Scalability of applications has become a lot more complex than buying a bigger Windows server or z/OS mainframe.  It is not just a system’s ability to handle user requests well as the numbers grow, but has mostly to do with managing system load, handling priorities, remaining responsive under high-load situations, while scaling as linearly as possible. A key problem of large-scale systems is data integrity as the number of transactions with write operations grows and the danger of data locks slowing down the system increases. Often the maximum number of concurrent users is seen as a measurement of scalability, but that is a dangerous shortcut. The number and kind of transactions that are required within a certain time frame is the only true measure of scalability. Response times for a transaction mix should remain below a limit at the expected peak transaction load.

For Java applications is difficult and expensive to achieve predictable, scalable performance. Simple switching from the single-server environments to multi-server can cause overall application throughput to collapse. Adding more data can cause the database to need progressively more resources. Java applications that work instantly in test, can be twenty times slower in response time when loaded with concurrent users.

The only (very complex) way to truly scale Java applications is to use three server tiers. The first is a HTTP-driven GUI, the second a Java server that de/serializes the tables of the third database tier into Java go to Download >>