The namespace backlog is something different. In Pulsar, a subscription maintains a state of message acknowledged. A subscription backlog is the set of messages which are unacknowledged. A subscription backlog size is the sum of size of unacknowledged messages(in bytes). A topic can have many subscriptions. A topic backlog is defined as the backlog size of the subscription which has the oldest unacknowledged message. Since acknowledged messages can be interleaved with unacknowledged messages, calculating the exact size of that subscription can be expensive as it requires I/O operations to read from the messages from the ledgers. For that reason, the topic backlog is actually defined to be the estimated backlog size of that subscription. It does so by summarizing the size of all the ledgers, starting from the current active one, up to the ledger which contains the oldest unacknowledged message (There is actually a faster way to calculate it, but this is the definition of the estimation). So a namespace backlog is one level up from that. I need to read the code to verify