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MySQL 5.7 Reference Manual
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17.1.3.3 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

The next Chapter covers technical aspects of Group Replication.


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