I'm building a simple live chat into a web application running on Django, but one thing I'm confused about is how I should store the messages between users.

The chat will support multiple users, and a chat "session" is composed of users connected to one user that is the "host." The application is a sort of online document collaboration thing, so user X has a document, and users Y and Z would connect to user X to talk about the document, and that would be one chat session.

If user Y disconnected for five minutes, and then signed back in and reconnected to user X, he should not get any of the messages shared between users X and Z while he was away.

if users X, Y, and Z can have a chat session about user X's document, then users X and Y can connect to a simultaneous, but separate discussion about user Z's document.

How should I handle this? Should I keep each message in the database? Each message would have an owner user and a target user (the host), and a separate table would be used to connect users with messages (which messages are visible to what users).

Or should I store each session as an HTML file on the server, which messages get appended to?

The problem is, I can't just send messages directly between clients. They have to be sent to the server in a POST request, and then each client has to periodically check for the messages in a GET request. Except I can't just have each message cleared after a client fetches it, because there could be multiple clients. How should I set this up? Any suggestions?