MySQL 8.4 Release Notes
Based on the metrics gathered across all servers in the group, a throttling mechanism kicks in and decides whether to limit the rate a member is able to execute/commit new transactions.
Therefore, metrics acquired from all members are the basis for calculating the capacity of each member: if a member has a large queue (for certification or the applier thread), then the capacity to execute new transactions should be close to ones certified or applied in the last period.
The lowest capacity of all the members in the group determines the real capacity of the group, while the number of local transactions determines how many members are writing to it, and, consequently, how many members should that available capacity be shared with.
This means that every member has an established write quota based on the available capacity, in other words a number of transactions it can safely issue for the next period. The writer-quota is enforced by the throttling mechanism if the queue size of the certifier or the binary log applier exceeds a user-defined threshold.
The quota is reduced by the number of transactions that were delayed in the last period, and then also further reduced by 10% to allow the queue that triggered the problem to reduce its size. In order to avoid large jumps in throughput once the queue size goes beyond the threshold, the throughput is only allowed to grow by the same 10% per period after that.
The current throttling mechanism does not penalize transactions below quota, but delays finishing those transactions that exceed it until the end of the monitoring period. As a consequence, if the quota is very small for the write requests issued some transactions may have latencies close to the monitoring period.