Galera Node Operation when Only One Single Node Left



  • Official comment
    Krzysztof Ksiazek


    The main difference is between "a node has failed" and "a node has been stopped". When you stop the node, it leaves the cluster in a graceful manner. As a result, the cluster size is reduced (from 3 to 2 and then later to 1). This has an impact on the fault tolerance of the cluster. If you have 3 nodes, one can fail as remaining two will have majority. If you stop one node, you'll have two. Any failure will result in a cluster in the non-Primary state. So, to sum it up, if you have three nodes, you can stop gracefully two of them but only one failure will be tolerated. I hope this makes it clear.



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    You are correct.  I have performed a test yesterday.

    When two nodes were gracefully shut down, the one single node could operate normally (Primary status).

    When one node failed, the two remaining nodes still operate normally (Primary status).  When one more node failed, the single node left will be changed into non-Primary.  It cannot be operate anymore, even 'select' operation was not allowed.

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