The log of Azure Function on Application Insights changed customDimensions name from prop__FunctionName to prop__functionName

Repro steps

1. Create a new Azure Function project (HTTP trigger) with following codes

        [FunctionName("Function1")]
        public static IActionResult Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", Route = null)] HttpRequest req,
            ILogger log)
        {
            using (log.BeginScope(new Dictionary<string, object> { ["FunctionName"] = "Test" }))
            {
                log.LogInformation("C# HTTP trigger function processed a request.");
            }

            return new OkResult();
        }

1. Create a new Azure Function (Windows) resource with Application Insights enabled, deploy the project. (When I test all deployed to West US 2).

2. Test run and wait for the log available.

Expected behavior

The trace looks like customDimensions={ "prop__FunctionName" : "Test" } (Uppercase for F)

Actual behavior

The trace looks like customDimensions={ "prop__functionName" : "Test" } (lowercase for f).

Known workarounds

Not found.

Related information

I noticed this issue recently (our other Azure resources read the prop_functionName) and found that the behavior changed since 2/4, with different SDK versions on that day.

I ran following query on Application Insights around 2/4

traces
traces
| where not (message startswith "Executing '" or message startswith "Executed '" or message startswith "Stopped the listener" or message startswith "Stopping the listener")
| summarize f = countif(tostring(customDimensions.prop__functionName) != ''), F = countif(tostring(customDimensions.prop__FunctionName) != '') by sdkVersion, bin(timestamp, 1d)
| order by timestamp asc

Here is the result

timestamp [UTC]	skdVersion	f	F
2/2/2021, 12:00:00.000 AM	azurefunctions: 3.0.15193.0	0	12,017
2/3/2021, 12:00:00.000 AM	azurefunctions: 3.0.15193.0	0	12,580
2/4/2021, 12:00:00.000 AM	azurefunctions: 3.0.15371.0	567	0
2/4/2021, 12:00:00.000 AM	azurefunctions: 3.0.15193.0	0	12,232
2/5/2021, 12:00:00.000 AM	azurefunctions: 3.0.15371.0	10,893	0
2/6/2021, 12:00:00.000 AM	azurefunctions: 3.0.15371.0	181	0

According to the result, the change happens along with the SDK version update from 3.0.15193 to 3.0.15371.

My question is that is the behavior change expected (e.g., functionName is a preserved word now)? If the change is expected, we would change our project to use another value to replace FunctionName.

[mathewc]

Guys - I just sent a PR https://github.com/Azure/azure-webjobs-sdk/pull/2691 fixing this issue. Please have a look. We’ll expedite this into our next release.

[FanBaoMS]

Verified that the issue was fixed since 4/26 for our Azure Function resources. Thanks for the fix and close the issue.