Don't worry, clustering is simple in theory, but VERY complicated in practice, as we found out when building 5!
With v4 and earlier, we maintained a persistent connection to the database on the SQL Server which worked just great as long as the server stays running - which of course if 99.999% of the time ideally! This persistent connection was the usual way of connecting to databases for SQL2005 (which was the main version at the time!) plus essential for users still needing to run with the old Jet file based databases.
But with clustering, when SQL-A fails, the cluster realises it needs to spin up SQL-B, (which yes, takes a short delay) but when it's running, SQL-B has no record of the connections that SQL-A had back before it died, so Myriad can no longer get any data from that server.
With v5 we changed to a per-request connection system which is how all modern DB systems expect connections, and is essential for things like clustering to work. This would actually be very slow if every time a connection was needed a new one had to be created, so In the middle there is actually a very clever SQL Connection Cache (managed by Windows itself) which keeps track of connections and re-uses them between requests, so the performance is kept incredibly fast. This is "cluster aware" so if the server dies then the cache knows to kill off it's contents and start working with the new server.
Myriad v5 also has a fairly extensive multi-retry system baked into the code as well so that in the event of a SQL Server running slow (or in the case of a cluster, taking a little while to spin up the new server) Myriad actually retries certain queries multiple times to try and get some data to keep running - this is why v5 is much more tolerant when run on flaky hardware or over slow links that often have packet timeouts as we see on large WAN systems in some overseas countries.
Hope that makes sense?
Peter Jarrett, Technical Director
Broadcast Radio Ltd.
Bill Bailey: No win, no fee, no basis in reality. Just a room above a minicab office in Acton and a steady stream of greedy simpletons whose delusion is only matched by their clumsiness