From the help on mongoexport:
On a sharded cluster this can lead to tragedy:
- It would be possible for the same document to appear twice (repeated _id) or even many times (orphans) which could break a subsequent import.
- Some documents can be missing entirely, even ones that have been present for years and never modified (an in-progress migration can cause this).
The default should not allow reading from secondaries. If we are concerned about a change in behavior without notice then make the -k parameter required when connected to a mongos (as opposed to optional).
It is better to fail totally and noisily than to fail partially but silently.