[GitHub] [druid] a2l007 commented on a change in pull request #11135: Create dynamic config that can limit number of non-primary replicants loaded per coordination cycle

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a2l007 commented on a change in pull request #11135:
URL: https://github.com/apache/druid/pull/11135#discussion_r617042227



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File path: docs/configuration/index.md
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@@ -822,6 +823,7 @@ Issuing a GET request at the same URL will return the spec that is currently in
 |`decommissioningMaxPercentOfMaxSegmentsToMove`|  The maximum number of segments that may be moved away from 'decommissioning' servers to non-decommissioning (that is, active) servers during one Coordinator run. This value is relative to the total maximum segment movements allowed during one run which is determined by `maxSegmentsToMove`. If `decommissioningMaxPercentOfMaxSegmentsToMove` is 0, segments will neither be moved from _or to_ 'decommissioning' servers, effectively putting them in a sort of "maintenance" mode that will not participate in balancing or assignment by load rules. Decommissioning can also become stalled if there are no available active servers to place the segments. By leveraging the maximum percent of decommissioning segment movements, an operator can prevent active servers from overload by prioritizing balancing, or decrease decommissioning time instead. The value should be between 0 and 100.|70|
 |`pauseCoordination`| Boolean flag for whether or not the coordinator should execute its various duties of coordinating the cluster. Setting this to true essentially pauses all coordination work while allowing the API to remain up. Duties that are paused include all classes that implement the `CoordinatorDuty` Interface. Such duties include: Segment balancing, Segment compaction, Emission of metrics controlled by the dynamic coordinator config `emitBalancingStats`, Submitting kill tasks for unused segments (if enabled), Logging of used segments in the cluster, Marking of newly unused or overshadowed segments, Matching and execution of load/drop rules for used segments, Unloading segments that are no longer marked as used from Historical servers. An example of when an admin may want to pause coordination would be if they are doing deep storage maintenance on HDFS Name Nodes with downtime and don't want the coordinator to be directing Historical Nodes to hit the Name Node with API req
 uests until maintenance is done and the deep store is declared healthy for use again. |false|
 |`replicateAfterLoadTimeout`| Boolean flag for whether or not additional replication is needed for segments that have failed to load due to the expiry of `druid.coordinator.load.timeout`. If this is set to true, the coordinator will attempt to replicate the failed segment on a different historical server. This helps improve the segment availability if there are a few slow historicals in the cluster. However, the slow historical may still load the segment later and the coordinator may issue drop requests if the segment is over-replicated.|false|
+|`maxNonPrimaryReplicantsToLoad`|This is the maximum number of non-primary segment replicants to load per Coordination run. This number can be set to put a hard upper limit on the number of replicants loaded. It is a tool that can help prevent long delays in new data being available for query after events that require many non-primary replicants to be loaded by the cluster; such as a Historical node disconnecting from the cluster. The default value essentially means there is no limit on the number of replicants loaded per coordination cycle.|`Integer.MAX_VALUE`|

Review comment:
       It would be useful if we add some info regarding what could be a good starting value to set this to.

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File path: server/src/main/java/org/apache/druid/server/coordinator/duty/RunRules.java
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@@ -128,6 +130,18 @@ public DruidCoordinatorRuntimeParams run(DruidCoordinatorRuntimeParams params)
       boolean foundMatchingRule = false;
       for (Rule rule : rules) {
         if (rule.appliesTo(segment, now)) {
+          if (
+              stats.getGlobalStat(
+                  "totalNonPrimaryReplicantsLoaded") >= paramsWithReplicationManager.getCoordinatorDynamicConfig()
+                                                                                   .getMaxNonPrimaryReplicantsToLoad()
+              && !paramsWithReplicationManager.getReplicationManager().isLoadPrimaryReplicantsOnly()
+          ) {
+            log.info(
+                "Maximum number of non-primary replicants [%d] have been loaded for the current RunRules execution. Only loading primary replicants from here on.",

Review comment:
       Since this behavior is valid only for the present coordinator run, the log message might be clearer with something like "Only loading primary replicants from here on for this coordinator run period"




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