In general, MongoDB will replicate from a Primary to Secondaries asynchronously, based on number of write operations, time and other factors by shipping oplog from primary to secondaries.
When describing WriteConcern options, MongoDB documentation states "...primary waits until the required number of secondaries acknowledge the write before returning write concern acknowledgment". This seems to suggest that a WriteConcern other than "w:1" would replicate to at least some of the members of the replica set in a blocking manner, potentially avoiding log shipping.
The basic question I'm trying to answer is this: if every write is using WriteCocnern of "majority", would MongoDB ever have to use log shipment? In other words, is using WriteCocnern of "majority" also controls replication timing?
I would like to better understand how MongoDB handles WriteConcern of "majority". A few obvious options:
- Primary sends write requests to every Secondary, and blocks the thread until majority respond with acknowledgment
or
- Primary pre-selects Secondaries first and sends requests to only those secondaries, blocking the thread until all chosen secondaries respond with acknowledgment
or
- Something much smarter than either of these options
If Option 1 is used, in most cases (assuming equidistant placement of secondaries) all secondaries will have received the write operation by the time Write completes, and there's high probability (although not a guarantee) all secondaries will have applied it. If true, this behavior enables use cases where writes need to be reflected on Secondaries quicker than typical asynchronous replication process.
Obviously WriteConcern of "majority" will incur performance penalty, but this may be acceptable for specific use cases where read operations may target Secondaries (e.g. ReadPreference of "nearest") and desire more recent data.