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pt-heartbeat is a service running on MariaDB servers (except for multi-instance hosts). A process on the source server writes a timestamped row to the heartbeat table every second. A process on a replica compares the most recent entry in the table to the host 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).