Replication lag
Replication lag is the elapsed delay between a change being applied or committed at a source and becoming applied or visible at a replication target.
PostgreSQL Mistakes and How to Avoid Them
Replication lag is the delay between a change being committed on a primary database server and being applied or visible on a replica.
- How it is used
- Frames replica-read and primary-write patterns as consistency and failover hazards.
- How deep it goes
- Explains stale-read consequences and WAL timeline divergence, while deferring detailed lag measurement and recovery procedures.
- Examples
- Uses a delayed standby account balance and promotion of a lagging replica after primary failure.
- What you can do after
- Evaluate/understand when stale replica reads or lagging promotion can violate consistency and recovery expectations.
Read how PostgreSQL Mistakes and How to Avoid Them explains replication lag →
MongoDB 8.0 in Action, Third Edition
Replication lag is the delay between an operation being recorded or applied on the primary and being applied on a replica.
- How it is used
- Monitors whether replicas keep pace with primary operations and helps diagnose replication problems.
- How deep it goes
- Explains topology-related causes and demonstrates status commands, but does not develop a formal lag calculation.
- Examples
- Uses db.getReplicationInfo() and db.printSecondaryReplicationInfo() outputs showing oplog timing and per-secondary lag.
- What you can do after
- Diagnose replication status with MongoDB shell commands and identify secondaries that trail the primary.
Read how MongoDB 8.0 in Action, Third Edition explains replication lag →
Related concepts
- alertingconfigured by · Alerting is the generation and delivery of notifications when monitored conditions or events meet defined criteria or require attention or action.
- asynchronous replicationcontrasts with · Asynchronous replication propagates data changes to replicas without requiring the primary operation to wait for replica confirmation.
- read replicationused by · Read replication is the replication of database data from a writable primary instance to one or more instances that serve read operations.
- synchronous replicationcontrasts with · Synchronous replication is a replication method in which an operation is acknowledged or completed only after required replicas have confirmed the change.
- performancekind of · Performance is the measurable effectiveness or operational behavior of a system under specified conditions, assessed through relevant qualities such as task results, speed, responsiveness, throughput, or resource use.
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