What is the mechanism for loading and recovering Rocket…

The message storage file loading and recovery mechanism of RocketMQ mainly consists of two aspects: file loading and file recovery.

1. File loading: RocketMQ uses MmappedFile to load message storage files. MmappedFile is a type of memory-mapped file that improves file read and write performance by mapping the file into memory. When RocketMQ starts, it scans the storage directory for files and loads them into memory for subsequent read and write operations.
2. File recovery: RocketMQ saves message data in CommitLog files, which are sequentially written files. After messages are written to CommitLog files, they are asynchronously flushed to disk. During the startup process of RocketMQ, the integrity of CommitLog files is checked. If the file is damaged or some data is lost, RocketMQ will automatically perform file recovery. The file recovery process mainly includes two steps: file recovery and data recovery.
3. File recovery: File recovery refers to restoring damaged CommitLog files starting from the last complete file until the newest file, RocketMQ will validate the files based on their magic code and size, skipping or deleting any damaged files if necessary.
4. Data recovery: Data recovery refers to restoring data in damaged files. RocketMQ will use file recovery modes such as overwriting, restoring, deleting, etc., to handle damaged data in order to ensure message integrity.

RocketMQ can automatically detect and restore the integrity of storage files during the startup process, ensuring the consistency and reliability of message data.