Integration / Apache Pulsar Interview questions
What is the difference between backlog quota and retention policy?
Backlog quota caps how much unacknowledged data a subscription is allowed to accumulate; once the limit is hit, Pulsar applies the configured action - typically blocking producers or evicting the oldest unacked messages - to protect the cluster from an unbounded backlog caused by a stalled or slow consumer.
Retention policy, by contrast, governs already-acknowledged data - by default that data is eligible for deletion the moment every subscription has consumed it, and retention overrides that to keep it around longer anyway, for replay or late-joining subscriptions.
In short, they protect against opposite risks: backlog quota against too much unread data piling up, retention against read data disappearing too soon.
| Backlog Quota | Retention Policy |
| Governs unacknowledged (unread) data | Governs already-acknowledged data |
| Triggered by a slow/stalled consumer | Applied when you choose to keep read data longer |
| Typical action: block producer or evict oldest unacked | Data simply persists until the retention window expires |
Backlog quota exists to protect against:
Retention policy governs:
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