MongoDB historical data storage - best practice?

Question

Assuming I have an user who has an ID and I want to store a historical record (document) on this user every day, what is better:

• create a new document for each record and search for the user id; or
• keep updating and embedding that data into one single user document which keeps growing over time?

Mostly I want to retrieve only the current document for the user but all records should be accessible at any time without a super long search/query.

Answer 1

There are a lot of variables that can affect such a decision. One big document seems most obvious provided it doesn't grow to unpractically large or even disallowed sizes (mind you, a document can be at most 16MB in size).

Using document per entry is also perfectly viable and provided you create the appropriate indexes should not result in slow queries.

Answer 2

There is a limit to how big a document can be. It's (as of v1.8) 16 MB. So you can simply run out of room if you update & embed. Also, mongo allocates document space based on average document size in a collection. If you keep adjusting/resizing this might have negative performance implications.

I think it's much safer to create new documents for each record and if/when you want to collate that data, you do it in a map/reduce job.