With respect to the case of component granularity, the
blueprint states that the entity bean should correspond
to a "logical row". Granted, in DB modeling jargon, a
logical row may represent an aggregation of physical rows.
I have found in practice that using a denormalized layer
within the DB schema so that coarse entity beans still correspond
to only one denormalized table can be optimal in large apps.
I will admit that I have only used this strategy in cases where
the DB vendor had alot of support for replication and resolution
of the denormalized layer back into the actual E-R schema.
>From: Thibault Cuvillier <[EMAIL PROTECTED]>
>Reply-To: Thibault Cuvillier <[EMAIL PROTECTED]>
>To: [EMAIL PROTECTED]
>Subject: Re: Coarse grained and Fine grained beans
>Date: Wed, 3 Jan 2001 13:45:03 -0600
>
>Samuel,
>
>There is 2 granularity problem when you design Entity EJBs:
> - Component granularity
> - Accessors granularity
>
>
>1-- Component granularity
>---------------------------------
>There is two extrem solutions when you create EJbs
>
>1- Create small, fine grained EJBs.
> - Usually, you create 1 EJB per table
> - i.e.: PersoneEJB and AddressEJB
>
>2- Create BIG, coarse grained EJBs.
> - Represent an aggregat of data stored in different tables
> - i.e.: PersoneEJB with a method getAddress
>
>When you design an EJB entity, you must keep in mind that you are designing
>a remote object interface, not only a way to access the data without
>writing
>JDBC code.
>
>An EJB should not be too small, or too BIG. Easy to say...... so, here is a
>few clue:
>
>Syndroms if you design too small EJBs:
> - Performace will be low because you will need to many remote calls
> - System supervision may be challenging
> - Communication between EJBs will be unmanageable
> - A lot of small SQL queries to the database
> - You will need to write more code to implement your system
>
>Syndroms if your design too BIG EJBs:
> - ejbLoad and ejbStore will take too long time
> - The component use too much memory
> - System modularity will be low as well as it capability to evolve.
>
>When you create EJB, you must always respect the rule described in the EJB
>spec:
>"A dependent object should never been modelized as an EJB". (see the spec
>for more detailed)
>
>
>2- Methods granularity
>-------------------------------
>Accessors ( get and set methods ) also have a granularity:
>
>Fine grained:
>
> EJB Person:
> getFirstName
> getLastName
> getZipCode
> ...
>
>Coarse Grained:
> EJB Person:
> PersonStateHolder getStateHolder
>
> The state holder ius a serializable object containing a copy of the
>attributes.
>
> class PersonStateHolder implements Serializable {
> String firstName;
> String lastName;
> String zipCode;
> ...
> }
>
>Usually, an Entity EJBs contain fine grained and coarse grained methods.
>
>
>3-- So...
>There is 2 common errors when designing Entity EJBs:
> - Create one Entity per Table
> - Create one Entity per conceptual class
>
>Entity EJB design must:
> - Optimize the data load/store
> - Optimize the remote calls
> - Optimize the memory/CPU usage on the server
> - Allow an easy deployement and system management
> - Allow re-use, if it's required on the project
> ...
>
>Finding the right component granularity require experience: there is no
>magic-generic rules. You have to understand what's happen if you define too
>small grained or too large grained components.
>
>Hope this help
>
>Thibault Cuvillier
>http://www.valtech.com
>
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