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Date: Wed, 30 Jun 2010 23:38:13 -0400
Message-ID: <AANLkTim38T07jyuN20dwxoZ9U0-e1tkPkQ8GgAqQn9yT@mail.gmail.com>
Subject: Re: "Application" vars -
From: Shay Rojansky <roji@roji.org>
To: Tomcat Users List <users@tomcat.apache.org>
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Hi Eric.

On Wed, Jun 30, 2010 at 9:20 PM, Eric P <eric.maillist@gmail.com> wrote:

> Shay,
>
> Forgive all potential newbness in my responses below.  I'm still learning
> this stuff.
>
>
No worries for newbiness, your responses all make sense.


>
> Shay Rojansky wrote:
>
>> Hi Eric.
>>
>> Would making your servlet reload all application vars not be akin to
>> simply
>> reloading your servlet altogether, by changing context/init params in your
>> web.xml or context.xml?
>>
>>
> Do you mean "reloading your 'application' altogether"?  If so wouldn't that
> cause disruption to users currently on the app?  This disruption is what I'd
> like to avoid.


So it makes sense to go into what "disruption" means. I'm not 100% sure
about the following, it would be good if a tomcat heavyweight would
confirm/refute what I say.

When you initiate a webapp reload, Tomcat waits for requests that have
already started processing to terminate. This ensures that people who
accessed your app just before the webapp get a complete response. Once
that's done, the application is reloaded and your servlets' init methods are
called if necessary. During this time, incoming requests aren't denied, they
are just paused until the reload is complete.

So the only disruption people see is your application freezing up for the
time it takes to reload (which is going to depend on what you your
initialization consists of). No ugly server unavailable errors or anything
of the sort.

If you don't like the idea of your app freezing, think about this. Rereading
environment params without reloading has its own risks, namely potential
race conditions. Imagine you have 5 parameters, and requests are coming in
as you are reading these in and initializing your webapp. A request might be
handled while 2 params have been read, but 3 still contain the old values.
If you start to think about locking/synchronization to solve this you're
definitely better off just using Tomcat's reload mechanism.

So my answer would be, trust Tomcat's reloading process unless you
absolutely want to avoid your webapp freezing for the time it will take for
it to init (this depends on the webapp). If you want to do your own
"reloading", think long and hard about potential race conditions (which will
occur in all except the simplest cases).

Again, all this should probably be verified, you can set up very simple test
cases with a JSP that  sleeps, etc.


>
>  If you really want to avoid an application reload, why not just have your
>> app read its values from a properties config file instead of a DB? It
>> would
>> be much more lightweight and standard.
>>
>>
> That's an idea.  But wouldn't file I/O every time a servlet needs an
> application value be way more expensive than storing settings in a record,
> reading and setting them to the application scope once, and only resetting
> these vars manually when needed?
>

I wasn't proposing to perform I/O and parse the properties file upon every
request, simply to parse it once upon init and load it into
application-scoped vars. This is exactly the same as what you were planning
to do, except that instead of pulling the values from a DB record you pull
them from a properties file.

Note that using a property file will almost surely be faster (and not to
mention simpler) than a database, as connecting to a database usually means
network activity, authentication, etc. It would also make changing those
values easier - editing a properties file is more straightforward than a
database with its schema.

But at the end of the day, it all depends on your app. If you already have
everything in your DB and it makes sense to have those config values in
there too, why not.


>
> Is this standard documented somewhere?
>

What exactly? :)


>
> Thanks... I appreciate the ideas.
> Eric
>
>
>
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