That all depends on the setup. The "standard" setup (not specific to MySQL) is:
- Single Write Leader per partition
- Backup Write Leader that is setup with synchronous replication (so WL -> WLB and waits for commit)
- Read Followers all connected asynchronously using either binlog replication (not recommended anymore) or GTID-based row replication (recommended)
In the above scenario, the odds of loss are pretty small since the Write Leader has a direct backup, and any of the Read Followers can be promoted to a Write Leader/Backup. DDIA calls the above semi-synchronous replication, although MySQL now supports a similar-but-slightly different version out of the box: https://dev.mysql.com/doc/refman/8.4/en/replication-semisync...
- Single Write Leader per partition
- Backup Write Leader that is setup with synchronous replication (so WL -> WLB and waits for commit)
- Read Followers all connected asynchronously using either binlog replication (not recommended anymore) or GTID-based row replication (recommended)
In the above scenario, the odds of loss are pretty small since the Write Leader has a direct backup, and any of the Read Followers can be promoted to a Write Leader/Backup. DDIA calls the above semi-synchronous replication, although MySQL now supports a similar-but-slightly different version out of the box: https://dev.mysql.com/doc/refman/8.4/en/replication-semisync...