DDL locks are currently used to serialize DDLs on the same namespace on sharded clusters.

Considering that on replica sets there are some DDLs such as collMod that acquire/release/re-acquire the collection lock during their execution, it would be useful to use the DDL locks also on replica sets.

Benefits:

• Prevent concurrent DDLs from sneaking in
• Check preconditions before acquiring db/collection locks in strong modes in order to avoid unnecessary lock contention and have the guarantee that preconditions will hold during the whole duration of the operation

NB: if some DDL at replica set level is not atomic, this would still not prevent DDLs from getting half-committed upon stepdown/crash.