Enable diagnostic logging for apps in Azure App Service

Azure provides built-in diagnostics to assist with debugging an Azure App Service app. In this article, you learn how to enable diagnostic logging and add instrumentation to your application. You also learn how to access the information that Azure logs.

The following video complements the steps in this article by illustrating how to enable diagnostic logging for apps.

This article uses the Azure portal and the Azure CLI to work with diagnostic logs. For information on working with diagnostic logs by using Visual Studio, see Troubleshoot an app in Azure App Service by using Visual Studio.

In addition to the logging instructions in this article, you can use the Azure Monitor integrated logging capability. The Send logs to Azure Monitor section in this article provides more information about this capability.

Overview of logging types

When logs are stored in the App Service file system, they're subject to the available storage for your pricing tier. For more information, see App Service limits.

Enable application logging (Windows)

To enable application logging for Windows apps in the Azure portal:

1. Go to your app and select App Service logs.

2. Select On for either or both of these options:

   • Filesystem: This option is for temporary debugging purposes. It turns itself off in 12 hours.

   • Blob: This option is for long-term logging. It needs a blob storage container to write logs to.

     The Blob option includes additional information in the log messages, such as the ID of the origin virtual machine instance of the log message (InstanceId), the thread ID (Tid), and a more granular time stamp (EventTickCount).

3. For Level, select the level of details to log. The following table shows the log categories included in each level:

   Level	Included categories
   Disabled	None
   Error	Error, Critical
   Warning	Warning, Error, Critical
   Information	Info, Warning, Error, Critical
   Verbose	Trace, Debug, Info, Warning, Error, Critical (all categories)

4. Select Save.

If you write logs to blobs, the retention policy no longer applies if you delete the app but keep the logs in the blobs. For more information, see Costs that might accrue after resource deletion.

Currently, only .NET application logs can be written to blob storage. Java, PHP, Node.js, and Python application logs can be stored only in the App Service file system (without code modifications to write logs to external storage).

If you regenerate your storage account's access keys, you must reset the respective logging configuration to use the updated access keys:

1. On the Configure tab, set the respective logging feature to Off. Save your setting.

2. Enable logging to the storage account's blob again. Save your setting.

Enable application logging (Linux or container)

To enable application logging for Linux apps or custom containers in the Azure portal:

1. Go to your app and select App Service logs.

2. In Application logging, select File System.

3. In Quota (MB), specify the disk quota for the application logs.

4. In Retention Period (Days), set the number of days to retain the logs.

5. Select Save.

Enable web server logging

To enable web server logging for Windows apps in the Azure portal:

1. Go to your app and select App Service logs.

2. For Web server logging, select Storage to store logs in blob storage, or select File System to store logs in the App Service file system.

3. In Retention Period (Days), set the number of days to retain the logs.

4. Select Save.

If you write logs to blobs, the retention policy no longer applies if you delete the app but keep the logs in the blobs. For more information, see Costs that might accrue after resource deletion.

If you regenerate your storage account's access keys, you must reset the respective logging configuration to use the updated keys:

1. On the Configure tab, set the respective logging feature to Off. Save your setting.

2. Enable logging to the storage account's blob again. Save your setting.

Log detailed errors

To save the error page or failed request traces for Windows apps in the Azure portal:

1. Go to your app and select App Service logs.

2. Under Detailed Error Logging or Failed Request Tracing, select On.

3. Select Save.

Both types of logs are stored in the App Service file system. Up to 50 errors (files or folders) are retained. When the number of HTML files exceeds 50, the oldest error files are automatically deleted.

By default, failed request tracing captures a log of requests that failed with HTTP status codes between 400 and 600. To specify custom rules, you can override the <traceFailedRequests> section in the Web.config file.

Add log messages in code

In your application code, you use the usual logging facilities to send log messages to the application logs. For example:

• ASP.NET applications can use the System.Diagnostics.Trace class to log information to the application diagnostic log. For example:

  System.Diagnostics.Trace.TraceError("If you're seeing this, something bad happened");
  

  By default, ASP.NET Core uses the Microsoft.Extensions.Logging.AzureAppServices logging provider. For more information, see ASP.NET Core logging in Azure. For information about WebJobs SDK logging, see Get started with the Azure WebJobs SDK.

• Python applications can use the OpenCensus package to send logs to the application diagnostic log.

Stream logs

Before you stream logs in real time, enable the log type that you want. App Service streams any information written to the console output or files ending in .txt, .log, or .htm that are stored in the /home/LogFiles directory (D:\home\LogFiles).

Azure portal

To stream logs in the Azure portal, go to your app and select Log stream.

Cloud Shell

To stream logs live in Azure Cloud Shell, use the following command.

az webapp log tail --name appname --resource-group myResourceGroup

To filter specific log types, such as HTTP, use the --provider parameter. For example:

az webapp log tail --name appname --resource-group myResourceGroup --provider http

Local terminal

To stream logs in the local console, install the Azure CLI and sign in to your account. After you're signed in, follow the instructions for Cloud Shell.

Access log files

If you configure the Azure Storage blobs option for a log type, you need a client tool that works with Azure Storage. For more information, see Microsoft client tools for working with Azure Storage.

For logs stored in the App Service file system, the easiest way to access the files is to download the ZIP file in the browser at:

• Linux or custom containers: https://<app-name>.scm.azurewebsites.net/api/logs/docker/zip
• Windows apps: https://<app-name>.scm.azurewebsites.net/api/dump

For Linux or custom containers, the ZIP file contains console output logs for both the Docker host and the Docker container. For a scaled-out app, the ZIP file contains one set of logs for each instance. In the App Service file system, these log files are the contents of the /home/LogFiles directory. Deployment logs are stored in /site/deployments/.

For Windows apps, the ZIP file contains the contents of the D:\Home\LogFiles directory in the App Service file system. It has the following structure:

Send logs to Azure Monitor

With Azure Monitor integration, you can create diagnostic settings to send logs to storage accounts, event hubs, and Log Analytics workspaces. When you add a diagnostic setting, App Service adds app settings to your app, which triggers an app restart.

Supported log types

For a list of supported log types and their descriptions, see Supported resource logs for Microsoft.Web.

Network considerations

For information about restrictions for diagnostic settings, see Destination limits.

Related content

• Log queries in Azure Monitor
• Azure App Service quotas and alerts
• Troubleshoot an app in Azure App Service by using Visual Studio
• Tutorial: Run a load test to identify performance bottlenecks in a web app