Build a pub-sub system

• Create topics; publishers send messages to a topic, which appends them to an ordered log with increasing offsets
• Subscribers register with a topic and a consumer group; each group receives every message once, and members of the same group share the work (a message goes to one member)
• Delivery is asynchronous: publish returns immediately after the append, and a slow subscriber does not block publishers or other subscribers
• Each group tracks its own committed offset, so a new group can start from the beginning (replay) or from the latest offset
• At-least-once delivery: a message is redelivered if the handler throws or does not acknowledge within a timeout; handlers should be idempotent, and the system exposes the delivery attempt number
• A failing message is retried a limited number of times, then moved to a dead-letter topic
• unsubscribe stops delivery and releases resources; publishing to an unknown topic fails clearly

Only after every requirement works.

• Partitions with key-based routing so messages for the same key stay ordered
• Message retention by count or age
• Backpressure: bounded in-flight messages per subscriber