component timing.JPG

Information:

Environment

  • Dynatrace Classic Portal
  • Backbone Chart

 

Symptoms

I created a Performance chart for one of my Backbone tests. When I click the "View components chart" icon to check the components chart, I find that the "1st Byte Time" is longer than the "Response Time". Why?

Solution

First byte time is the time between the completion of the TCP connection with the destination server that will provide the displayed page's objects and the reception of the first packet (first byte) for the objects.

 In the Components Chart, as there are multiple objects on one page, when you chart the test's 1st byte time,  the value is the summary of all first byte time of objects downloaded parallel.

While the value of test response time is the end-to-end response time, so times for first byte time may be greater than end-to-end response time.

This is because objects are downloaded in parallel (at the same time) during a test execution, which is the standard behavior of a browser. As we can see in the attached screenshot, the waterfall chart from a single test execution shows multiple objects often having a first byte time occurring at the same time during a test. Summing these events leads to a longer time than either of their individual duration and may exceed the overall test time.

Root Cause

The value of 1st byte time in the Components Chart is the summary of all first byte time of objects downloaded parallel.

 

 

  1. Anonymous (login to see details)

    What people assume when they go for "1st Byte Time" is usually how long it takes to get the first byte after a succesful request.

    What is shown is actually NOT the first byte time, but the "SUM of all first byte times".

    Hence the report name is misleading and causes confusion, even though it mnight have it's own value.

  2. Anonymous (login to see details)

    I have to agree with Ulf. This repeatedly trips me up when charting. While I can imagine that somebody might find the sum of all first bytes useful, I find it completely useless and misleading.

  3. Anonymous (login to see details)

    Agree with Ulf and Joe. It is misleading.

  4. Anonymous (login to see details)

    I agree, useless and misleading metric.