WebSphere Application Server V7.0: Concepts ... - IBM Redbooks

Database data caching
To minimize direct reads from the database, database systems usually offer one
or more of the following options:
► Fetch ahead constructs attempt to anticipate that additional pages from that
table are required and then pre-loads those pages into memory pools.
► Buffer pools offer the ability to keep the data loaded into memory, assuming
that it is likely the same data will be requested again.
Both of these constructs reduce disk access, opting instead for reading the data
from the memory, increasing performance. These facilities assume that the data
is predominately read only. If the data has been written, the copy in memory can
be stale, depending on the write implementation of the database. Also, memory
buffers can be used to store data pages, reducing disk access. The key is to
make sure that the system has enough memory to provide to the database.
Application data caching
Another option is to cache some of the database or Web page data inside an
application by creating objects that are instantiated when the application server is
started. Those objects pull the necessary information in memory, improving
performance as the query is against an object in memory. The key is to make
sure there is some kind of synchronous or asynchronous mechanism (or both) to
update this cache on a timely basis according to the system requirements. This
approach, however, can create additional memory requirements, especially if a
dynamic cache that might grow over time is implemented.
EJB persistence implies loading the data into an EJB after a call to the data
provider. This is similar to database caching, except that caching takes place in
the application space, not the database server memory. The EJB has an access
intent, which indicates the rules used to determine the currency of the data in the
bean. From a performance standpoint, avoiding a call to an external database in
favor of a local bean creates significant gains.
Chapter 7. Performance, scalability, and high availability 259