Log stream not outputting anything in certain situations

[guidojw]

I'm experiencing very odd behaviour with the log stream functionality.
I implemented tests using az webapp log tail to keep checking the function app's output for a specific log in our tests. This works fine on our dev environment, but no logs seem to appear whatsoever when using this command on our staging function app. I get the usual "Now listening on ..." startup logs and a few other ones around that, but no request logs are outputted.

In the log stream tab of this function app on Azure Portal, I see the desired logs showing up when using the Application Insights setting. When using the Filesystem logs setting, I only see the same startup logs as earlier. I think this is related to the problem.
Using Kudu I ssh'd into the OS and checked out the log files in /appsvctmp/volatile/logs/runtime/*.log, these show the same problem, only startup logs and nothing more.
On our dev function app, both places show the desired other logs too.

I spent some time debugging and comparing all the function app settings, but have not found what is causing this so far.

Investigative information

The problem is not coupled to a specific invocation, but I attached them for debugging purposes.
staging: no logs
dev: working correctly

• Timestamp: staging=2024-04-15 15:06:29.900 dev=2024-04-15 14:17:49.044
• Function App version: ~4
• Function App name: disclosed via Invocation ID
• Function name(s) (as appropriate): all, testing on health_status
• Invocation ID: staging=a2a4cb58-9d39-4716-a091-de7d12cce8c7 dev=8c1da834-a1ff-47d2-9161-7ed8b93be9af
• Region: eu-west-1

Repro steps

1. az webapp log tail --resource-group <resource_group_name> --name <function_app_name>

Expected behavior

See function execution logs.

Actual behavior

See only the start up logs.

Known workarounds

None, I don't think Application Insights has a log stream functionality.

Related information

Python 3.9,

host.json:

{
  "version": "2.0",
  "logging": {
    "fileLoggingMode": "always",
    "logLevel": {
      "Host.Results": "Error",
      "Function.health_status": "Debug",
      "Host.Aggregator": "Trace"
    },
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      }
    }
  },
  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[3.3.0, 4.0.0)"
  }
}

Application Insights is coupled, ftp publishing credentials are off but scm is still on. Kudu dashboard is accessible.

[guidojw]

New information:

• On the dev environment a diagnostic setting with category "FunctionAppLogs" was clicked together which was not configured in the IaC, this is an important difference with the other non-functional environments.
• I tested out configuring "App Service logs", it seemed like this worked but this was only when the Log Stream was open somewhere recently.

Knowing this information, I tried out configuring App Service logs and adding a diagnostic setting pointing to a dummy storage account with a management policy deleting blobs after a day, this works like a charm.

The storage account solution however sounds a bit odd and like a non-solution to me.
I'll keep this issue open to ask for confirmation if this is really the way to go.

[raushan2998]

Hello @guidojw, Thanks for the detailed breakdown.

Yes, the difference in behavior is due to the absence of a diagnostic setting with the FunctionAppLogs category on the staging app. The az webapp log tail command relies on file system logging, which only works when App Service logs are explicitly enabled. Application Insights captures logs separately and doesn’t stream via this command.
For a workaround, you can add a diagnostic setting with FunctionAppLogs enabled or ensure App Service filesystem logging is turned on. Using a storage account is a valid approach, not ideal for real-time, but works for persistence and streaming when properly configured.
Let us know if enabling that resolves the issue.

[raushan2998]

Closing the issue as there is no feedback.