Compaq AA-Q88CE-TE manual Primary and Secondary Roles, Automatic Features, Shadow Events

B

Server Shadowing and Recovery

RTR shadowing gives you the ability to recover from a site disaster without the need for special coding within your application program. This appendix is an introduction to RTR shadowing.

A server for a database partition is said to be shadowed when two copies of the same server perform identical actions on identical database copies on separate nodes. It is possible to declare servers to be members of a pair of shadow sites for any particular RTR key-range. Shadowed sites can either be two nodes within a single cluster, or can be two separate clusters.

Note that concurrent servers handle similar transactions, (that is, in the same key-range but not the same transactions). Standby servers do not handle transactions at all (for the given key-range) and shadow servers handle the same transactions.

B.1 Primary and Secondary Roles

There is a concept of primary and secondary role for the shadow server pair, although in most cases this is transparent to the user when the processing is the same on both sites.

Initial role assignment is arbitrary, in that the first server of a shadow pair to start is given the primary role, and the second the secondary. The assigned roles may change, as servers come and go. Roles are required, since RTR needs to determine the voting order on the primary site before the transaction is presented to the secondary site.

B.2 Automatic Features

Shadow sites each have an identical copy of the customer's database.

Transactions are sent by RTR to both sites, and RTR ensures that they are processed by the servers in equivalent order on each of these sites, so that both copies of the customer database remain up-to-date.

A transaction is sent to the secondary site only after the primary has accepted it, or if the primary fails before being asked to vote.

RTR suppresses replies and broadcasts issued by the secondary shadow server.

B.2.1 Shadow Events

RTR provides the following shadowing events:

•RTR_EVTNUM_SRPRIMARY - Server is in primary mode

•RTR_EVTNUM_SRSTANDBY - Server is in standby mode

•RTR_EVTNUM_SRSECONDARY - Server is in secondary mode

•RTR_EVTNUM_SRSHADOWLOST - Server has lost its shadow partner

Server Shadowing and Recovery B–1