When deciding to run multiple Web servers, the general practice is to implement some form of load balancing, Under this scenario, all your Web servers serve identical content, but as far as the Web-browsing public disconcerting site is accessed through a single host name. You then implement a system for causing user requests to your site to be distributed across the multiple servers in some fashion, with the goal of evenly spreading load across the servers. From the user’s perspective, there is a single server (or address) and they don’t need to know that they could be receiving each page during their visit to your site from a different server. Several techniques can be used for load balancing:
• Round-robin DNS
• Load-balancing software
• Load-balancing hardware
DNS is the domain name service. This is an Internet-wide distributed database that translates host names such as www. grande. due into IP addresses such as 188.8.131.52 .
This is important because computers on the Internet use IP addresses to direct communication to the correct desalination Generally, each organization that owns its domain name runs its own name server for the domain to handle translation of host names within their domain. DNS can be extended to implement basic load-balancing. Through special changes to a DNS server, you can make it deliver different IP addresses each time a name is looked up. For instance, if you have two Web servers, then each time a name lookup is received for your Web server host name, alternating IP addresses will be provided during the name lookup. At first glance this may seem like a simple way to evenly distribute load across multiple servers. However, this technique does not take into account actual load on the servers. It is not unlikely that one server will end up handling a disproportionate number of CPU-intensive requests. resulting in distinctly unbalanced loads across the servers.
Also, if a system is down, the DNS server doesn’t detect this and at least some of the attempts to connect to your site Will be directed to the system that is down, resulting in failed connection attempts.
Specialized load-balancing software exists that takes things a step beyond the crude functionality of round-robin DNS. ColdFusion Server includes Bright Tiger’s Cluster- _ATS load-balancing software (the name comes from the concept of a cluster: multiple servers appearing to act as one server). Other products are also available.
Specialized load-balancing software can perform several jobs:
• Replicating content across multiple servers in a duster
• Handling situations in which a server suffers failure or a state of massive load
• Monitoring actual load on the various servers to ensure the optimal distribution of incoming requests on the basis of available server resources on different servers .
However, load-balancing software has its limits. These include the need for o.,e server in the duster to play the central role of managing the duster, which introduces a single point of failure (if the primary server goes ,the cluster ceases to function), and lack of built-in support for security features such as network address translation.
The most complete form of load balancing comes in dedicated hardware systems such as Cisco’s ~coal Director or Hyderabad. These systems completely hide a cluster from t e outside v world, masking the duster behind a single IP address. The hardware manages the distribution of requests by negotiating with servers in the duster to find one that appears to be most ready to handle a new request. Of all load-balancing approaches, this is the most complex to install and manage but offers the best capabilities for large-scale Web installations. Hardware load balancing offers man) advantages, including the best distribution of load across servers, extra security in form of network address translation, and the best capability to handle system failures. It is also possible to provide redundancy of the cluster management system, which is generally not possible with software-based solutions. On the other hand, hardware systems generally don’t handle content replication and tend to be pricey. move view of C lust CATS The Enterpris edition of ColdFusion Server 4.5 ships with ClusterCATS from software.
This is a software-based load-balancing solution integrated into the C old Fusion server environment. ClusterCA TSworks through URL redirection: Your application server is monitored and requests are redirected to another server in the cluster when load gets too high. This is a fairly-basic form of load balancing but it generally works. The version of Carter IS.