At the beginning of each replicated single phase index build, the primary writes a noop oplog entry with a msg field that includes the namespace of the collection. This noop write, introduced in SERVER-34172, is intended to provide a timestamp for the catalog update that sets the index entry's state to ready: false.

In SERVER-48010, the msg field value was modified to "Creating indexescoll:<ns>".

The original msg format was introduced in SERVER-34172: "Creating indexes. Coll: <ns>".

Since there is no other identifying information in the noop oplog entry, it would be prudent to keep this message stable for users that may have be relying on this since 4.0.

Starting in 4.4, this is less of an issue with two phase index builds because we have distinct oplog entries for both index build start and completion.