Simulated DNS Load Balancing Improves Testing Accuracy - Web Performance

Simulated DNS Load Balancing Improves Testing Accuracy

With the release of Load Tester 4.1, we’ve been discussing some of the new features available in this release. One of those features is Simulated DNS Load Balancing. This feature offers improved testing accuracy over previous versions of Load Tester.

This feature is designed for systems which may use DNS to alias a single hostname to many different IP addresses, allowing the DNS system to provide load balancing. This works effectively when the users or load engines greatly outnumber the number of servers, and are well distributed and not in a common location. For users that wish to test their system with only enough load engines to accurately simulate load, this can be a limitation, as software network stacks and even some DNS servers can cache DNS responses, forcing all load coming from a single machine or office to a specific server rather than diversifying it.

For example, lets suppose we have an internal hostname under test, which resolves to two possible IP addresses: and DNS servers can provide round-robin load balancing, so that each call to resolve the hostname alternates which server is used. In other testing tools, as well as previous versions of Load Tester, attempting to load test this host from a single engine would resolve and cache the name once and then simulate all users hitting that hostname. Overcoming this meant knowing the IP addresses in advance, and using a host redirect or direct replacement on the hostname in the URLs to redirect the load to one of the desired IP addresses; a workaround that may be ineffective if the server did not accept it’s own IP address as a valid virtual host.

Load Tester 4.1 makes this easy by automatically simulating a unique DNS cache for each virtual user, and load balancing DNS lookups automatically. This allows a each load engine to provide greater accuracy when attempting to simulate load which is expected to originate from many locations. Under the Web Performance – Preferences, simply verify that the “Simulate even distribution for DNS load balanced hostnames” is checked.
Web Performance Preferences
This option is enabled by default, and ensures accuracy for not only your own DNS load balanced hostnames, but also spreads load more accurately to linked URLs, such as resources hosted by a CDN. This feature may be disabled, particularly for those who wish to experiment with alternate load balancing strategies. To disable this feature, simply uncheck the option in Load Tester, and the change will automatically be reflected by the Load Engines during your next load test.

We hope that this feature will help to remove complexity during the testing cycle, and provide faster and more accurate results. Happy Testing!

Engineer at Web Performance

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