MACROMEDIA ClusterCATS, Руководство пользователя

Страница: 38 / 156

28 Chapter 2 Scalability and Availability Overview
Creating scalable and highly available sites
When you understand the issues of scalability and availability, the next step is to learn the
techniques you can use to achieve scalable and highly available websites.
This section describes the following topics:
•
“What is clustering?” on page 28
•
“Hardware-based clustering solutions” on page 29
•
“Software-based clustering solutions” on page 30
•
“Combining hardware and software clustering solutions” on page 32
What is clustering?
Clustering is a technique in which two or more web servers supporting one or more
domains (such as www.yourcompany.com) are grouped as a
cluster of servers, to
collectively accommodate increases in load and provide system redundancy.
The following figure shows an example of a server cluster for a website:
Clustering for scalability works by distributing load among servers in the cluster (load
balancing) using an unintelligent-but-regular distribution sequence (round-robin DNS
and routers) or a predefined threshold or algorithm (specialized clustering software) that
you specify and can adjust for each server in the cluster.
Clustering for failover relies on redundant servers to ensure that business-critical
applications remain available if one of the servers in a cluster fails. Intelligent
software-based failover solutions can detect when a server has failed and automatically
redirect new incoming HTTP requests to available cluster members. Some
hardware-based failover devices that have less built-in intelligence require an
administrator’s intervention when a failure is detected.
Clustering can be accomplished using software-based solutions, such as round-robin
DNS alone or together with a third-party package; a hardware-based solution, such as a
packet router; or a combination of the two.