I understand that doing an database cluster is a hard problem and even harder to make easy to use, but this seems really complicated to set up. I'm not a fan of needing "transaction managers", "coordinators" and data nodes, I would like these to all run on the same server.
Of cause Postgres-XL may not exactly be designed for my use case where I just want to be able to have one node fail or take a server down to do upgrades. Ideally it would be as simple as the built in Postgresql replication as become.
It may be that coordinators and datanodes, while the same binary, become one and the same as the same process. At the moment that is not the case.
The Global Transaction Manager here ensures that transaction ids and snapshots (the context in which statements are run, the other running transactions), are exactly the same across the entire cluster. Without it, there are theoretical possibilities of inconsistencies based on the timing of concurrently running transactions and when connections to other data nodes are taken.
The GTM ensures cluster-wide Multi-version Concurrency Control whereby readers do not block writers and writers do not block readers and ACID properties are respected globally.
As for taking a server down for upgrades, well, that is beyond the scope of the project. There have been people looking at XL for simple load balancing and HA, we should perhaps consider making it easier for this use case, but I think some use pgpool for that.
Of cause Postgres-XL may not exactly be designed for my use case where I just want to be able to have one node fail or take a server down to do upgrades. Ideally it would be as simple as the built in Postgresql replication as become.