lucasbru commented on code in PR #23454:
URL: https://github.com/apache/kafka/pull/23454#discussion_r4034924120
##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/AssignmentRefinerImpl.java:
##########
@@ -288,6 +286,240 @@ private static boolean isReady(
.anyMatch(holder -> holder.processId().equals(targetProcessId) &&
holder.caughtUp());
}
+ /**
+ * Decides which of the staged migrations get a warm-up task, under the
warmup budget.
+ *
+ * <p>There is different scenarios:
+ * <ul>
+ * <li>A warm-up task already restoring keeps its warm-up slot if the
target assignment didn't change, and the
+ * warmu-up task is not caught up yet. It could also get revoked if
the warmup budget was reduced and keeping
+ * the warmup would now exceed the budget.
+ * <li>A <b>fresh plant</b> puts a warm-up task on a target owner
whose process holds nothing for the task,
+ * and spends a warm-up slot.</li>
+ * <li>When the target owner <em>itself</em> already holds a standby
of the task we can <b>borrow</b> it,
+ * and no warmup budget is used: that standby warms-up the task
anyway.</li>
+ * <li>If a target member's <em>sibling</em> hold a standby, we cannot
borrow but, but need to move the
+ * standby to its new owner, and putting a warmup on the target
member, spending a warm-up slot.
+ * (Cf case (2) of {@link #isReady(CurrentAssignmentIndex, TaskId,
String, String)} </li>
+ * </ul>
+ *
+ * <p>Everything else <b>parks</b> -- the task keeps running on its
current owner with nothing warming up, and a
+ * later refinement step picks it up once a warm-up slot frees.
+ *
+ * @param decisions
+ * What the case analysis decided, from {@link #analyzeTasks}.
+ * @param members
+ * All members of the group, used to resolve which process a task's
current owner runs in.
+ * @param processLoad
+ * The load of each process, from {@link #indexProcessLoad}.
+ * @param numWarmupReplicas
+ * How many copies beyond the target assignment may exist at once,
group-wide.
+ *
+ * @return Which warm-up tasks the intermediate assignment places, and how
each staged migration is warmed.
+ */
+ static WarmupPlan planWarmups(
+ final TaskDecisions decisions,
+ final Map<String, StreamsGroupMember> members,
+ final Map<String, ProcessLoad> processLoad,
+ final int numWarmupReplicas
+ ) {
+ if (numWarmupReplicas == 0) {
+ return WarmupPlan.EMPTY;
+ }
+
+ final SortedMap<TaskId, String> warmupTasks = new TreeMap<>();
+ final SortedSet<TaskId> borrowedMigrations = new TreeSet<>();
+ final SortedSet<TaskId> parkedMigrations = new TreeSet<>();
+
+ final List<FundingCandidate> keptWarmups = new ArrayList<>();
+ final List<FundingCandidate> newWarmupCandidates = new ArrayList<>();
+
+ for (final StagedMigration migration : decisions.stagedMigrations()) {
+ final Warming warming = warmingOf(migration);
+ switch (warming) {
+ case PARK -> parkedMigrations.add(migration.task());
+ case BORROW -> borrowedMigrations.add(migration.task());
+ case KEEP -> keptWarmups.add(fundingCandidate(migration,
members, warming));
+ // Both put a warm-up task on the target owner and both cost a
warm-up slot, so they share one
+ // candidate list -- but a plant is funded ahead of a sibling
move (see comparePriority).
+ case PLANT, SIBLING_MOVE ->
+ newWarmupCandidates.add(fundingCandidate(migration,
members, warming));
+ }
+ }
+
+ // Warm-up tasks already restoring are funded first. If {@code
num.warmup.replicas} config was reduced, we might
+ // be over warmup budget and have to give up some warmup tasks.
Evicting in reverse funding order
+ // keeps which ones deterministic rather than dependent on iteration
order.
+ // Note: revocation of warmup task happens implicitly by not adding
them to the assignment patch again
+ keptWarmups.sort((left, right) -> comparePriority(left, right,
processLoad, Map.of()));
+ for (int i = 0; i < keptWarmups.size(); i++) {
+ final FundingCandidate keptWarmup = keptWarmups.get(i);
+ if (i < numWarmupReplicas) {
+ warmupTasks.put(keptWarmup.task(), keptWarmup.targetOwner());
+ } else {
+ parkedMigrations.add(keptWarmup.task());
+ }
+ }
+
+ // New warm-up tasks raises its target process's load, so we need to
update it while we go, and find a new
+ // `best` from scratch each time
+ // note: this nested-loop is bounded by the number of unused warm-up
slots; so while it's O(unused * candidate)
+ // it's effectively not quadratic (we can consider `unused` a constant)
+ final Map<String, Integer> newWarmupsByProcess = new HashMap<>();
+ int used = Math.min(keptWarmups.size(), numWarmupReplicas);
+
+ while (used < numWarmupReplicas && !newWarmupCandidates.isEmpty()) {
+ int best = 0;
+ for (int candidate = 1; candidate < newWarmupCandidates.size();
candidate++) {
+ final int comparison = comparePriority(
+ newWarmupCandidates.get(candidate),
+ newWarmupCandidates.get(best),
+ processLoad,
+ newWarmupsByProcess
+ );
+ if (comparison < 0) {
+ best = candidate;
+ }
+ }
+
+ final FundingCandidate funded = newWarmupCandidates.remove(best);
+ warmupTasks.put(funded.task(), funded.targetOwner());
+ newWarmupsByProcess.merge(funded.targetProcessId(), 1,
Integer::sum);
Review Comment:
Seems like a valid concern.
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