MongoDB: best design for messaging app [duplicate]

Question

A very simple design problem. Say I want to build Facebook Messenger. Let's say John and Marry are chatting, which is a better approach?

1) 1 document per conversation, messages is an array of message object

{ participants: ['john', 'marry'],    messages: [        { sender: 'john', content: 'howdy', time_created: new Date() },       { sender: 'marry', content: 'good u', time_created: new Date() },       ...   ] } 

2) 1 document per message

{ participants: ['john', 'marry'], sender: 'john', message: 'howdy', time_created: new Date() } // document 1 { participants: ['john', 'marry'], sender: 'marry', message: 'good u', time_created: new Date() } // document 2 ....  

Which approach has better performance in terms of inserting a new message (updating a conversation vs. creating a new document) ?

or are there any better approach (as in my 2nd approach, i'm not sure if it's a good design to specify the participants field in each document)?

Thanks!

Answer 1

Based on your example data for the messaging app, what you could do is having two collections: Conversation and Messages. Where the relationship is one Conversation have many Messages.

Conversation: { id: 123   participants: ['john', 'marry'], }   Message: { sender: 'john',    content: 'howdy',    time_created: new Date(),   converstationId: 123 }, { sender: 'marry',    content: 'good u',    time_created: new Date(),   converstationId: 123  }, 

Creating a new document message would be better in this case, as you can then have two applications (1 for john and 1 for marry) without handling the possibility of the two of them updating the same document. They just happens to be sharing the same conversation session.

Also, if a conversation is a single document, you might end up with a very large document. (Document growth concern)

You can find out more about data modelling for this mongodb doc

http://docs.mongodb.org/manual/core/data-modeling-introduction/

Also see MongoDB: Socialite for examples/discussion for social network use case.

Hope it helps. Cheers.