[Jdo Wiki] Update of "Failover" by ErikBengtson

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The following page has been changed by ErikBengtson:
http://wiki.apache.org/jdo/Failover

New page:
= Failover in JDO =

JDO needs the capability to recover from connection failures switching to backup servers upon failure.

== Requirements ==

 * Switch to backup server when JDO is unable to obtain a connection to a database server/engine, or the connection breaks while in use.

 * User may be able to configure an ordered list of datasources.
 
 * JDO will default to the first datasource in the list, and if unable to obtain a connection, will attempt to obtain a connection from the next datasource in the list.

 * JDO will raise JDODatastoreException when unsuccessful attempts to obtain connection in all datasources are exausted.
 
 * If a database connection in use breaks, JDO may try to recover using the same datasource before switching to the next datasource in the list.
 
 * If a database connection in use breaks, JDO will apply changes made previously to the new connection during JDO transaction commit or flush. The broken connection has changes rolled back.
 
 * Each datasource in the list, may have a test statement that verifies if the connection is healthy.
 
 * For every new transaction, JDO will switch back to the first datasource in the list.
 
 * JDO, in background, may track which datasources are heathy to do faster switching.