Plan for Site Selector

Site Selector works with a domain name server to load balance among a group of servers that uses measurements and weights that are gathered. You can create a site configuration to load balance traffic among a group of servers that are based on the domain name that is used for a client request.

CAUTION:
The DNS queries that Site Selector supports are type A queries only. Any other query types results in a return code of NOTIMPL (Not Implemented). If an entire domain is delegated to Site Selector, ensure that the domain receives type A queries only.
Example of a DNS environment.Example of a DNS environment

When you set up a subdomain for Site Selector within your DNS environment, ensure that Site Selector has authority over its own subdomain. For example, your company is granted authority over company.com domain, as shown in the Example of a DNS environment figure. Within the company, there are several subdomains. Site Selector would have authority for siteload.company.com, while the DNS servers would still maintain authority for atlanta.company.com and boston.company.com.

For a company name server to recognize Site Selector as having authority for the sideload subdomain, add a name server entry to its named data file. For example, on AIX® systems, a name server entry would look like the following example:
siteload.company.com IN NS siteselector.company.com 
Where siteselector.company.com is the host name of the Site Selector server. Equivalent entries must 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 server. Site Selector then resolves the domain name to the IP address of one of the servers that is 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 returns an error if it does not resolve the domain name request.)

Refer to the figure in the Site Selector component topic, which illustrates a site in which Site Selector is used with a DNS system to load balance across local and remote servers.

Site Selector consists of the following functions:
  • ssserver handles request from the command line to the name server, manager, and advisors.
  • The name server function supports the load balancing of incoming name server requests. You must start the name server function for Site Selector to begin providing DNS resolution. Site Selector listens on port 53 for incoming DNS requests. If the requesting site name is configured, then Site Selector returns a single server address (from a set of server addresses) associated with the site name.
  • The manager sets weights that are used by the name server that is based on:
    • Feedback from the servers that are provided by the advisors
    • Feedback from a system-monitoring program, such as Metric Server.
    Using the manager is optional. However, if the manager is not used, load balancing uses weighted round-robin scheduling that is based on the current server weights, and advisors are not available.
  • The Metric Server is a system monitoring component of Load Balancer that you install on the backend server. If you collocate Load Balancer on a server that is being load balanced, then you would install Metric Server on the Load Balancer server.

    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 that is 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 that the manager uses to determine weights:
    • CPU
    • memory
    • port
    • system
    CPU and memory values are all supplied by Metric Server, which is recommended with the Site Selector component. Read the Getting advice with Metric Server topic to learn how to use Metric Server with the Site Selector component.
  • The advisors query the servers and analyze results by protocol before calling the manager to set weights as appropriate. It might not make sense to use some of these advisors in a typical configuration. You also have the option of writing your own advisors. Using the advisors is optional, but recommended. Read the List of advisors topic to learn about the advisors that are provided with Load Balancer.
  • To configure and manage the name server, advisors, Metric Server, and manager, use the command line (sscontrol) or the graphical user interface (lbadmin).

The four key functions of Site Selector (name server, manager, Metric Server, and advisors) interact to balance and resolve the incoming requests between servers.

TTL

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 name server caches 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 you choose a TTL value, carefully consider 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.

Configure TTL by using the sscontrol sitename [add | set] command.

Network Proximity feature

Network proximity is the calculation of each server's nearness to the requesting client. To determine network proximity, the Metric Server agent (which must be present 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:
  • Cache life: The amount of time a proximity response is valid and saved in the cache.
  • Proximity percent: The importance of the proximity response versus the health of the server (as input from the manager weight).
  • Wait for all: Determines whether to wait for all proximity (ping) responses from the servers before responding to the client request.

    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 timeout 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 supplies the client with the server IP address with the best combined weight. It is expected that most client name servers have a 5-second timeout. 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 that 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 the IP address for this server to the client for its second request.

Set network proximity options with the sscontrol sitename [add | set] command. Read the Commands reference topic to learn about Site Selector commands.


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