Mastering MongoDB 4.x - Second Edition

Read isolation and consistency

MongoDB read operations would be characterized as read uncommitted in a traditional RDBMS definition. What this means is that, by default, reads may get values that may not finally persist to the disk in the event of, for example, data loss or a replica set rollback operation.

In particular, when updating multiple documents with the default write behavior, lack of isolation may result in the following:

• Reads may miss documents that were updated during the update operations
• Non-serializable operations
• Read operations are not point-in-time

These can be resolved by using the $isolated operator with a heavy performance penalty.

Queries with cursors that don't use .snapshot() may also, in some cases, get inconsistent ...