I have been using (and loving) App Insights a lot recently and one of the things that have really impressed me is the capability and power of the queries when analysing usage patterns. One thing that caught me out however, was counting the number of requests when the sampling was active – in my case when the site was getting a lot of traffic during load testing.

Creating a simple chart showing number of requests per minute over the last hour using:

1 requests
2 | where timestamp > ago(1h)
3 | summarize count() by bin(timestamp, 1m) 
4 | render timechart
5 

1 requests
2 | where timestamp > ago(1h)
3 | summarize count() by bin(timestamp, 1m) 
4 | render timechart

Was showing far less than anticipated after my load tests?

Turns out (if you actually read the docs) this is directly called out:

https://docs.microsoft.com/en-us/azure/application-insights/app-insights-analytics-reference#count

So remember to use the sum(itemCount) approach:

1 requests
2 | where timestamp > ago(1h)
3 | summarize sum(itemCount) by bin(timestamp, 1m) 
4 | render timechart

Fairly significant difference!

1 requests
2 | where timestamp > ago(1h)
3 | summarize count() by bin(timestamp, 1m) 
4 | render timechart
5