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From Les Hazlewood <lhazlew...@transdynatlanta.com>
Subject RE: Why a cached object must be Serializable
Date Fri, 23 May 2003 13:05:46 GMT
That's correct.

I like having such flexibility (even thought it means doing a little extra
work), rather than being restricted by something I may not use.  It means
that _I_ can determine when and how safeguarding code is implemented.  

It would be nice to see the JCS team implement a thread safe version that
can be turned on or off, depending what the user wants.  We'll just have to
wait to see what the JCS folks decide. 

Les

> -----Original Message-----
> From: Hanson Char [mailto:HChar@realm.com] 
> Sent: Friday, May 23, 2003 8:02 AM
> To: 'Turbine JCS Users List'
> Subject: RE: Why a cached object must be Serializable
> 
> 
> What you describe is how to get around the problem, and of 
> course there are
> many ways one can achieve that.
> 
> But the bottom line is items cached in JCS are NOT thread safe.
> 
> A cached item retrieved from JCS can now be simultaneously 
> modified by a
> hundred threads, and the user of JCS must provide layers of code to
> safeguard this concurrency chaos.
> 
> Hanson
> 
> -----Original Message-----
> From: Les Hazlewood [mailto:lhazlewood@transdynatlanta.com]
> Sent: Friday, May 23, 2003 7:47 AM
> To: 'Turbine JCS Users List'
> Subject: RE: Why a cached object must be Serializable
> 
> 
> > Therefore, an item put into the cache must be by value, and an item
> > retrieved from the cache must also be by value.  In order to 
> > achieve this, a
> > cache item must be deeply cloned into copies.  The best way 
> > to do so is by
> > serializing and deserializing.
> 
> Hanson,
> 
> This is unnecessary behavior to be enforced.  Not everyone 
> will want or need
> such functionality.
> 
> Due to the performance/memory overhead of cloning everything, its not
> advisable to do so automatically without the user being able 
> to tell JCS.
> That is, JCS should not force this behavior.
> 
> Perhaps, it would be better to have a switch in the .ccf file 
> that allowed
> this behavior.  Something that "turned on" caching objects by 
> value rather
> than by reference.  That will be up to the JCS developers.
> 
> 
> At least for now, its better for JCS to cater to any possible 
> use, rather
> than enforce something many may find restrictive.
> 
> 
> Using proper design patterns, you can easily complement JCS 
> to provide the
> functionality you describe.
> 
> For example, in my company, we use the DAO (Data Access Object) design
> pattern to proxy access to the cache, and the VO (Value Object) design
> pattern to transfer information to/from the cache.
> 
> The DAO has an internal, private static reference to the 
> cache.  Any actions
> done to an object in the cache is done via the DAO.  That is, 
> no "user" of
> the DAO ever gets a reference to any objects inside the DAO's 
> JCS cache.
> Instead, Value Objects containing the information we care 
> about (to and
> from) are used for transfer of information.
> 
> For example, lets say we're working on some product-driven 
> system (e.g.
> amazon.com or any other ecommerce site).  Caching a Product 
> object would be
> a good idea since the product information rarely changes.  But if this
> Product needed to be updated, naturally, it would be a bad 
> idea to just
> update it immediately, without regard to other threads that might be
> accessing it.
> 
> The dao does the "work" involved on the objects in the cache:
> 
> public ProductInfo getProductInfo(String productId);
> 
> This method returns a "clone" of the product's information as 
> a ProductInfo
> object (the Value Object)...its nothing but a JavaBean with 
> getters and
> setters to hold information.  It contains no logic.
> 
> Or the dao could have:
> 
> public void setProductName(String productId, String newName) 
>     throws ProductUnavailableException;
> 
> This method would naturally set the name on the actual 
> Product object inside
> the DAO's JCS cache.
> 
> For example:
> 
> //in class attributes:
> private static JCS productCache = JCS.getInstance("productCache");
> 
> public void setProductName(String productId, String newName) ...
> {
>     productCache = JCS.getInstance("productCache");
> 
>     Product aProduct = productCache.get(productId);
> 
>     if (aProduct == null)
>         throw new ProductUnavailableException("not in cache");
> 
>     synchronized (aProduct)
>     {
>         aProduct.setName(newName);
>     }
> }
> 
> Using synchronization and proper design patterns you can 
> easily achieve what
> you're looking for without worrying about threading blunders.
> 
> The above technique (DAO/VO) should absolutely be used in a 
> J2EE environment
> due to J2EE Container managed threads accessing the DAO (e.g. 
> through a
> Stateless Session Bean, SLSB).  The J2EE developer has no 
> prior knowledge
> when the container might enable a SLSB and access the 
> underlying DAO, so the
> synchronization code is necessary.  Value Objects are also a 
> wise choice for
> the J2EE developer due to RMI's performance when transferring many
> fine-grained objects (String, int, etc) over a network 
> connection.  The VO
> must be serializable, and offers the best RMI performance 
> when remotely
> transferring a single large-grained object.
> 
> I hope this helps in sidestepping your problems with using 
> JCS.  It works
> great for us.
> 
> Les Hazlewood
> 

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