How GTM determines server liveness with multiple liveness tests

When you configure multiple tests, servermonitor aggregates the scores across the tests. You can configure how these scores are aggregated by choosing the following methods:

  • mean
  • median
  • worst (Control Center default creating new properties)
  • best

If the aggregation type is mean, and a server returns an HTTP test object in 2 seconds and an HTTP test object in 4 seconds, servermonitor reports a score of three seconds. More significantly, if one test succeeds in 5 seconds and the other incurs an HTTP error, servermonitor aggregates the mean of 5 nd 75, which is 40.

In the case in which one test succeeds and the other fails, the server will usually be considered down. However, if the second test is failing on all servers in the property, they will all have a similar mean score, and that test will be discounted in the liveness algorithm. If any server is failing both tests, its mean will be 75; it will be considered down (because its score is more than 1.5 times the best score), while all the servers with scores around 40 will be considered up.

Note how the result of the previous example changes if you switch to using worst as the aggregation method. As an example, if one of the tests is failing on all servers, they will all have a score of 75. As a result, they will all appear to be down. If there's a backup CNAME, GTM hands that out; otherwise, GTM hands them all out (including the server failing both tests). When using this option, it is important that all the tests succeed on live servers. A problem that causes a test to fail on all servers, such as a typo in the configuration of the test object, effectively renders all the liveness tests unusable.