Databases Reference
In-Depth Information
This type of transaction resiliency is a hallmark of the older TP monitors such as
Tuxedo, and the newer application servers and standards offer similar features.
Transaction routing
The logic in the middle tier can direct transactions to specific database servers,
increasing scalability.
Heterogeneous transactions
Application servers can manage transactions across multiple heterogeneous data‐
base servers—for example, a transaction that updates data in Oracle and DB2.
While developing three-tier OLTP systems is complex and requires specialized skills,
the benefits are substantial. Systems that use application servers provide higher scala‐
bility, availability, and flexibility than the simpler two-tier systems. Determining which
architecture is appropriate for an OLTP system requires (among other things) careful
evaluation and consideration of costs, available skills, workload profiles, scalability re‐
quirements, and availability requirements.
Figure 9-3
illustrates a three-tier system using an application server.
Figure 9-3. Three-tier architecture
Application Servers and Web Servers
The middle tier of web-based systems is usually an application server and/or a web
server. These servers provide similar services to the application server previously de‐
scribed, but are more web-centric, dealing with HTTP, HTML, CGI, and Java.
