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Tom,
Tom Kobialka wrote:
> I have a web service which is running under the Tomcat container.
[snip]
> I need to implement some sort of serialization such that only one
> request is processed from each instance at a time.
Is your web service synchronous or asynchronous? If it's asynchronous,
then you have the option of queuing all requests and having a single
processor thread process them in order.
If you have th handle these requests synchronously, then you will need
to use some sort of synchronization monitor to control access to the
hardware device. This is not as simple as merely marking a method as
"synchronized", as another respondent suggested.
You need to make sure that the object being used as the synchronization
lock (the "monitor") is the one that is always being used. You have
several options:
1. Throw an object in the appropriate scope (probably 'application') and
do something like:
synchronized(application.getAttribute("MY_HARDWARE_LOCK"))
{
/// do your hardware call
}
2. Better than that, you could use a "hardware access" class that knew
how to manage its own synchronization, and put *that* into the
application scope:
application.getAttribute("MY_HARDWARE_CONTROLLER").doSomething();
In this case, make sure that you are only creating one instance of your
hardware controller and that you make the doSomething method
synchronized (synchronizing a method makes /this/ the monitor for the
synchronization).
3. Use a singleton like #2 above, which won't require you to have a
scope like the application available for storage. This is useful when
you have to access something like this from deep within different
components and you don't have a convenient place to store your hardware
controller.
> Is it possible to serialize between service instances, which have
> been created by tomcat?
Yes, you can do this too. Although I recommend using a single instance
of this hardware service, you can use multiple instances of you wish.
You just need to make sure that all instances are using the same monitor
for synchronization.
> Is there some external library to tomcat shared memory object which
> will allow me to do this?
No.
- -chris
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