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fix(consumer): stop roundrobin balancer looping on an unsubscribed topic - #3740
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roundRobinBalancer.Plan searched members[i%n] forever when the topics map held a topic no member subscribes to. Bound the search to one pass over the members and leave such a topic unassigned, as the range and sticky strategies do. When a subscriber exists the same member is chosen and the cursor advances by the same amount, so existing plans do not change. Signed-off-by: Jeff Lenamon <jlenamon@gmail.com>
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puellanivis
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Sep 15, 2026
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| probe := 0 | ||
| for probe < n && !members[(i+probe)%n].hasTopic(tp.topic) { | ||
| probe++ | ||
| } | ||
| if probe == n { | ||
| continue // no member subscribes to this topic | ||
| } |
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🤔 We could probably do this by reworking the existing control flow instead:
m := member[i%n]
for range len(members) {
if m.hasTopic(tp.topic) {
break
}
i++
m = member[i%n]
}
if !m.hasTopic(tp.topic) {
continue // no member subscribes to this topic
}… cursor Rework the subscriber search as the reviewer suggested: advance the existing member cursor inside a loop bounded by the member count and skip the topic when the cursor comes back without a subscriber. Signed-off-by: Jeff Lenamon <jlenamon@gmail.com>
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Reworked to your form in the second commit; the same member is chosen and the cursor advances by the same amount, so the added test row and the existing rows still pass. |
dnwe
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Sep 20, 2026
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Thanks! Great contribution – lgtm
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roundRobinBalancer.Plannever returns when the topics map holds a topic no member subscribes to: the subscriber search walksmembers[i%n]forever. The range and sticky strategies leave such a topic unassigned.The search is now bounded to one pass over the members and a topic with no subscriber is skipped. Whenever a subscriber exists the same member is chosen and the cursor advances by the same amount, so existing plans are unchanged.
Sarama's own consumer group builds the topics map from the members' subscriptions, so this reaches direct callers of
Plan.A row added to
TestBalanceStrategyRoundRobincovers it; on main that row hangs until the test timeout.