Mackerel sub-producer 
id:Songmu here.
On Thursday, October 12th, we will be adjusting specifications in relation to the URL external monitoring response time measurement. These adjustments are scheduled to be released between the hours of 2:00pm - 4:00pm (Japan Standard Time). The following two points are the issues to be adjusted.
- DNS name resolution time will not longer be included in URL external monitoring response time measurement
- Response time will no longer be recorded if the HTTP response does not return
DNS name resolution time no longer included in URL external monitoring response time measurement
Although the DNS name resolution time is currently included in the response time measured by URL external monitoring, a phenomenon has been occurring in which that segment of the response time increases when the DNS cache TTL expires.
In order to resolve this specification issue, name resolution time will no longer be included in the response time.
Response time no longer recorded if HTTP response is not returned
Currently, the response time is measured and posted in Mackerel, regardless of whether or not the HTTP response has returned from the request target server.
With this current situation, cases like the following occur.
- Values close to 0 milliseconds are measured when name resolution fails or the TCP connection is denied
- 15000 milliseconds are measured when the connection times out (15000 milliseconds is the configured value for timeout in the URL external monitoring client specifications)
We believe that these are not intuitive behaviors. Therefore we will make changes so that the response time is not measured and only recorded when the server returns an HTTP response.
Behavior for alerts when an HTTP response is not returned will not change.
How this will effect the user
We do not believe there will be any major impact on the user, but please adjust thresholds as necessary when monitoring response times.
Future adjustments for URL external monitoring
URL external monitoring is a mechanism to send requests from the servers used by the Mackerel system directly to the user’s URL and it is easily susceptible to various influences such as system load, intermediate network routing, etc. Because of this, measurement fluctuations can occur.
In order to stabilize quality, we will make fine adjustments from time to time as needed.
Thank you for your cooperation.
