Managing HP Serviceguard for Linux, Sixth Edition, August 2006

Designing Highly Available Cluster Applications
Restoring Client Connections
Appendix B 315
the retry to the current server should continue for the amount of
time it takes to restart the server locally. This will keep the client
from having to switch to the second server in the event of a
application failure.
• Use a transaction processing monitor or message queueing software
to increase robustness.
Use transaction processing monitors such as Tuxedo or DCE/Encina,
which provide an interface between the server and the client.
Transaction processing monitors (TPMs) can be useful in creating a
more highly available application. Transactions can be queued such
that the client does not detect a server failure. Many TPMs provide
for the optional automatic rerouting to alternate servers or for the
automatic retry of a transaction. TPMs also provide for ensuring the
reliable completion of transactions, although they are not the only
mechanism for doing this. After the server is back online, the
transaction monitor reconnects to the new server and continues
routing it the transactions.
• Queue Up Requests
As an alternative to using a TPM, queue up requests when the server
is unavailable. Rather than notifying the user when a server is
unavailable, the user request is queued up and transmitted later
when the server becomes available again. Message queueing
software ensures that messages of any kind, not necessarily just
transactions, are delivered and acknowledged.
Message queueing is useful only when the user does not need or
expect response that the request has been completed (i.e, the
application is not interactive).