Database Driven and Entity Tier (On Clustering III of VII)

Database Driven
Database provides query, indexing, transaction, storage management, data security and other support. Database driven application is very common. Database tier is being inserted in between the application and storage. Query optimization and indexing enhanced performance and help the application to achieve higher volume. However, its power and convenience is usually turned to extra feature to the application and rarely leave user impression of performance. Share-nothing strategy can be used. Bandwidth from the application tiers and database tiers are very critical to the overall performance. Before gigabyte ethernet becomes common, high-volume application utilize multiple 100Mbps ethernet card per machine to achieve the desired bandwidth.

The scaling and tuning of database its an art of itself. Different optimization models and expertise are widely available, some for signficiant cost.

When performance is absolutely critical, a subset of the table can be divided into memory-only database, like TimesTen (now a Oracle product) It provides advanced functionality like other DBMS. However, its keeps all its data in-memory, and requires no disk access for query.

Entity Tier
In the J2EE world, EJB was famous on its EntityBean concept. EntityBean is Component model representation of a single entity (relational db) in a Database. (loosely speaking, component is an object which lifecycle managed by a Container.) With this model, an application into two tiers: the presentation tier and the business logic tier. The EJB model represents the later. Direct access (thru RMI or RPC) by client pretty much went out of fashion. The presentation tier most likely is a web server that serve HTML or expose the Business logic functionalty as Web Service. The scaling and load-balancing of Entities are enabled by the remote Stub and Skeleton model. Clustering is provided by that application server with EJB container. The Stub on the client side, when instantiate, was assigned to one of the server.

The database access (persistent) of the entity is sometimes provided by the container. However, no serious application developer didn't complains about the performance of this container managed persistent (or CMP). The first EJB design was simply too heavy-weighted. EJB 3.0 moves to a much ligher-weight model. It appears to be good enough for long-term sucess. However, a very wide selection of programming model and framework exists.

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