Using ClusterCATS
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?"
• "Hardware-based clustering solutions"
• "Software-based clustering solutions"
• "Combining hardware and software clustering solutions"

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.

Hardware-based clustering solutions

A common and reliable hardware-based clustering solution is a packet router. One of the most popular routers is Cisco Systems' LocalDirector. A router, in front of a cluster of web servers, directs incoming HTTP requests to available web servers in the cluster. A router works by assessing the rate and volume of IP packet flow to and from web servers, and selecting the best server to accommodate the traffic. This process is fast and efficient. The router and clustered web servers comprise a virtual server.

Routers are considered semi-intelligent devices because they can detect a server failure and redirect requests to other servers. If a web server fails or stops responding, the router stops sending packets to the unresponsive server. Routers are not considered fully intelligent because, while they can redirect requests upon discovering a failure, they do not let you configure redirection thresholds for individual servers. They also do not support application-aware load balancing.

The following figure shows a router distributing requests in round-robin fashion to the available servers in a web server cluster:

Advantages of hardware-based solutions

A hardware-based clustering solution, such as a router, is an attractive solution for the following reasons:

• It uses proven technology
• It is has relatively low complexity
• There are no recurrent licensing fees
• It is semi-intelligent; routers can load balance in a round-robin fashion, detect failures, redirect traffic, and remove failed servers from a cluster.

Note:   Load-balancing devices offer different features and capabilities.

Considerations

Carefully evaluate the following issues against a router's attributes:

• Expense - hardware devices can be expensive relative to some software solutions, even without yearly licensing fees.
• Single point of failure - if a problem develops on the load-balancing device itself and it fails, your load-balancing and failover strategies do not work. Although some load-balancing devices come with secondary systems for this reason, this additional equipment often inflates the overall price of a hardware solution.
• Lack of application-awareness - the device cannot be tuned for particular types of web applications (static vs. dynamic sites) or for the development tools used to build them (scriptlets vs. JSP vs. CGI vs. ASP and so on). Consequently, a router cannot measure the performance of a web application server.
• Limited intelligence - the device does not let you configure individual load and redirection thresholds for each server in a cluster, so it cannot effectively manage load to prevent failures.

Software-based clustering solutions

There are several kinds of software-based clustering solutions on the market. As with hardware-based clustering solutions, there are strengths and weaknesses associated with each. These software solutions include:

• Round-robin DNS - a very popular choice because of its relative simplicity and low implementation cost, but does not include intelligence for load balancing or failover.
• Primary/backup clustering - cloned systems provide redundancy for one another. This type of clustering does not provide parallel server load balancing.
• Smart clustering - combines the advantages of round-robin DNS and backup clustering to provide simplicity with intelligence and redundancy.

ClusterCATS lets you easily create, optimize, and maintain "smart" clusters to support your web applications. ClusterCATS runs on Windows, Solaris, and Linux platforms and works with leading mission-critical web servers, including Microsoft IIS, Netscape Enterprise Server, and Apache. It is easily administered from remote locations and provides robust features, including:

• Configuring load and redirection thresholds by server
• Optimizing the load-balancing scheme with application-aware and session-aware load balancing
• Automatically detecting failures
• Automatically redirecting traffic to available servers
• Automatically notifying administrators of problems

Advantages

The following benefits make a software-based clustering solution attractive:

• Relatively low expense - compared to the cost of hardware devices, such as routers or switches, software-based clustering solutions are relatively inexpensive. In fact, you can cheaply implement Internet DNS on UNIX and Windows platforms for initial load-balancing needs, and augment with third-party clustering software.
• Flexibility - some clustering software can augment existing hardware devices, providing a more robust load-balancing and failover solution. By integrating hardware with software, you diminish, if not eliminate, losses on capital expenditures that your organization has already made. For more information, see "Combining hardware and software clustering solutions" and "Load-balancing devices".
• Intelligence - some software solutions provide a level of intelligence that enables preventive load-balancing measures that actually minimize the chance of servers becoming unavailable. If a server does becomes overloaded or actually fails, some software can automatically detect the problem and reroute HTTP requests to available servers in the cluster.
• No single point of failure - by distributing the load-balancing and failover capabilities among multiple servers in a cluster or multiple clusters, as opposed to relying on only a single device, no individual server failure can disable your application.

Considerations

Consider the following issues when evaluating software-based solutions for your environment:

• Differences among feature sets - software-based clustering solutions differ in their of capabilities and features. For instance, some lack automatic failure detection, notification, or IP address assumption, and others have significantly delayed detection. Some let you configure load thresholds to enable preventive measures, and some don't. Determine your scalability and failover needs in advance and pick your solution accordingly.
• Platform constraints - determine whether the software solution is available on your platform and operates with your preferred web server. When reviewing data sheets and other marketing collateral from vendors, ensure that the robust features you want are available on the platform you need.
• Level of complexity - some software-based clustering solutions relatively simple. Others introduce more complexity because of the features offered, the amount of initial configuration and subsequent administration, or the amount of integration that must occur between other systems and devices.

Combining hardware and software clustering solutions

Instead of having to choose either a hardware solution or a software solution, you can combine both types of clustering choices. Combining hardware and software solutions certainly provides the greatest scalability and availability capabilities for a site. A combined solution is an attractive option if your organization has already invested in one, but is looking for more comprehensive coverage. Having the flexibility to integrate hardware with software means that your organization won't necessarily have to absorb a capital loss on a previous technology investment if you decide to purchase additional clustering technology.

However, as already discussed, all hardware or software solutions are not equal. Many have different features and capabilities, and not all hardware and software integrate well together. Investigate thoroughly when purchasing technology to augment your current solution.

For a visual representation of hardware and software clustering solutions working together, see "Hardware-based clustering solutions".

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