How synchronous is galera cluster

Question

Actually I have couple of questions here.

1) When I call insert from my application using Mysql connector, its answered by one of the Master node, but does that master node waits before the insert is applied on all the nodes and then reply to the client. If it waits for all the nodes to insert before replying to the client then how is wsrep_sst_method=xtrabackup helps, will it make it reply to client immediately or will it make no difference. Maybe I understood this variable wrong.

2) What about read, I guess it is just answered by one of the master node. In case wsrep_sync_wait is set only in that case it waits for a reply from all the nodes. Thanks

Answer 1

"How synchronous"? Synchronous enough, but with one exception: "Critical read".

The "fix" is during reading, not writing.

1. When writing the heavyweight checking is done during COMMIT. At this point, all other nodes are contacted to see if "this transaction will eventually commit successfully". That is, the other nodes say "yes" but don't actually finish the work enough for a subsequent SELECT to see the results of the write. The guarantee here is that, the cluster is in a consistent state and will stay that way, even if any one node dies.

2. "Critical read" is, for example, when a user posts something, then immediately reads the database and expects to see the posting. But, if the read (SELECT) hits a different node, the "almost" synchronous nature of Galera may not have committed the data to the reading node. The data is there, and will be successfully written to disk, but maybe not yet. The workaround is to use wsrep_sync_wait when reading to assure that replication is caught up before the SELECT. No action is taken when writing.

(I don't see the relevance of wsrep_sst_method=xtrabackup. That relates to recovering from a dead node.)