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Looking into using Galera and MariaDB across 3 data centers. We have a metroE comnection between the sites. Connectivity is pretty stable, but my concern is if a WAN link goes down at one of the sites, and I am writing to the local nodes.

How does it handle that situation?

How many nodes at minimum at each site is recommended?

This question was similar, but different: https://stackoverflow.com/questions/27777134/galera-cluster-replication-between-two-data-centers/27793927#27793927

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If the WAN link breaks, the cluster will split into 2 clusters. The bigger cluster will be a PRIMARY cluster, meaning that it will work properly. The other partition won't accept any write until it re-joins with the PRIMARY cluster and the data in the nodes are updated.

Note that it is very important to set a good value for evs.suspect_timeout. Before this timeout occurs, the whole cluster will wait a response from unreachable node. After that timeout occurs, the cluster is partitioned. If the value is too low, slow nodes may cause a cluster partitioning. If the timeout is too high, working nodes may wait uselessly for a response from unreachable nodes.

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  • How does the smaller cluster know it is the smaller cluster?
    – Andrew Brennan
    Commented Sep 2, 2016 at 9:15
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    If cluster_size > all_nodes / 2, it is a PRIMARY cluster. If not, it isn't. That's the reason why you should never have a 2 nodes cluster: if one crashes, the other one cannot be PRIMARY.
    – Federico Razzoli
    Commented Sep 3, 2016 at 16:46

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