Galera, MariaDB and multiple datacenter
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.