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Each user stops rereading data, producing an (application-specific) linear reduction in the number of database hits.



  • Across multiple users, the more users there are, the more they tend to be reading a lot of the same data, so the number of database hits increases slower than linearly.

  • Multitier Caching Example

    In this example, a hardware load-balancing device--a sort of specialized router--is used to mediate between the incoming request, which arrives at one IP address, and the farm of Web servers. A cache is needed because the load balancer can cause a user session to migrate between multiple application server virtual machines (VMs) at the same IP address.

    When the browser initiates a request that requires data, the Web server looks for the data in the top-tier (VM) cache. If it isn't there, it invokes its cache loader to locate the needed data. In the case of a JMS cache loader, a subscription with a reply-to address is sent to the midtier cache, which attempts to resolve the reference and send back the information.

    Figure 4: Cache Loading in a Multitier Cache

    If necessary, the same thing happens down the hierarchy until a cache that can resolve the item is located. The cache loader for the "bottom" cache does some database lookup and publishes the data on the appropriate channel. All the caches that are subscribing for the same object receive the data. In this case, a single midtier cache gets the data. It then publishes the data to all the VM caches on its box. They get it because they all use a more generic topic. The other VM caches don't get it because they aren't connected to the same local JMS bus.

    At first glance, the temptation is to ask "Why do I need a midtier cache?" The answer is quite simple. By using this approach, we can use a technique called "traffic shaping," which enables us to manage traffic flow based on the type of traffic and on which application or database is its intended destination.

    Conclusion

    The bottom line is that Web application performance depends on efficient data distribution. Since almost all server interactions involve data access, it's crucial to ensure fast data access for maximum application performance.

    Because of the latency that exists between the Web server and the underlying data--and given that much of data access involves read-only information--it pays to build a cache and avoid time-consuming round trips. By reducing repetitive traffic between the different layers of a Web application, you substantially diminish the size and cost of the installation and greatly enhance the system's responsiveness.

    -4-

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