Using DNS round robin for request distribution

When you would use this method

If you are using either of the following client access methods, you can use DNS round robin to distribute requests over NetCache appliances:

Pointing clients to a single NetCache appliance
Using a nontransparent access method for streaming media

Compare this discussion of using DNS round robin for request distribution to Using a Server Load Balancer for request distribution.

Type of DNS round robin described

This section applies to browsers that use DNS round robin to choose between NetCache appliances. It does not apply to NetCache appliances using DNS round robin to choose between Web servers.

DNS round robin described

DNS round robin distributes traffic to NetCache appliances in a predetermined cyclical pattern. For example, the first request is sent to NetCache A, the second to NetCache B, the third to NetCache C, and so on. When the end of the list of NetCache appliances is reached, the cycle starts again at NetCache A. This method, therefore, gives each NetCache appliance an equal number of requests. There is no possibility, for example, to have more requests sent to a high-end NetCache appliance than to a low-end appliance.

Setting up DNS round robin

You set up DNS round robin so that the IP addresses of the different NetCache appliances correspond to a single DNS host name. You should set up the DNS nameserver so that it periodically changes the primary IP address that it gives out for the requested host name, for example, for every request or every other request. The result is that different clients are pointed to the NetCache appliance that is next in the round-robin cycle.

Additionally, you configure the client's Web browser proxy settings with the host name, not the IP address, of the NetCache appliance.

Advantages of this method for request distribution

This method has the following advantages:

It works with all NetCache products and with third-party Web caches.
It is an inexpensive method of request distribution because you do not have to purchase hardware.

Disadvantages of this method for request distribution

Network Appliance does not recommend this method because you can experience the following problems when using DNS round robin for caching service:

DNS cannot determine the availability or performance of a given server. Therefore, it continues to direct client requests to failed or overloaded servers. If one NetCache appliance goes down, DNS continues to send requests to it. Clients receive error messages; for Web caching, the messages indicate that those URLs are unreachable.
In contrast, if an L4 or L7 switch, a WCCP router, a Server Load Balancer (SLB), or an automatic proxy configuration file is used and one appliance goes down, requests are sent to another appliance.
Because all requests can be sent through any NetCache appliance, the same objects can be cached in more than one appliance. To achieve the maximum cache hit rate, requests for the same objects should be sent to the same NetCache appliance.
For Web caching, problems with cookies can occur if the proxy route changes.
Cookies are often used to store authentication credentials. When cookies are used for this purpose, it is not always necessary to reauthenticate future requests. The cookies commonly encode the IP address from which the request originates and the user name. If the IP address in the request changes because a different Web cache is used to send a request from a user, a cookie might be rendered invalid.

This problem is unlikely to occur unless you are using multiple levels of caches to handle requests, as described in Section A, Request resolution hierarchies.

Where to get more information about DNS round robin

You can obtain more details about DNS round robin in technical report TR3007, "Scaling Web sites for the Expanding Internet Universe." You can find this report at the Network Appliance Web site, http://www.netapp.com/. To find the report quickly, enter TR3007 as the search keyword in the Web site's search engine.