WriteBackListener] MessagingPort say send() errno:32 Broken pipe 10.121.14.5:27018
Sun Oct 10 22:05:46 [WriteBackListener] WriteBackListener exception : St9exception

At this point the application could not read or write to the DB

Workaround:

• bounce the mongos
• bounce the apache server (running the PHP driver)

The bounce of the mongos did not solve the problem, the apache server had to be bounced as well. Possibly a connection pooling / persistent connection issue?

Business case:

• User experience
  During a failover, the monogs/drivers should deal with this gracefully.