clintropolis commented on a change in pull request #7185: Avoid many
unnecessary materializations of collections of 'all segments in cluster'
cardinality
URL: https://github.com/apache/incubator-druid/pull/7185#discussion_r262811871
##########
File path:
server/src/main/java/org/apache/druid/server/coordinator/DruidCoordinator.java
##########
@@ -94,10 +93,29 @@
@ManageLifecycle
public class DruidCoordinator
{
- public static Comparator<DataSegment> SEGMENT_COMPARATOR =
Ordering.from(Comparators.intervalsByEndThenStart())
-
.onResultOf(DataSegment::getInterval)
-
.compound(Ordering.<DataSegment>natural())
-
.reverse();
+ /**
+ * This comparator orders "freshest" segments first, i. e. segments with
most recent intervals.
+ *
+ * It is used in historical nodes' {@link LoadQueuePeon}s to make
historicals load more recent segment first.
+ *
+ * It is also used in {@link DruidCoordinatorRuntimeParams} for {@link
+ * DruidCoordinatorRuntimeParams#getAvailableSegments()} - a collection of
segments to be considered during some
+ * coordinator run for different {@link DruidCoordinatorHelper}s. The order
matters only for {@link
+ * DruidCoordinatorRuleRunner}, which tries to apply the rules while
iterating the segments in the order imposed by
+ * this comparator. In {@link LoadRule} the throttling limit may be hit (via
{@link ReplicationThrottler}; see
+ * {@link CoordinatorDynamicConfig#getReplicationThrottleLimit()}). So
before we potentially hit this limit, we want
+ * to schedule loading the more recent segments (among all of those that
need to be loaded).
+ *
+ * In both {@link LoadQueuePeon}s and {@link DruidCoordinatorRuleRunner}, we
want to load more recent segments first
+ * because presumably they are queried more often and contain are more
important data for users, so if the Druid
+ * cluster has availability problems and struggling to make all segments
available immediately, at least we try to
+ * make more "important" (more recent) segments available as soon as
possible.
+ */
Review comment:
Thanks for javadoc :+1:
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