Provisioning For High Service Availability
The priority field determines the precedence of use of the record's data. Clients always use the SRV record with the lowest-numbered priority value first, and fallback to other records of equal or higher priority if the connection to the host fails.
If a service has multiple SRV records with the same priority value, clients use the weight field to determine which host to use. The weight value is relevant only in relation to other weight values for the service, and only among records with the same priority value.
In the following example, both the priority and weight fields are used to provide a combination of load balancing and backup service.
_sip._tcp.example.com. 86400 IN SRV 10 60 5060 bigbox.example.com. _sip._tcp.example.com. 86400 IN SRV 10 20 5060 smallbox1.example.com. _sip._tcp.example.com. 86400 IN SRV 10 10 5060 smallbox2.example.com. _sip._tcp.example.com. 86400 IN SRV 10 10 5066 smallbox2.example.com. _sip._tcp.example.com. 86400 IN SRV 20 0 5060 backupbox.example.com.The first four records share a priority of 10, so the weight field's value will be used by clients to determine which server (host and port combination) to contact. The sum of all four values is 100, so bigbox.example.com will be used 60% of the time. The two hosts smallbox1 and smallbox2 will be used for 20% of requests each, with half of the requests that are sent to smallbox2 (i.e. 10% of the total requests) going to port 5060 and the remaining half to port 5066. If bigbox is unavailable, these two remaining machines will share the load equally, since they will each be selected 50% of the time.
If all four servers with priority 10 are unavailable, the record with the next highest priority value will be chosen, which is backupbox.example.com. This might be a machine in another physical location, presumably not vulnerable to anything that would cause the first four hosts to become unavailable.
The load balancing provided by SRV records is inherently limited, since the information is essentially static. Current load of servers is not taken into account.
Read more about this topic: SRV Record
Famous quotes containing the words high, service and/or availability:
“To that high Capital, where kingly Death
Keeps his pale court in beauty and decay,
He came.”
—Percy Bysshe Shelley (1792–1822)
“Television could perform a great service in mass education, but there’s no indication its sponsors have anything like this on their minds.”
—Tallulah Bankhead (1903–1968)
“Since ... six weeks ago, there has been no day in which I have not had letters and visits on the subject of my nomination for the Presidency.... I say very little. I have in no instance encouraged any one to work to that end.... I have said the whole talk about me is on the score of availability. Let availability do the work then.”
—Rutherford Birchard Hayes (1822–1893)