Technical data

Using DNS to Balance Work Load
6.3 Load Broker Concepts
6.3.1 How the Load Broker Works
When the load broker starts, it reads its configuration file and starts polling
DNS cluster members. The load broker exchanges messages with DNS cluster
members that run the metric server. The metric server (Section 6.3.2) calculates
the current rating and reports it when polled by the load broker. Periodically,
the load broker sorts the list of addresses based on metric rating reports, drops
the systems that are not responding after being polled three times, and takes
a subset of the list and compares it to the name server information. To do the
comparison, the load broker sends a host lookup request to the specified name
server. If the lists are the same, the load broker does not make any change. If
the lists are different, the load broker updates the name server data by sending a
dynamic update request to the specified name server.
The name server uses round-robin scheduling to further balance the load across
the members of a DNS cluster. So every consecutive request for translating the
DNS cluster name results in a list being returned, rotated by one.
The
dns-ttl value stored in the load broker configuration file governs how long
the record is to be cached by other name servers. If some intermediate name
server caches the "A" resource records for a given DNS cluster name, it caches it
for the period of time defined by the dns-ttl value. The default dns-ttl value is
45 seconds. If less time is required, you can set
dns-ttl to a smaller value. To
suppress any caching, you can set the dns-ttl to 0.
The dns-refresh time specifies how often the DNS information for a given DNS
cluster is refreshed. The default is 30 seconds. If you want to quickly pick up
changes in the system load (reported by metric servers), set
dns-refresh to a
smaller number.
If the load broker has not received a response from a metric server after three
polling intervals, the load broker marks the address for removal from the DNS
alias. This removal will occur at the next dns-refresh interval.
For earliest possible detection of this failure, the polling-interval should be set
to a value that is less than one-third of the value specified as the
dns-refresh
period.
polling-interval < (dns-refresh) / 3
The masters list specifies the name server to use for dynamic updates. This
must point to the master name server. The name server must be set up to allow
dynamic updates from the system that runs the load broker. For how to configure
dynamic updates, see Section 5.3.6.
TCP/IP Services supports dynamic updating of only one master server in a DNS
cluster environment.
6.3.2 How the Metric Server Calculates Load
The metric server calculates the current load on a DNS cluster host by using the
following equation:
rating = availablity + workload - penalty
In the equation, the variables are calculated by:
• Availability
6–4 Using DNS to Balance Work Load