MySQL 9.4 Release Notes
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 |