WebSphere eXtreme Scale - How many catalog servers do I need?
JonMarshall 1000009QM4 Visits (3455)
The WebSphere eXtreme Scale (WXS) catalog server is an interesting component.
It can be a very lowly utilised component of a WebSphere eXtreme Scale grid. After all, it is only used when a WXS client tries to initially connect to a grid (typically a one-off connection) or when a grid needs to rebalance due to a planned or unplanned grid container restart.
However it is a very important part of the grid. The grid simply cannot be used without one. A WXS client could not connect and the grid wouldn't know how to organise itself. So, how many catalog servers do I need?
Of course, the answer will depend on your availability requirements.
The minimum you need to have in order to have some availability is 2 catalog servers.
But our recommendation would be to have 3 or more. (3 is decidedly uneven and unsymmetrical, so computer scientist types seem far more happy with 4!)
This has the small overhead of another Java process, but gives better availability, even in the case of an outage
Making use of quorum
Related to this is the idea of quorum This isn't enabled by default, but gives you an important capability preventing catalog servers becoming disconnected (due to a network issue) and making separate, potentially different placement decisions. This would leave the grid in an inconsistent state.
To prevent this happening, when we place catalog servers into quorum, a catalog server will not make a placement decision until it is contact with all of the other catalog servers
There is an operational side effect to this. If you use quorum, and a catalog server fails (unplanned stop), quorum is broken and the grid will not be issued any further placement decision or rebalance until the catalog server retu