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Work in progress
pt-heartbeat is a service running on MariaDB servers. It writes a timestamped row to the heartbeat table on the primary server every second. On the replicas the most recent entry in the table is compared to the real time clock to evaluate the replication lag. MediaWiki load-balancer will avoid replicas that don't have an up-to-date heartbeat entry.
The [upstream version] is developed by Percona, we use a modified version (pt-heartbeat-wikimedia).