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 |