Load balancing and redundancy

[Leon Nicholls <Le...@sonusnet.com>]

I managed to get my Apache server configured with two Tomcat servers
with load balancing. Everything works as expected with the requests
evenly handled by the two Tomcat servers.

I was thinking of using this architecture for redundancy, but I have
discovered a problem. If the first Tomcat server goes down, the
round-robin algorithm works fine and the requests go to the second
Tomcat server only (as expected). If I try and bring the first Tomcat
server up again, Apache (through mod_jk) does not pick up the new server
and always sends the request to the second server. Seems the only way to
get both Tomcat servers back up for Apache is to restart Apache. I was
hoping that this architecture could be used to ensure that I would never
have to restart Apache if the Tomcat servers goes down, therefore
ensuring continuous up-time.

I had a look at the mod_jk source code and it seems that if Apache
cannot connect to the Tomcat server, the server is marked as dead. There
does not seem to be a mechanism to check if the server is back up again.
Therefore "dead" servers are not used in the round-robin algorithm even
if they come up again.

My question is do I understood the problem correctly? Has somebody else
experienced these issues and what have they done about it?

Hope you can shed some light on the problem. Thanks for you time.
Leon


Leon Nicholls			INtelligentIP Division
Sonus Networks, Inc.		Tel: (972) 728-8953
Suite 3000			Fax: (972) 301-4901
1701 N. Collins Blvd.		www.sonusnet.com
Richardson, TX 75080		www.inippowered.com