Frequently asked questions (FAQ) - Azure App Service on Linux

Summary

This article answers common questions about App Service on Linux.

Built-in images

What are the expected values for the Startup File section when I configure the runtime stack?

The following table lists the expected values for each stack:

These commands or scripts run after the built-in Docker container starts, but before your application code starts.

Management

What happens when I select the restart button in the Azure portal?

This action is the same as a Docker restart.

Can I use Secure Shell (SSH) protocol to connect to the app container virtual machine (VM)?

Yes, you can connect through the Azure source control management (SCM) site.

How can I create a Linux App Service plan through an SDK or Azure Resource Manager (ARM) template?

Set the reserved field of the App Service to true.

Continuous integration and deployment

My web app still uses an old Docker container image after I updated it on Docker Hub. Do you support continuous integration and deployment (CI/CD) of custom containers?

Yes, CI/CD is supported. For more information, see Continuous Deployment with Web App for Containers.

Do you support staging environments?

Yes.

Can I use WebDeploy/MSDeploy?

Yes, set the WEBSITE_WEBDEPLOY_USE_SCM setting to false.

Git deployment fails using Linux web apps. How can I work around the issue?

Use one of the following options:

• Use Azure Continuous Delivery with Azure DevOps or GitHub.
• Use the zipdeploy API and run the following command from your local machine:

curl -X POST -u <user> --data-binary @<zipfile> https://{your-sitename}.scm.azurewebsites.net/api/zipdeploy

Language support

I want to use web sockets in my Node.js application. Are there any special settings or configurations to set?

Yes, disable perMessageDeflate in your server-side Node.js code. For example, if you're using socket.io, use the following code:

const io = require('socket.io')(server,{
  perMessageDeflate: false
});

Do you support uncompiled .NET Core apps?

Yes.

Do you support Composer as a dependency manager for PHP apps?

Yes. During a Git deployment, Kudu should detect that you're deploying a PHP application (due to the presence of a composer.lock file). Kudu then triggers a composer install.

Custom containers

Can I use managed identities when pulling images from Azure Container Registry?

Yes. Use Azure CLI (not Azure portal). You can use system-assigned or user-assigned identities.

I'm using my own custom container. I want the platform to mount a Server Message Block (SMB) share to the /home/ directory. Is this doable?

If the WEBSITES_ENABLE_APP_SERVICE_STORAGE setting is either unspecified or set to false, the /home/ directory won't be shared across scale instances, and files written won't persist across restarts. Explicitly setting WEBSITES_ENABLE_APP_SERVICE_STORAGE to true enables the mount. Once this is set to true, to disable the mount, you need to explicitly set WEBSITES_ENABLE_APP_SERVICE_STORAGE to false.

My container fails to start with a "no space left on device" message. What does this error mean?

App Service on Linux uses two different types of storage:

• File system storage: The file system storage is included in the App Service plan quota. It's used when files are saved to the persistent storage that's rooted in the /home directory.
• Host disk space: The host disk space is used to store container images. It's managed by the platform through the Docker storage driver.

The host disk space is separate from the file system storage quota. It's not expandable and there is a 15 GB limit for each instance. It's used to store any custom images on the worker. You might be able to use larger than 15 GBs depending on the exact availability of host disk space, but this isn't guaranteed.

If the container's writable layer saves data outside of the /home directory or a mounted Azure Storage path, the host disk space is also consumed. The platform routinely cleans the host disk space to remove unused containers. If the container writes a large quantity of data outside of the /home directory or uses bring your own storage (BYOS), it results in startup failures or runtime exceptions once the host disk space limit is exceeded. Keep your container images as small as possible and write data to the persistent storage or BYOS when running on App Service for Linux. If this isn't possible, split the App Service plan because the host disk space is fixed and shared between all containers in the App Service plan.

My custom container takes a long time to start, and the platform restarts the container before it finishes starting up. How do I fix it?

You can configure the amount of time the platform waits before restarting the container. To do so, set the WEBSITES_CONTAINER_START_TIME_LIMIT app setting to the value you want. The default minimum value is 230 seconds and the maximum value is 1800 seconds.

What's the format for the private registry server URL?

Provide the full registry URL including https://.

What is the format for the image name in the private registry option?

Add the full image name, including the private registry URL (for example, myacr.azurecr.io/dotnet:latest). Image names that use a custom port can't be entered through the portal. To set docker-custom-image-name, use the az command-line tool.

Can I expose more than one port on my custom container image?

This isn't supported.

Can I bring my own storage (BYOS)?

Yes. BYOS is currently in preview.

Why can't I browse my custom container's file system or running processes from the SCM site?

The SCM site runs in a separate container. You can't check the file system or running processes of the app container.

Do I need to implement HTTPS in my custom container?

No. The platform handles HTTPS termination at the shared front ends.

Do I need to use WEBSITES_PORT for custom containers?

Yes. This setting is required for custom containers. To manually configure a custom port, use the EXPOSE instruction in the Dockerfile and the app setting WEBSITES_PORT with a port value to bind on the container.

Can I use ASPNETCORE_URLS in the Docker image?

Yes. Be sure to overwrite the environmental variable before the .NET core app starts. For example, in the init.sh script, use: export ASPNETCORE_URLS={Your value}.

Multi-container with Docker Compose

How do I configure Azure Container Registry to use with multi-container?

To use Azure Container Registry with multi-container, host all container images on the same Azure Container Registry registry server. When the images are on the same registry server, create application settings and update the Docker Compose configuration file to include the Azure Container Registry image name.

Create the following application settings:

• DOCKER_REGISTRY_SERVER_USERNAME
• DOCKER_REGISTRY_SERVER_URL (Use a full URL. For example: https://<server-name>.azurecr.io.)
• DOCKER_REGISTRY_SERVER_PASSWORD (Enable admin access in Azure Container Registry settings.)

Within the configuration file, reference your Azure Container Registry image like the following example:

image: <server-name>.azurecr.io/<image-name>:<tag>

How do I know which container is internet accessible?

• You can open only one container for access.
• Only ports 80 and 8080 are accessible (exposed ports).

Here are the rules for determining which container is accessible (in the order of precedence):

• Application setting WEBSITES_WEB_CONTAINER_NAME set to the container name.
• The first container to define port 80 or 8080.
• If neither of the preceding rules is true, the first container defined in the file is accessible (exposed).

How do I use depends_on?

App Service doesn't support the depends_on option and ignores it. Like the control startup and shutdown recommendation from Docker, App Service multi-container apps should check dependencies through application code, both at startup and disconnection.

The following example code shows a Python app checking to see if a Redis container is running:

          import time
          import redis
          from flask import Flask
          app = Flask(__name__)
          cache = redis.Redis(host='redis', port=6379)
          def get_hit_count():
              retries = 5
              while True:
                  try:
                      return cache.incr('hits')
                  except redis.exceptions.ConnectionError as exc:
                      if retries == 0:
                          raise exc
                      retries -= 1
                      time.sleep(0.5)
          @app.route('/')
          def hello():
              count = get_hit_count()
              return 'Hello from Azure App Service team! I have been seen {} times.\n'.format(count)
          if __name__ == "__main__":
              app.run(host="0.0.0.0", port=80, debug=True)

Are WebSockets supported?

WebSockets are supported on Linux apps. The webSocketsEnabled ARM setting doesn't apply to Linux apps since WebSockets are always enabled for Linux.

Pricing and service license agreement (SLA)

What is the pricing now that the service is generally available?

Pricing varies by SKU and region. See more details at our pricing page: App Service Pricing.

Other questions

How does the container warmup request work?

When App Service starts your container, the warmup request sends an HTTP request to the /robots933456.txt endpoint of your application. This endpoint is a placeholder, but your application must reply by returning any status code (including 5xx). If your application logic doesn't reply by sending an HTTP status code to nonexistent endpoints, the warmup request can't receive a response. Therefore, it perpetually restarts your container.

To change this default behavior, customize the warmup endpoint path and status codes that consider the site to be warmed up. Set the WEBSITE_WARMUP_PATH and WEBSITE_WARMUP_STATUSES application settings.

The warmup request also might fail because of port misconfiguration.

Is it possible to increase the container warmup request time-out?

The warmup request by default fails after waiting 240 seconds for a reply from the container. You can increase the container warmup request time-out by adding the application setting WEBSITES_CONTAINER_START_TIME_LIMIT with a value between 240 and 1800 seconds.

How do I specify the port in my Linux container?

The following table describes how to specify the port in your Linux container:

Can I use a file based database (like SQLite) with my Linux web app?

The file system of your application is a mounted network share. This enables scale out scenarios where your code needs to be run across multiple hosts. However, this blocks the use of file-based database providers like SQLite since it's not possible to acquire exclusive locks on the database file. Use a managed database service like Azure SQL, Azure Database for MySQL, or Azure Database for PostgreSQL.

What are the supported characters in application settings names?

You can use only letters (A-Z, a-z), numbers (0-9), and the underscore character (_) for application settings.

Where can I request new features?

You can submit your idea at the Web Apps feedback forum. Add "[Linux]" to the title of your idea.

References

• What is Azure App Service on Linux?
• Set up staging environments in Azure App Service
• Continuous Deployment with Web App for Containers
• Things You Should Know: Web Apps and Linux
• Environment variables and app settings reference