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Programming Languages Bug Report

Java ConcurrentModificationException removing from an ArrayList in for-each

Daniel Tran · May 13, 2025 · 8 min read

Our webhook dispatcher threw ConcurrentModificationException a few times a day and silently skipped deliveries the rest of the time. Two bugs shared one list: removal inside a for-each loop and an admin thread editing it concurrently.

Banner: Kafka consumers repeatedly reshuffling partitions in a rebalance loop while lag grows

Data Engineering Bug Report

Kafka consumer group stuck in a rebalance loop: max.poll.interval.ms and slow processing

Daniel Tran · Feb 13, 2025 · 9 min read

Our enrichment consumers rebalanced every few minutes, lag climbed past two million messages and some records were processed twice. A slow external API pushed batch processing past max.poll.interval.ms. How we proved it, fixed the poll loop and made rebalances cheap.