This document discusses MongoDB's oplog and logging operations. It covers: 1. The createOplog function which creates the oplog as a capped collection in the local database. 2. The logOp function which logs operations to the oplog in BSON format with fields like timestamp, operation type, namespace, and operation object. 3. The _logOp and _logOpOld functions which handle actually writing to the oplog, with _logOpOld writing directly to the data file.