System design interview question
"Design a publish/subscribe messaging service — something like Google Pub/Sub or SNS/SQS. Publishers send to topics, many independent subscribers receive copies, each subscriber acknowledges messages individually, and undelivered messages are retried. Millions of messages per second."
per-message delivery state at scale, which is the crucial difference from a log-based queue. Kafka tracks one offset per consumer group; here each subscriber acknowledges each message independently, which means the broker must hold and manage per-message-per-subscriber state — and that changes the storage, redelivery, and ordering story completely.
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