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How is the replication lag calculated in MongoDB?
Replication lag is calculated between the last operation time on the primary and the time of the last operation received by the secondary. If a replica set is only written to once every 10 minutes, the replication lag will be 10 minutes just after the write is made to the primary and just prior to the next write being replicated to the secondary.
Why are replicas not supported by MongoDB profiler?
They do not depend on the log levels (either at the system or component level), or the profiling level, or the slow operation sample rate. The profiler does not capture slow oplog entries. All members of a replica set must be able to connect to every other member of the set to support replication.
How is replication lag related to cache pressure?
Replication lag refers to the amount of time that it takes to copy (i.e. replicate) a write operation on the primary to a secondary. Some small delay period may be acceptable, but significant problems emerge as replication lag grows, including building cache pressure on the primary.
How many nodes are in a MongoDB replica set?
Using MongoDB with a well-designed schema and lovingly-tuned indexes, your application back-end has been handling thousands of transactions per second without breaking a sweat. You’ve got multiple nodes arranged in a replica set with no single point of failure.
How to check the length of replication lag?
To check the current length of replication lag: In a mongosh session that is connected to the primary, call the rs.printSecondaryReplicationInfo () method. Returns the syncedTo value for each member, which shows the time when the last oplog entry was written to the secondary, as shown in the following example:
How to check the status of a replica set in Mongo?
To display the current state of the replica set and current state of each member, run the rs.status () method in a mongo shell connected to the replica set’s primary. For descriptions of the information displayed by rs.status (), see replSetGetStatus. The rs.status () method is a wrapper that runs the replSetGetStatus database command.