Site Selector acts as a name server that works with other
name servers in a domain name system to load balance among a group
of servers by using measurements and weights that are gathered. You
can create a site configuration to load balance traffic among a group
of servers based on the domain name used for a client's request.
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
is configured under the site name. Site Selector returns the IP address
of the selected server to the name server. The name server returns
the IP address to the client.
Metric Server is a system monitoring component of Load Balancer
that must be installed in each load-balanced server within your configuration.
Using 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. By customizing system metric script files, you can control
the type of measurements that are 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).
Features of Site Selector:
- Collocation
- High availability
- Client to server affinity
- Rules-based load balancing
- Wide area load balancing
- Alerts
Functions that provide load balancing
The
primary functions of Site Selector interact with each other and your
server configuration to balance network traffic in your environment.
Site Selector consists of the following functions:
- The ssserver command handles requests from
the command line to the name server, manager, and advisors.
- The nameserver command works with other name
servers in a domain name system to load balance among a group of servers.
The name server always runs when the Site Selector component is being
used for load balancing.
- The manager sets weights that are used by the executor, which
is based on:
- Internal counters in the executor
- Feedback from the servers that are provided by the advisors
- Feedback from a system-monitoring program, such as Metric Server
or WLM.
- The advisors query the servers and analyze results by protocol
before it calls the manager to set weights as appropriate. To configure
and manage the name server, advisors, and manager, use the command
line (sscontrol) or the graphical user interface
(lbadmin).
Managing local and remote servers with Site Selector
and Metric Server
A client submits a request for resolution
of a domain name to a client name server. The client name server forwards
the request through the DNS to the Site Selector machine (Path 1).
Site Selector then resolves the domain name to the IP address of one
of the servers. Site Selector returns the IP address of the selected
server to the client name server. The name server returns the IP address
to the client. After the client receives the IP address of the server,
the client routes application requests directly to the selected server
(Path 2).
Important: In this example, the Metric Server
provides system load information to the Site Selector machine. The
Metric Server agent is installed on each backend server. Use Metric
Server with Site Selector; otherwise Site Selector can use only a
round-robin selection method for load balancing.
This figure
illustrates a site in which the Site Selector component is used to
answer requests. Server1, Server2, and Server3 are local. Server4,
Server5, and Server6 are remote.
