Best way to use PersistenceManagers in Web Applications

Question

Beacuse each PersistenceManager manages it's own cache , and I would like
to benefit from this cache is it a good practice to keep in each user's
session a PersistenceManager instance? By keeping this cache , wouldn't be
a problem when many users access the application at the same time.
Another approach would be to keep it in the servlet context but because
each instance of a PersistenceManager makes a transaction at once, that a
performance problem shall appear.
Of course I'd like to decouple the persistence operations so I wrote a
stateless session bean (as recomended), and when each metod is called I
obtain a new PersistenceManager instance I use it and that at finish I
close it.
Lets say that I have a one-2-many relationship between 2
PersistenceCapable objects Company and Employee. Let's say that I call the
method findCompanyByPK(Object pk) in the session bean. I obtain the PM
instance, I use pm.getObjectById() than I close the PM instance and return
the result. What if I want to access the company employees form the
instance returned. The PM is closed so I shall get an Exception. And
anpther thing, if I close the PM instance , how do I benefit from the
cache??
Can anybody show me a better approach?

Answer 1

Patrick Linskey wrote:
In article <[email protected]>, Danix wrote:
How can I achieve this PersistenceManagerFactory cache?
Seehttp://www.solarmetric.com/Software/Documentation/2.3.4/docs/datastore_cache.html
for details on using our PersistenceManagerFactory cache.One question here: Each object that I use and store in cache must be
pinned and unpinned from the cache manually by me or is it made
automatically by PMF?
One thing I liked about JDO was lazy loading. Lets say that my company has
a few thousand employees , as I know objects are loaded from datastore
only when they are iterated, so as you say wouldn' t be a solution to
retrieveAll employees and make all of them transient. Isn't there a better
alternative?
Well, not really, if you are set on using session beans. One of the important
functions that session beans play in many EJB architectures is that of a data
firewall, ensuring that only authorized clients can access certain data. So,
the data that the session bean methods return cannot be associated with a
live connection to the database, as this would cause your client tier to
directly access the database, which the EJB folks say is A Bad Thing.
Now, because this can lead to performance problems, and because most people
don't need this type of security between their web server and their database,
the EJB folks often propose what Sun refers to as the Fast Lane pattern, in
which data is read directly from the database by the web server to realize
performance improvements.
Many J2EE applications can be best implemented (from the standpoints of
simplicity, performance, and cost) by only using session beans whenabsolutely
necessary, rather than as an integral part of your three-tiered design.
You can easily enough design an architecture that separates presentation
logic from business logic, but only delegates that business logic to asession
bean when the services of a session bean are necessary. That is, you do not
necessarily need to use a session bean just because you want this separation.
So, if you were to go that route, and perform business logic in the same JVM
as your servlet/JSP, then you could see the benefits of lazy loading, since
you could keep the PM open until the presentation logic finished executing.
But, if you need the extra facilities of session beans, then you're pretty
much stuck with statically returning all the data that the session bean plans
to make available to the client.You are right maybe stateless session beans are not the best approach. Can
you explain to me a little about PerristentManagers? My opinion was that
the PMF when is instantiated creates a pool of PMs. Each of them uses a
cache that is initialized from datastore when is obtained first time from
PMF and when PM is closed , it is actually returned to the PMF pool and
its cache is not lost? Is it somethisng like this?
And thank you for your quick answers so far.
Dan
-Patrick
Patrick Linskey [email protected]
SolarMetric Inc. http://www.solarmetric.com