Multiple instances vs multiple hosts

[lightbulb432 <ve...@hotmail.com>]

In production, for organizations that run multiple web applications, would
they run it on different instances of Tomcat (1 instance per web application
or group of related web applications), or all web applications as different
hosts on one instance of Tomcat? (Not taking into account clustering, where
everything in whatever setup is chosen would be duplicated.)

What are the tradeoffs in terms of performance, resource usage,
maintainability, etc, and what are your comments based on past experience?
What's common in production and what isn't as much?

Thanks.
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Re: Multiple instances vs multiple hosts

[Christopher Schultz <ch...@christopherschultz.net>]

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lb,

lightbulb432 wrote:
> In production, for organizations that run multiple web applications, would
> they run it on different instances of Tomcat (1 instance per web application
> or group of related web applications), or all web applications as different
> hosts on one instance of Tomcat? (Not taking into account clustering, where
> everything in whatever setup is chosen would be duplicated.)

The is completely dictated by the deployment team and includes multiple
factors. We prefer the one-Tomcat-instance-per-webapp because we want to
insulate them from each other. If one application leaks memory (for
instance), then only one JVM will be disabled when that happens.

Our current needs are limited, so we can run all our applications on a
single physical machine. Moving to multiple machines would be trivial
since the applications are already deployed separately.

To deploy in this way, with everything running over port 80 (and 443),
we need to front these Tomcat instances with a routing web server (we
chose Apache httpd).

> What are the tradeoffs in terms of performance, resource usage,
> maintainability, etc, and what are your comments based on past experience?
> What's common in production and what isn't as much?

Memory usage is certainly the most significant resource "waste" when
considering the above strategy. All of our applications fit comfortably
in a quarter gigabyte or so, so running multiple JVMs on the same
hardware isn't a big deal for us.

I would venture a guess that in terms of CPU performance, the difference
is negligible, depending on how your operating system treats threads
versus processes. We are deployed on Linux 2.6 which treats them about
the same, so I would expect the performance impact to be minimal.

I find this setup to be very maintainable, since sometimes Tomcat
versions and built-in libraries interfere with each other. In the
described setup, each application has its own Tomcat environment that
can be separately tested, upgraded, etc. If you need a new XML parser
but Tomcat's built-in version is interfering (used to happen... might
not after the 4.1 versions), no problem: replace the built-in XML
parser, test, deploy. No need to interfere with other applications.

In other production environments, I have typically seen large
applications clustered. One machine runs one application, and that
machine is duplicated for other members in the cluster.

Depending on your hardware, more applications per physical server might
make more sense. These days, you can get some pretty beefy commodity
64-bit hardware that can handle a lot of traffic. You will quickly find
out that SSL traffic is the most expensive thing to manage, not your
application ;)

- -chris

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