What query could you use to check replication lag on MySQL?
The “SHOW SLAVE STATUS”: The MySQL DBA’s Mantra. In some cases, this is the silver bullet when dealing with replication lag and it reveals mostly everything the cause of an issue in your MySQL database. Simply run this SQL statement in your slave node that is suspected experiencing a replication lag.
What is read replication?
A read replica is a copy of the primary instance that reflects changes to the primary in almost real time, in normal circumstances. You can use a read replica to offload read requests or analytics traffic from the primary instance. For more information about how replication works, see Replication in Cloud SQL.
Why does replication lag occur in a replica?
Each replica pulls changes from its master and replays them as fast as it can. There is a nonzero delay between the point in time where changes are made visible on a master and the time where those changes are visible on some replica or on all replicas. This delay is the replication lag.
Why does MySQL lag when writing to Read Replica?
If a high volume of writes to the source DB instance occurs in parallel, the writes to the read replica are serialized using a single SQL_THREAD. This can cause a lag between the source DB instance and read replica. Multi-threaded (parallel) replication is available for MySQL 5.6, MySQL 5.7, and higher versions.
What to look for if your PostgreSQL replication is lagging?
PostgreSQL standby nodes, which are in recovery mode, does not report to you the exact state of what’s happening of your replication. Not unless you view the PG log, you can gather information of what’s going on. There’s no query you can run to determine this.
Can you tell the lag of a commit from a replica?
You can’t do it from the replica because the replica has no idea how far ahead the master is. Additionally this won’t tell you the lag in seconds. In current (as of 9.4 at least) PostgreSQL versions there’s no timestamp associated with a commit or a WAL record. So there’s no way to tell how long ago a given LSN (xlog position) was.