poorbarcode commented on code in PR #20469: URL: https://github.com/apache/pulsar/pull/20469#discussion_r1217407879
########## pip/pip-269.md: ########## @@ -0,0 +1,239 @@ +# Background knowledge + +The Cursor is used to read data from ledgers, there are three scenarios for reading messages(we can call these scenarios Read-Scenario): +- **Sequentially Read**: reading some entries backward from the `read position` and updating the read position to the next position to be read when reading is finished. + - Read entries from `read position`(include), after the read is complete, set `read position` to the next position of the last entry of this read, then the next round of **Sequentially Read**. +- **Message Replay Read**: Normally consumers will acknowledge messages all they received, but sometimes the consumer can not handle messages and asks the broker to redeliver these messages(these messages are always earlier than `read position`). E.g. call `consumer.negativeAcknowledge`; close a consumer and there are some messages held by this consumer, these messages will be redelivered to other consumers. Message Dispatcher will cache the position of messages which should be redelivered, and Cursor will read messages which were cached in the memory of Message Dispatcher in the next round. +- **Specified Read**(Read directly using Managed Ledger): there are some APIs that can read messages at a specified position, but generally do not affect the attributes of the Cursor or Message Dispatcher. E.g. Review Comment: Correct. In fact, this scenario has nothing related to the current proposal -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
