dan benanav wrote:
> Richard,
>
> I presented a very simple example of an Account bean and analyzed how one would
> implement an increment(int x) method. I specifically avoided "general" remarks like
> "Entity beans represent data and behavior".
>
> My conclusion was that to implement the increment method I would like to have calls
>to
> the increment method be serialized and that pessimistic concurrency achieves that.
>But
> since pessimistic concurrency is not standard the situation becomes more complicated.
>
> In addition, I said that I would be tempted to not have read only methods in the
>Account
> object (for example to return the account detail) because of the inefficiency of the
>ejb
> load-call business method-ejbStore cycle. I want to have a way to declare certain
> methods read only to avoid the extra call to the database.
>
>From what I understand, this is exactly the kind of optimization on which EJB servers
>can
compete. Whether all the possible optimizations should be in the specs or not is an
open
issue, but what is sure is that these kinds of optimizations already exist in several
EJB
servers.
For example, Weblogic allows to specify an "isModified()" method; IAS can automatically
detect if the bean state changed since the beginning of the tx and only update the
fields
that have changed, and (I think) WebSphere allows you to specify a method as
read-only. Of
course, your code and designed can be influenced by the EJB server you choose.
>
> >From a programming point of view I agree with you. I would prefer to put the read
> methods in the Entity bean. However given the current state of the EJB spec if I
>want
> to avoid the extra SQL calls I might be tempted to use a stateless bean for read only
> methods.
>
> Also, pessimistic concurrency means that read only methods will block, but really
>that
> is probably not so bad. It might be bad if lots of things happen to be reading the
>same
> object often.
>
Once again, you have the choice here. Weblogic will use pessimistic concurrency
whereas IAS
will use optimistic concurrency. Here again, your code will change depending on the
choice
you make (which is a bad thing). For IAS, you will probably have to write more
exception
handling code, but your performance will probably be better.
If your goal is to be able to port your application to any EJB server, then you will
probably have to code everything for the worst case, and hope that the spec leaves as
few
holes as possible, but that will also leave less place for optimization, performance
and
innovation.
IMO, given that EJB1.1 is quite new and that most EJB servers will continue to evolve
and
offer more and more choices for optimization, I would not code for the worst case. We
will
(hopefully) have soon EJB servers which
- let you choose between optimistic and pessimistic concurrency
- automatically detect if a store is necessary or not
- automatically detect which fields must be updated
- let you specify if the DB is shared or not (to avoid unnecessary loads)
- ...
In fact, IAS already does all that except for the first point. (Yeah, yeah, I like IAS
;-))
Best regards.
JB.
>
> So the problem is that pessimistic concurrency is what I want for "mutators" and
> optimistic concurrency is what I want for read only methods. Furthermore read only
> methods have to go through the ejbLoad-> call method -> ejbStore cycle even though no
> store is required.
>
> I would like to hear more specific criticisms to my conclusions. Remember also that
>all
> my remarks were based on the current state of the ejb spec (namely 1.1) not on what
> might be if the spec improves.
>
> Take your Patient example below for example. Do you care that read only methods (even
> one returning data for just that one patient) on such a bean would have to visit the
> database twice? What about the fact that read calls will block on the same object if
> pessimistic concurrency is used? If pessimistic concurrency is not used then how
>would
> you handle the problem of serializing calls to the mutator methods?
>
> dan
>
> Richard Monson-Haefel wrote:
>
> > The idea that entity beans should only be used for updates is misguided.
> >
> > Entity beans (enterprise beans) represent data and behavior. The behavior part is
> > important to this discussion because it justifies the use of both accessor and
> > mutator (update) methods in the bean's remote interface. Limiting entity beans to
> > only mutator methods reduces them to data objects, not business components. As a
> > business component, an entity bean represents the full spectrum of behaviors
> > associated with a persistent business concept. An entity bean that represents a
> > Patient, for example, provides methods for reading the patient's benefit data while
> > processing a claim; reading a patient's medical history for counter indications;
> > determining a patient's preferred care provider for scheduling appointments, etc.
> > If you remove the behavior for accessing data from an entity, you loose the ability
> > to manage a patient individually. EJB allows thousands of client applications to
> > concurrently manage entity business objects individually. This is what makes entity
> > beans so powerful. Without it, you're better off using something like JDBC
> > directly.
> >
> > There are many situations where the use of enterprise beans is inappropriate. If
> > your system presents lists and tables of data representing many entities for read
> > only access, then you should consider using a mechanism other than EJB. For
> > example, a commercial web site that is used for shopping, like Amazon.com or
> > eToys.com, would not use EJB to populate web pages with product information in
> > response to customer navigation and searches. A better approach in this situation
> > is to access the data more directly from Servlets using JDBC (proper connection
> > management is assumed). When, however, the client is processing a purchase, which
> > is a task applied to an individual purchase order, EJB should be used. (EJB might
>be
> > accessed indirectly by clients through Servlets.) In this scenario a stateful
> > session bean might be used to represent the client's shopping cart, while entity
> > beans represent the client, the chosen products, and the order itself.
> >
> > A hospital system, on the other hand, would use EJB every time a Patient record was
> > accessed so that the behavior (both declarative and business logic) could be
>applied
> > in the context of an individual's data. This is important when reading/updating
>data
> > specific to an individual. If, however, you needed to generate reports on all the
> > patients you would not use EJB. Instead of EJB, use a reporting product; something
> > that is designed to search, filter, and organize data for reporting. In the
>medical
> > system, EJB ensures that very large user populations can concurrently manage
> > thousands of individual patients safely and appropriately -- it's not a reporting
> > tool.
> >
> > EJB is not a silver bullet; it's only one part of a complete solution. The use of
> > both JDBC and EJB from Servlets, as illustrated in the shopping cart scenario, is
>an
> > example of how to properly use EJB in the context of a larger solution. EJB is a
> > powerful new weapon in our enterprise arsenal. Use it properly, within the right
> > context, and it will serve you well.
> >
> > If you find yourself making component design decisions that are counter to the EJB
> > programming model -- like using entity beans for updates only -- then you should
> > reevaluate your entire architecture because you are probably using EJB
> > inappropriately.
> >
> > Thanks,
> >
> > Richard
> >
> > --
> > Author of Enterprise JavaBeans
> > Published by O'Reilly & Associates
> >
> > EJB FAQ
> > http://www.jguru.com/faq/EJB
> >
> > EJBNow.com
> > http://www.ejbnow.com
> >
> > ===========================================================================
> > To unsubscribe, send email to [EMAIL PROTECTED] and include in the body
> > of the message "signoff EJB-INTEREST". For general help, send email to
> > [EMAIL PROTECTED] and include in the body of the message "help".
>
> ===========================================================================
> To unsubscribe, send email to [EMAIL PROTECTED] and include in the body
> of the message "signoff EJB-INTEREST". For general help, send email to
> [EMAIL PROTECTED] and include in the body of the message "help".
--
Jean-Baptiste Nizet
[EMAIL PROTECTED]
R&D Engineer, S1 Belgium
Excelsiorlaan 87
B-1930 Zaventem
+32 2 714 45 42
===========================================================================
To unsubscribe, send email to [EMAIL PROTECTED] and include in the body
of the message "signoff EJB-INTEREST". For general help, send email to
[EMAIL PROTECTED] and include in the body of the message "help".
