epugh commented on code in PR #4171: URL: https://github.com/apache/solr/pull/4171#discussion_r2962172468
########## solr/solr-ref-guide/modules/deployment-guide/pages/user-managed-index-replication.adoc: ########## @@ -575,6 +575,63 @@ A snapshot with the name `snapshot._name_` must exist or an error will be return `location`::: The location where the snapshot is created. +[[monitoring-follower-replication-lag]] +== Monitoring Follower Replication Lag + +In a leader-follower deployment it is important to know whether followers are keeping pace with the leader. +Solr's health-check endpoint supports a `maxGenerationLag` request parameter that lets you assert that each follower core is within a specified number of Lucene commit generations of its leader. +When the follower is lagging more than the allowed number of generations the endpoint returns HTTP 503 (Service Unavailable), making it straightforward to integrate into load-balancer health probes or monitoring systems. + +The `maxGenerationLag` parameter is an integer representing the maximum acceptable number of commit generations by which a follower is allowed to trail its leader. +A value of `0` requires the follower to be fully up to date. +If the parameter is omitted, the health check returns `OK` regardless of replication lag. + +[WARNING] +==== +Because a follower's generation can only increase when a replication from the leader actually completes, `maxGenerationLag=0` may return `FAILURE` immediately after a follower starts or after a period of network instability even though the follower will catch up on the next poll cycle. Review Comment: yeah, it was generated, and yes, you should DEFINITLY ask about these statemetns. It is documented in the javadocs in `HealthCheckHandler`. I'll be honest, I can't super vouch for this as well since I've never actually personally run leader/follower and then used this... I wonder if we are better off not having a WARNING on something that we only know from reading javadocs and haven't personally tested? I.e err on just documenting the "what we know", not "what we believe". -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
