Hi community I am starting a DISCUSS for making the default value of the parameter "--get-subscription-backlog-size" of admin API "topics stats" true.
In the PR https://github.com/apache/pulsar/pull/9302, the property backlog size of each subscription returned in the response of the API topics stats, by default this property is always equal to 0 in response, and this will confuse users. Since the calculation of backlog size is done in broker memory, there is no significant overhead(the process is described in the following section), so I think the correct values should be displayed by default. ### The following two APIs should be affected: In Pulsar admin API ``` pulsar-admin topics stats persistent://test-tenant/ns1/tp1 --get-subscription-backlog-size pulsar-admin topics stats persistent://test-tenant/ns1/tp1 -sbs ``` the default value of parameter `--get-subscription-backlog-size` will be `true` In Pulsar Rest API ``` curl GET "http://127.0.0.1:8080/test-tenant/ns1/tp1/stats "?subscriptionBacklogSize=true ``` the default value of parameter `subscriptionBacklogSize ` will be `true` ### The following is the process of calculating backlog size: - Divide `PersistentTopc.ledgers` into two parts according to the ledgerId of the mark delete position of the cursor. The second part is ledgers indicating the messages still need to be consumed, aka backlogSizeInLedgers. - Find the LedgerInfo whose ledgerId is the same as the ledgerId of the mark delete position of the cursor, and we can also divide the ledger into two parts, the second part is entries indicating the messages still need to be consumed, multiply the average size of each entry in metrics by the number of still need to be consumed entries we can get the backlog size in this ledger. aka backlogSizeInEntries. - `backlogSizeInLe dgers` + `backlogSizeInEntries` Thanks Yubiao Feng
