clintropolis commented on a change in pull request #11257:
URL: https://github.com/apache/druid/pull/11257#discussion_r648885387
##########
File path:
server/src/main/java/org/apache/druid/server/coordinator/duty/BalanceSegments.java
##########
@@ -180,22 +181,29 @@ private void balanceTier(
int maxSegmentsToMove
)
{
+ if (maxSegmentsToMove <= 0) {
+ return new Pair<>(0, 0);
+ }
+
final BalancerStrategy strategy = params.getBalancerStrategy();
final int maxIterations = 2 * maxSegmentsToMove;
final int maxToLoad =
params.getCoordinatorDynamicConfig().getMaxSegmentsInNodeLoadingQueue();
int moved = 0, unmoved = 0;
+ Iterator<BalancerSegmentHolder> segmetnsToMove =
strategy.pickSegmentsToMove(
Review comment:
nit: typo `segmetnsToMove` -> `segmentsToMove`
##########
File path:
server/src/main/java/org/apache/druid/server/coordinator/ReservoirSegmentSampler.java
##########
@@ -31,6 +32,42 @@
private static final EmittingLogger log = new
EmittingLogger(ReservoirSegmentSampler.class);
+ static List<BalancerSegmentHolder> getRandomBalancerSegmentHolders(
+ final List<ServerHolder> serverHolders,
+ Set<String> broadcastDatasources,
+ int k
Review comment:
does it make sense to retain the `percentOfSegmentsToConsider`
functionality to allow short-circuiting iterateAllSegments across all servers?
We update the docs
https://github.com/apache/druid/blob/master/docs/configuration/index.md#dynamic-configuration
accordingly either way
##########
File path:
server/src/main/java/org/apache/druid/server/coordinator/BalancerStrategy.java
##########
@@ -71,13 +71,60 @@
* @return {@link BalancerSegmentHolder} containing segment to move and
server it currently resides on, or null if
* there are no segments to pick from (i. e. all provided
serverHolders are empty).
*/
+ @Deprecated
Review comment:
this doesn't really seem deprecated so much as no longer called at all,
maybe we should remove it?
##########
File path:
server/src/main/java/org/apache/druid/server/coordinator/ReservoirSegmentSampler.java
##########
@@ -31,6 +32,42 @@
private static final EmittingLogger log = new
EmittingLogger(ReservoirSegmentSampler.class);
+ static List<BalancerSegmentHolder> getRandomBalancerSegmentHolders(
+ final List<ServerHolder> serverHolders,
+ Set<String> broadcastDatasources,
+ int k
+ )
+ {
+ List<BalancerSegmentHolder> holders = new ArrayList<>(k);
+ int numSoFar = 0;
+
+ for (ServerHolder server : serverHolders) {
+ if (!server.getServer().getType().isSegmentReplicationTarget()) {
+ // if the server only handles broadcast segments (which don't need to
be rebalanced), we have nothing to do
+ continue;
+ }
+
+ for (DataSegment segment : server.getServer().iterateAllSegments()) {
+ if (broadcastDatasources.contains(segment.getDataSource())) {
+ // we don't need to rebalance segments that were assigned via
broadcast rules
+ continue;
+ }
+
+ if (numSoFar < k) {
+ holders.add(new BalancerSegmentHolder(server.getServer(), segment));
+ numSoFar++;
+ continue;
+ }
+ int randNum = ThreadLocalRandom.current().nextInt(numSoFar + 1);
+ if (randNum < k) {
+ holders.set(randNum, new BalancerSegmentHolder(server.getServer(),
segment));
+ }
Review comment:
i think it would be worth adding some code comments here to describe
that this algorithm has 2 phases, where it picks the first `k` segments from
the servers, in order, then iterates through the server list randomly replacing
these picked segments with decreasing frequency the more segments have been
iterated.
Im also somewhat curious/nervous about random this pick method is compared
to the previous one, though I'm not entirely sure how it would be measured.
I think we should add a new coordinator dynamic config property,
https://github.com/apache/druid/blob/master/server/src/main/java/org/apache/druid/server/coordinator/CoordinatorDynamicConfig.java,
to allow falling back to the old algorithm in case there are any
unpredictable strange behavior caused by this new algorithm, maybe
`useBatchedSegmentSampler` or .. i'm not sure, naming is hard.
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