Why does PyMongo throw AutoReconnect?

Question

While researching some strange issues with my Python web application (in particular, issues regarding MongoDB connectivity), I noticed something on the official PyMongo documentation page. My web application uses Flask, but this shouldn't influence the issue I'm facing.

The PyMongo driver does connection pooling, but it also throws an exception (AutoReconnect) when a connection is stale and a reconnect is due.

It states that (regarding the AutoReconnect exception):

In order to auto-reconnect you must handle this exception, recognizing that the operation which caused it has not necessarily succeeded. Future operations will attempt to open a new connection to the database (and will continue to raise this exception until the first successful connection is made).

I have noticed that this actually happens constantly (and it doesn't seem to be an error). Connections are closed by the MongoDB server after what seems like several minutes of inactivity, and need to be recreated by the web application.

What I don't understand it why the PyMongo driver throws an error when it reconnects (which the user of the driver needs to handle themselves), instead of doing it transparently. (There could even be an option a user could set so that AutoReconnect exceptions do get thrown, but wouldn't a sensible default be that these exceptions don't get thrown at all, and the connections are recreated seamlessly?)

I have never encountered this behavior using other database systems, which is why I'm a bit confused.

It's also worth mentioning that my web application's MongoDB connections never fail when connecting to my local development MongoDB server (I assume it would have something to do with the fact that it's a local connection, and that the connection is done through a UNIX socket instead of a network socket, but I could be wrong).

Answer 1

You're misunderstanding AutoReconnect. It is raised when the driver attempts to communicate with the server (to send a command or other operation) and a network failure or similar problem occurs. The name of the exception is meant to communicate that you do not have to create a new instance of MongoClient, the existing client will attempt to reconnect automatically when your application tries the next operation. If the same problem occurs, AutoReconnect is raised again.

I suspect the reason you are seeing sockets timeout (and AutoReconnect being raised) is that there is a load balancer between the server and your application that closes connections after some period of inactivity. For example, this apparently happens on Microsoft's Azure platform after 13 minutes of no activity on a socket. You might be able to fix this by using the socketKeepAlive option, added in PyMongo 2.8. Note that you will also have to set the keepalive interval on your application server to an appropriate value (the default on Linux is 2 hours). See here for more information.