Re: "Application" vars -

[Eric P <er...@gmail.com>]

Shay,

Forgive all potential newbness in my responses below.  I'm still learning this stuff.

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.

> 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?

Is this standard documented somewhere?

Thanks... I appreciate the ideas.
Eric


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Re: "Application" vars -

[Shay Rojansky <ro...@roji.org>]

Just my comments on the dispatching idea proposed below...

If smooth processing with no freeze-time is important, I'm assuming there is
also some sort of cluster/high-availability functionality. If this is the
case, you are already running at least two Tomcats on different servers, and
using some load-balancing/dispatch mechanism - for example Apache with
mod_jk. If you have just one Tomcat you have much more to worry about than
Tomcat freezing during webapp reload :)

So, why not just use the load balancer's functionalities, instead of writing
a dispatcher yourself (webappX below)? In other words, you stop the webapp
on tomcat1 - the load balancer (or mod_jk or whatever) detects this
immediately and stops forwarding requests to it. You do whatever config
change, and start the app again. At this point the new app is added back to
the load balancer. Repeat same thing with tomcat2, tomcat3...

Seems simpler than deploying two copies of a webapp + a third dispatcher
that you need to write and maintain (and all this across the N servers in
your cluster)...

One last note: if I remember correctly, you can only forward requests within
the same servlet context (i.e. webapp), so "dispatching" from webappX to
webappA or webappB would be non-trivial. You could do redirections, but this
exposes your internal mechanism to clients and is really bad.

Shay

On Fri, Jul 2, 2010 at 4:27 AM, André Warnier <aw...@ice-sa.com> wrote:

> Eric P wrote:
>
>> 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.
>>>
>>>
>>>
>> Shay,
>> I think you made a good case for keeping app vars in web.xml (i.e., seems
>> pretty apparent now that's where they belong).
>>
>> Thanks for taking the time to respond.  I sincerely appreciate it!
>> Eric P.
>>
>>  +1
> Very neat explanation.
>
> While not being a great specialist, I would just like to expone what may be
> an alternative, if one really wants to diminish the (apparent) time it takes
> for a new version of a webapp to become available.  Since I am no great
> specialist, I will just outline the idea without Java/servlet specifics.
>
> (Maybe also I am barging through an open door, and there already exists a
> standard mechanism for doing this, but oh well..)
>
> Imagine that you have 2 identical versions of the same intrinsic webapp
> deployed, like
>
> catalina_base/webapps/webappA
> catalina_base/webapps/webappB
>
> You create a 3rd webapp webappX, which is the one the users see, and which
> is just a "dispatcher" to either one of the two webapps webappA and webappB.
> So you now have :
>
> catalina_base/webapps/webappX
> catalina_base/webapps/webappA
> catalina_base/webapps/webappB
>
> The users always use the URL "/webappX".
> When a call is made to webappX, /it/ decides, in function of some external
> parameter Y, whether to dispatch the call to either webappA or webappB.
>
> Say that initially the external parameter Y (which could be a system-wide
> property) is set so that all calls to "/webappX" are currently dispatched to
> /webappA.
> Now you want to update webappA.
> You change the parameter Y so that calls are now dispatched to (identical)
> /webappB.
> Then you update (redeploy) webappA.  Users see no delay, because they get
> directed to webappB.
> When webappA is deployed and started (which you could check by calling it
> directly),
> you can reset the parameter Y to its former value, and users will again be
> dispatched to webappA.  Now you can redeploy webappB, to keep it in sync.
>
>
>
>
>
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>

Re: "Application" vars -

[André Warnier <aw...@ice-sa.com>]

Eric P wrote:
>> 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.
>>
>>
> 
> Shay,
> I think you made a good case for keeping app vars in web.xml (i.e., 
> seems pretty apparent now that's where they belong).
> 
> Thanks for taking the time to respond.  I sincerely appreciate it!
> Eric P.
> 
+1
Very neat explanation.

While not being a great specialist, I would just like to expone what may be an 
alternative, if one really wants to diminish the (apparent) time it takes for a new 
version of a webapp to become available.  Since I am no great specialist, I will just 
outline the idea without Java/servlet specifics.

(Maybe also I am barging through an open door, and there already exists a standard 
mechanism for doing this, but oh well..)

Imagine that you have 2 identical versions of the same intrinsic webapp deployed, like

catalina_base/webapps/webappA
catalina_base/webapps/webappB

You create a 3rd webapp webappX, which is the one the users see, and which is just a 
"dispatcher" to either one of the two webapps webappA and webappB.
So you now have :

catalina_base/webapps/webappX
catalina_base/webapps/webappA
catalina_base/webapps/webappB

The users always use the URL "/webappX".
When a call is made to webappX, /it/ decides, in function of some external parameter Y, 
whether to dispatch the call to either webappA or webappB.

Say that initially the external parameter Y (which could be a system-wide property) is set 
so that all calls to "/webappX" are currently dispatched to /webappA.
Now you want to update webappA.
You change the parameter Y so that calls are now dispatched to (identical) /webappB.
Then you update (redeploy) webappA.  Users see no delay, because they get directed to webappB.
When webappA is deployed and started (which you could check by calling it directly),
you can reset the parameter Y to its former value, and users will again be dispatched to 
webappA.  Now you can redeploy webappB, to keep it in sync.




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Re: "Application" vars -

[Eric P <er...@gmail.com>]

> 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.
> 
> 

Shay,
I think you made a good case for keeping app vars in web.xml (i.e., seems pretty apparent now that's where they belong).

Thanks for taking the time to respond.  I sincerely appreciate it!
Eric P.


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Re: "Application" vars -

[Shay Rojansky <ro...@roji.org>]

Hi Eric.

On Wed, Jun 30, 2010 at 9:20 PM, Eric P <er...@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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> To unsubscribe, e-mail: users-unsubscribe@tomcat.apache.org
> For additional commands, e-mail: users-help@tomcat.apache.org
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>