Gabriele Bulfon wrote:
Hi, thanx for the interesting solution.
Anyway, not always possible to run 2 tomcat instances because of memory
requirements.
Do you see any different solution with 1 tomcat only (or tomcat + apache)?
Gabriele,
I think Peter gave you the only appropriate general answer.
You could imagine a scheme whereby
- you have under Tomcat 2 versions of your webapp (say /webappA and
/webappB), of which only one is the active one at any particular time
- your users access the application by a URL like /XYZ/..
- the front-end Apache, by using something like mod_rewrite, would
rewrite such URLs to either /webappA or /webappB, before forwarding them
to Tomcat. Say that for the moment the active one is webappA, and the
Apache front-end rewrites /XYZ to /webappA.
- then you update webappB
- then you change the Apache rewrite rule to rewrite /XYZ to /webappB,
and reload Apache (not restart, reload)
- then you can update webappA
...
BUT, how your Tomcat applications and the ongoing user sessions in them
will react to this, only you can tell, knowing your application and what
kind of change you make from one version to the next.
Clearly, if a change involves changing the content of what is stored in
a session, or the forms you are sending to the browsers (*), you would
be in trouble. My knowledge of Java and Tomcat is too limited to know
what other kind of trouble you may be in, but I can imagine it may be
plenty.
It may work, if every HTTP request to Tomcat is really independent of
any previous one, and you do not use sessions.
Peter's solution is the real solid one. Memory requirements can be
solved nowadays for very little money. The time you would spend
designing and debugging a scheme like the above would be many times more
expensive.
(*) example :
- webappA sends a form to a browser, with input fields a,b,c,d.
- you update webappB, and now the form has fields a,b,c,d,e, and webappB
expects that
- now you switch webappB for webappA
- then the user submits the form, still with fields a,b,c,d. But this
submit goes to webappB now.
Does webappB survive ?
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