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7.1.2: Availability management

7.1.2: Availability management

One of the benefits of scaling up to a multimachine configuration is that it improves the availability of the system. Applications hosted on multiple machines generally have less down time and are able to service client requests more consistently.

The following commonly used scaling techniques can be combined to take advantage of the best features of each topology and create a highly available system. (Note that this is not an exhaustive list of ways to improve availability.)

  • Eliminate single points of failure in the system by providing hardware and process redundancy:
    • Use horizontal scaling to distribute application servers over multiple physical machines. If a hardware or process failure occurs, clones are still available to handle client requests. Web servers and IP sprayers can also benefit from horizontal scaling.
    • Use backup servers for databases, Web servers, IP sprayers, and other important resources. This ensures that they remain available if hardware or process failure occurs.
    • Deploy an application in multiple administrative domains. If an entire domain goes offline, the others are still available to handle client requests.
    • Run administrative servers with workload management enabled. The failover support that workload management provides eliminates a single administration server as a point of failure.
  • Provide process isolation so that failing servers do not negatively impact the remaining healthy servers in the configuration. The following configurations provide some degree of process isolation:
    • Deploy the Web server onto a different machine from the application servers. This ensures that problems with the application servers do not affect the Web server, and vice versa.
    • Use horizontal scaling, which physically segregates application server processes onto different machines.
    • Deploy an application in multiple administrative domains. Problems are confined to one domain while the other remains available.
  • Use load-balancing techniques to make sure that individual servers are not overwhelmed with client requests. These techniques include the following:
    • Use workload management. It is automatically implemented for cloned application servers, but must be explicitly enabled for administrative servers.
    • Use an IP sprayer to distribute requests to the Web servers in the configuration.
    • Direct requests from high-traffic URLs to more powerful servers.
  • Provide failover support. The application must continue to process client requests when servers are stopped or restarted. Ways to provide failover support include the following:
    • Use horizontal scaling with workload management to take advantage of its failover support.
    • Use the HTTP transport to distribute client requests among application servers.
    • Enable the Session Manager to store session information in a persistent database. This preserves session state in case of server failure.
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Go to previous article: Scaling up WebSphere applications Go to next article: Multimachine topologies