Understanding the Two-Tier Architecture

A two-tier application generally includes a Java client that connects directly to the database through TopLink. The two-tier architecture is most common in complex user interfaces with limited deployment. The database session provides TopLink support for two-tier applications.

For more information, see Chapter 75, "Understanding TopLink Sessions".

Figure 2-4 Two-Tier Architecture

Description of "Figure 2-4 Two-Tier Architecture"

Although the two-tier architecture is the simplest TopLink application pattern, it is also the most restrictive, because each client application requires its own session. As a result, two-tier applications do not scale as easily as other architectures.

Two-tier applications are often implemented as user interfaces that directly access the database (see Figure 2-4). They can also be non-interface processing engines. In either case, the two-tier model is not as common as the three-tier model.

The following are key elements of an efficient two-tier (client-server) architecture with TopLink:

Minimal dedicated connections from the client to the database
An isolated object cache

Example Implementations

An example of a two-tier architecture implementation is a Java user interface (Swing/AWT) and batch data processing.

Advantages and Disadvantages

The advantage of the two-tier design is its simplicity. The TopLink database session that builds the two-tier architecture provides all the TopLink features in a single session type, thereby making the two-tier architecture simple to build and use.

The most important limitation of the two-tier architecture is that it is not scalable, because each client requires its own database session.

Technical Challenges

The current trend toward multitiered Web applications makes the two-tier architecture less common in production systems, but no less viable. Because there is no shared cache in a two-tier system, you risk encountering stale data if you run multiple instances of the application. This risk increases as the number of individual database sessions increases.

To minimize this problem, TopLink offers support for several data locking strategies. These include pessimistic locking and several variations of optimistic locking. For more information, see "Configuring Locking Policy".