Re: 5 important questions

On Sun, Jul 5, 2009 at 6:25 AM, Daniel
Lanciana<[email protected]> wrote:

> 1. Imagine two web servers, Tomcat and Apache HTTP.
> Tomcat has a web application using Prevayler, Apache HTTP has a web
> application in PHP.
> How do I access the live Tomcat "data" from the second web server (in PHP)?
> Manually created web services?

Perhaps you can't.  Usually the only way to accept the data in
Prevaylor is to go through your Java app.
If you make a web service out of your app then the PHP could access
the data that way.  But the PHP
is not going to access the database directly.  If you want to use your
database for application integration,
don't use Prevalor.  On the other hand, if you want to integrate
through web services, Prevaylor might
work very well.

> 3, How do you know when more RAM is needed? Does everyone need to set up
> their own monitoring?

My rule of thumb is that if a system pages, it needs more memory.  So,
use a utility that tells you how much the operating system is paging.
A Prevayler-type system only works well if all the data fits in
memory.

> 5.Example: There is a web application using Prevayler in production. A new
> field, "referrer", needs to be added to the User POJO.
> Do I just add the new field to the POJO and restart the web application?
> Will it automatically convert all the previously saved User objects (no
> referrer field) to the new User object? If not, please describe what steps
> would need to be done.

Any Prevayler-type system needs a way of storing data on the disk.
When you boot up the system, it will read in all the data, and
thereafter access the in-memory version of the data.  When you change
your schema, you have to change the program that reads in the data so
it can handle old data formats.  Usually this is easy, but sometimes
it is hard.  In this case, you have to define a default referrer for
all the old data objects, since they won't have a referrer field.

-Ralph Johnson

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