I'm currently using MongoDB to record application logs, and while I'm quite happy with both the performance and with being able to dump arbitrary structured data into log records, I'm troubled by the mutability of log records once stored.

In a traditional database, I would structure the grants for my log tables such that the application user had INSERT and SELECT privileges, but not UPDATE or DELETE. Similarly, in CouchDB, I could write a update validator function that rejected all attempts to modify an existing document.

However, I've been unable to find a way to restrict operations on a MongoDB database or collection beyond the three access levels (no access, read-only, "god mode") documented in the security topic on the MongoDB wiki.

Has anyone else deployed MongoDB as a document store in a setting where immutability (or at least change tracking) for documents was a requirement? What tricks or techniques did you use to ensure that poorly-written or malicious application code could not modify or destroy existing log records? Do I need to wrap my MongoDB logging in a service layer that enforces the write-only policy, or can I use some combination of configuration, query hacking, and replication to ensure a consistent, audit-able record is maintained?