What is the difference between a ledger and a segment in BookKeeper?

A ledger is BookKeeper's fundamental storage abstraction - an append-only sequence of entries that, once closed, is immutable; a Pulsar topic's data is really a chain of these ledgers over time, not one continuous file.

A segment, in Pulsar's terminology, generally refers to that same ledger viewed from the topic's perspective - the topic periodically "rolls over" to a new ledger (segment) based on size or time thresholds, and old segments are what tiered storage offloads once they age out.

This ledger-based structure is what makes operations like tiered storage and independent per-segment replication placement possible - each segment can, in principle, live on a different set of bookies or be offloaded to different storage entirely.

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