MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
MySQL Group Replication builds on an implementation of the Paxos
distributed algorithm to provide distributed coordination
between servers. As such, it requires a majority of servers to
be active to reach quorum and thus make a decision. This has
direct impact on the number of failures the system can tolerate
without compromising itself and its overall functionality. The
number of servers (n) needed to tolerate f
failures is then n = 2 x f + 1.
In practice this means that to tolerate one failure the group must have three servers in it. As such if one server fails, there are still two servers to form a majority (two out of three) and allow the system to continue to make decisions automatically and progress. However, if a second server fails involuntarily, then the group (with one server left) blocks, because there is no majority to reach a decision.
The following is a small table illustrating the formula above.
Group Size |
Majority |
Instant Failures Tolerated |
|---|---|---|
1 |
1 |
0 |
2 |
2 |
0 |
3 |
2 |
1 |
4 |
3 |
1 |
5 |
3 |
2 |
6 |
4 |
2 |
7 |
4 |
3 |