This chapter describes what the network planner should consider before installing and configuring the Site Selector component.
This chapter includes the following sections:
Site Selector works in conjunction with a domain name server to load balance among a group of servers using measurements and weights that are gathered. You can create a site configuration to let you load balance traffic among a group of servers based on the domain name used for a client's request.
Limitations: The DNS queries that Site Selector supports are Type A queries only. Any other query types will result in a return code of NOTIMPL (Not Implemented). If an entire domain is delegated to Site Selector, ensure that the domain receives only Type A queries.
When setting up a subdomain for Site Selector within your DNS environment, Site Selector should have authority over its own subdomain. For example (see Figure 21), your company has been assigned authority over company.com domain. Within the company, there are several subdomains. Site Selector would have authority for siteload.company.com, while the DNS server(s) would still maintain authority for atlanta.company.com and boston.company.com.
In order for company's name server to recognize Site Selector as having authority for the siteload subdomain, a name server entry will need to be added to its named data file. For example, on AIX® systems, a name server entry would look like the following:
siteload.company.com. IN NS siteselector.company.com.
Where siteselector.company.com is the hostname of the Site Selector machine. Equivalent entries would need to be made in any other named database files for use by DNS servers.
A client submits a request for resolution of a domain name to a name server within its network. Name server forwards the request to the Site Selector machine. Site Selector then resolves the domain name to the IP address of one of the servers that has been configured under the site name. Site Selector returns the IP address of the selected server to the name server. Name server returns the IP address to the client. (Site Selector acts as a non-recursive (leaf node) name server, and it will return an error if it does not resolve the domain name request.)
Refer to Figure 5 which illustrates a site in which Site Selector is used in conjunction with a DNS system to load balance across local and remote servers.
Site Selector consists of the following functions:
With Metric Server, Site Selector can monitor the level of activity on a server, detect when a server is the least heavily loaded, and detect a failed server. The load is a measure of how hard the server is working. The system Site Selector administrator controls the type of measurement used to measure the load. You can configure Site Selector to suit your environment, considering such factors as frequency of access, the total number of users, and types of access (for example, short queries, long-running queries, or CPU-intensive loads).
Load balancing is based on server weights. For Site Selector, there are four proportions which the manager uses to determine weights:
CPU and memory values are all supplied by Metric Server. Consequently, use of Metric Server is recommended with the Site Selector component.
See Metric Server for more information.
The four key functions of Site Selector (name server, manager, Metric Server, and advisors) interact to balance and resolve the incoming requests between servers.
Using DNS-based load balancing requires that caching of name resolutions be disabled. The TTL (time to live) value determines the effectiveness of DNS-based load balancing. TTL determines how long another nameserver will cache the resolved response. Small TTL values allow for subtle changes in the server or network load to be realized more quickly. However, disabling caching requires that clients contact the authoritative name server for every name resolution request, thus potentially increasing the client latency. When choosing a TTL value, careful consideration should be given to the impact that disabled-caching has on an environment. Also be aware that DNS-based load balancing is potentially limited by client-side caching of name resolutions.
TTL can be configured using the sscontrol sitename [add | set] command. See sscontrol sitename -- configure a sitename for more information.
Network proximity is the calculation of each server's nearness to the requesting client. To determine network proximity, the Metric Server agent (which must reside on each load-balanced server) sends a ping to the client IP address and returns the response time to Site Selector. Site Selector uses the proximity response in the load-balancing decision. Site Selector combines the network proximity response value with the weight from the manager to create a combined final weight value for the server.
Use of the network proximity feature with Site Selector is optional.
The Site Selector provides the following network proximity options that can be set per site name:
If set to yes, the Metric Server pings the client to obtain the proximity response time. Name server waits for all Metric Servers to respond or for a time-out to occur. Then, for each server, the name server combines the proximity response time with the weight the manager calculated to create a "combined weight" value for each server. Site Selector will supply the client with the server IP address with the best combined weight. (It is expected that most client name servers have a 5 second time-out. Site Selector tries to respond before that time-out is exceeded.)
If set to no, a name resolution is provided to the client based on the current manager weights. Then, the Metric Server pings the client to obtain the proximity response time. The name server caches the response time it receives from the Metric Server. When the client returns for a second request, the name server combines the current manager weight with the cached ping response value for each server to obtain the server with the best "combined weight." Site Selector returns this server's IP address to the client for its second request.
Network proximity options can be set on the sscontrol sitename [add | set] command. See Command reference for Site Selector for more information.