We're using DB2 9.5 on AIX and are looking at implementing HADR. HADR doesn't perform automatic failover so we need to use HACMP or TSA to automate it? Any suggestion which one to use? Are there any other options to automate the failover?

Veritas Cluster Manager makes a product that has an HADR agent. I have not personally used it, but looked at it about 3 years ago. TSA was not very good at the time, but has improved greatly since then in terms of HADR integration. My company uses Red Hat Cluster Manager (decision of the system admins).

Can HADR be used by itself? I mean do we really need to use some cluster software such as HACMP? If we don't use it, how do we detect primary is down and when to switch over to standby?

You can use HADR without a cluster manager. If you write your own scripts to monitor the databases you could issue the takeover command yourself. But in that case you have just written your own cluster manager (not that there is anything wrong with that). It would be best if the monitoring was done from a third server IMO.

This is exactly what I wanted to know