Application Insights Partially Missing Data

Question

During a recent traffic spike, I was looking at Application Insights telemetry in the Azure Portal. I noticed that some data is missing. For example, at the beginning of the spike, there were 3 failed requests. However, if I drill down to the details, only 1 of the 3 requests is shown. These screenshots illustrate the issue:

Is this a bug, or is my telemetry being throttled? And if it is throttled, how can I make sure that all errors make it through to the dashboard?

I am currently at the Free tier, and the "Quota + pricing configuration" settings blade states that 100% of the data samples received are being retained. I have only used about 3% of my monthly quota so far.

Answer 1

It is likely that the reason is SDK sampling. Assuming you are using .NET Web SDK, can you please check your ApplicationInsights.config file and see if your have adaptive sampling telemetry processor, which is enabled by default:

 <Add Type="Microsoft.ApplicationInsights.WindowsServer.TelemetryChannel.AdaptiveSamplingTelemetryProcessor, Microsoft.AI.ServerTelemetryChannel">
      <MaxTelemetryItemsPerSecond>5</MaxTelemetryItemsPerSecond>
 </Add>

To completely disable sampling, you can remove this processor, however in this case you want to make sure you don't exceed 500 events per second (200EPS for free pricing tier), otherwise you'll be throttled by our data collection endpoint. Going forward we'll allow to configure to selectively exclude telemetry types from sampling, for now if you're interested in how to do it, you can check this post.