How to troubleshoot "ghost" Azure Function events?

Question

I have Azure Application Insights enabled for an Event Grid-triggered Azure Function. Some of the App Insight events show "No results" even though the Function had an execution time of ~65 secs.

How do I troubleshoot these "ghost" events?

Example:

• Function run history shows no failures in last 30d days
• Selecting the event shown (run time of ~65secs) displays "No results":
• Selecting "Run query in Application Insights" also yields "No results found..."

So... what are these events and how do I determine what happened?

Answer 1

Coming back to this after some time... the issue was the samplingSettings listed in the Azure Function host.json file.

Specifically: Setting samplingSettings isEnabled to false.

"applicationInsights": {
            "samplingSettings": {
              "isEnabled": false,

With sampling enabled, some App Insights logs were not available. Now all appear as intended.

Answer 2

• If you know its not your code, check if it is one of the automatically tracked dependencies.
• If this is happening often enough, you can open Kudu and look at streaming log as well.
• Look at other categories of events (e.g. dependencies, custom metrics, custom events, etc.) in app insights during this period (dont just go with the generated query you get after clicking the "Run query..." button.
• Look into child/correlated events that have a a trackable "ParentID" or a "RequestID" field. See post here.