mongo db design of following and feeds, where should I embed?

Question

I have a basic question about where I should embed a collection of followers/following in a mongo db. It makes sense to have an embedded collection of following in a user o

Answer 1

In general, it's a bad idea to embed following/followed-by relationships into user documents, for several reasons:

(1) there is a maximum document size limit of 16MB, and it's plausible that a popular user of a well-subscribed site might end up with hundreds of thousands of followers, which will approach the maximum document size,

(2) followership relationships change frequently, and so the case where a user gains a lot of followers translates into repeated document growth if you're embedding followers. Frequent document growth will significantly hinder MongoDB performance, and so should be avoided (occasional document growth, especially is documents tend to reach a stable final size, is less of a performance penalty).

So, yes, it is best to split out following/followed-by relationship into a separate collection of records each having two fields, e.g., { _id : , oid : }, with indexes on _id (for the "who am I following?" query) and oid (for the "who's following me?" query). Any individual state change is modeled by a single document addition or removal, though if you're also displaying things like follower counts, you should probably keep separate counters that you update after any edge insertion/deletion.

(Of course, this supposes your business requirements allow you some flexibility on the consistency details: in general, if your display code tells a user he's got 304 followers and then proceeds to enumerate them, only the most fussy user will check that the followers enumerated tally up to 304. If business requirements necessitate absolute consistency, you'll either need a database that isolates transactions for you, or else you'll have to do the counting yourself as part of displaying all user identities.)