#### 18.1.4.3 Fault-tolerance

 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             |

