MongoDB Schema Design - Real-time Chat

Posted by Nick on Stack Overflow See other posts from Stack Overflow or by Nick
Published on 2010-05-29T21:02:38Z Indexed on 2010/05/29 21:22 UTC
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mongodb

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nosql

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schema-design

I'm starting a project which I think will be particularly suited to MongoDB due to the speed and scalability it affords.

The module I'm currently interested in is to do with real-time chat. If I was to do this in a traditional RDBMS I'd split it out into:

Channel (A channel has many users)
User (A user has one channel but many messages)
Message (A message has a user)

The the purpose of this use case, I'd like to assume that there will be typically 5 channels active at one time, each handling at most 5 messages per second.

Specific queries that need to be fast:

Fetch new messages (based on an bookmark, time stamp maybe, or an incrementing counter?)
Post a message to a channel
Verify that a user can post in a channel

Bearing in mind that the document limit with MongoDB is 4mb, how would you go about designing the schema? What would yours look like? Are there any gotchas I should watch out for?

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