Question on replication/high availability designs

Question

We're currently trying to work out a design for a high-availability system using Oracle 9i release 2. Having gone through some of the Oracle whitepapers, it appears that the ideal architecture involves setting up 2 RAC sites using Dataguard to synchronize the data. However, due to time and financial constraints, we are only allowed to have 2 servers for hosting the databases, which are geographically separate from each other in prevention of natural disasters. Our app servers will use JDBC pools to connect to the databases.
Our goal is to have both databases be the mirror image of each other at any given time, and the database must be working 24/7. We do have a primary and a secondary distinction between the two, so if the primary fails, we would like the secondary database to take over the tasks as needed.
The ability to query existing data is mission critical. The ability to write/update the database is less important, however we do need the secondary to be able to process data input/updates when primary is down for a prolonged period of time, and have the ability to synchronize back with the primary site when it is back up again.
My question now is which replication technology should we try to implement? I've looked into both Oracle Advanced Replication and Dataguard, each seems to have its own advantages and drawbacks:
Replication - can easily switch between the two databases using multimaster implementation, however data recovery/synchronization may be difficult in case of failure, and possibly will lose data (pending implementation). There has been a few posts in this forum that suggested that replication should not really be considered as an option for high availability, why is that?
Dataguard - zero data loss in failover/switchover, however manual intervention is required to initiate failover/switchover. Once the primary site fails over to the standby, the standby becomes the primary until DBA manually goes back in and switch the roles. In Oracle 10g release 2, seems that automatic failover is achieved through the use of an extra observer piece. There does not seem to be anyway to do this in Oracle 9i release 2.
Being new to the implementation of high-availability systems, I am at somewhat of a loss at this point. Both implementations seem to be a possible candidate, but we will need to sacrifice some efforts for both of them also. Would anyone shine some light on this, maybe point out my misconceptions with Advanced Replication and Dataguard, and/or suggest a better architecture/technology to use? Any input is greatly appreciated, thanks in advance.
Sincerely,
Peter Tung

Answer 1

Hi,
It sounds as if you're talking about the DB_TXN_NOSYNC flag, rather than DB_NOSYNC.
You mention that in general, you lose uncommitted transactions on system failure. I think what you mean is that you may lose some committed transactions on system failure. This is correct.
It is also correct that if you use replication you can arrange to have clients have a copy of all committed transactions, so that if the master fails (and enough clients do not fail, of course) then the clients still have the transaction data, even when using DB_TXN_NOSYNC.
This is a very common usage scenario for Berkeley DB replication/HA, used to achieve high throughput. You will want to pay attention to the configured ack policy, group size setting, setting of the 2SITE_STRICT option (if group size == 2).