App Service Linux WebApp Java 11/TomCat 9 readonly filesystem error

Question

App Service Linux WebApp Java 11/TomCat 9 readonly filesystem error

Frans Koning 66

Hi There,

I'm facing an error on an app service with Java 11 and Tomcat 9 runtime. The Linux container starts, but java cannot start due to errors on a read only filesystem.

java.nio.file.FileSystemException: /home/LogFiles/Application/catalina.pl0sdlwk000MX4.2023-03-03.log: Read-only file system

Also, when I SSH login on the container I can confirm the /home directory is somehow readonly:

b648da145290:/home# touch test
touch: test: Read-only file system

How can I resolve this problem?

brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2023-06-06T22:36:26.0066667+00:00

@Frans Koning Please let us know if the below answer is helpful by accepting it as the answer. If you have further concerns, please reply to this message so we can assist you further.

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brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2023-06-06T22:36:26.0066667+00:00

@Frans Koning Please let us know if the below answer is helpful by accepting it as the answer. If you have further concerns, please reply to this message so we can assist you further.

Answer 1

By design, the App Service on Linux uses a read-only root filesystem to ensure the integrity and security of the platform. However, it provides a persistent storage option for your application's data.

To resolve this problem, you should avoid writing or modifying files in the read-only filesystem. Instead, you can utilize the persistent storage provided by Azure App Service to store your application logs and other writable data.

In the case of Tomcat, you can configure it to write logs and other files to a writable location by specifying a different directory path. By default, Tomcat writes logs to the ${catalina.base}/logs directory, which resides in the read-only filesystem. You can override this configuration by setting the CATALINA_BASE environment variable to a writable path. For example, you can set it to /home/site/wwwroot which is the path to the persistent storage in the App Service.

To configure the CATALINA_BASE environment variable in your App Service, you can follow these steps:

Open the Azure portal and navigate to your App Service.
Go to the Configuration section.
Add a new application setting with the name CATALINA_BASE and the value /home/site/wwwroot.
Save the configuration.

After making this configuration change, restart your App Service for the changes to take effect. Tomcat will now write logs and other files to the /home/site/wwwroot directory, which resides in the persistent storage and is writable.

Remember that any changes made to the container filesystem during the application's runtime will be lost when the container restarts. Therefore, it's essential to store your application's data and logs in a persistent storage location like the /home/site/wwwroot directory or an external storage service.

By following these steps, you should be able to resolve the read-only filesystem issue in your App Service Linux Web App running Java 11 and Tomcat 9.

Frans Koning 66 Reputation points

2023-06-06T06:52:18.8466667+00:00

Thanks for your extended answer and explanation!

What I was wondering though, the /home/ directory is writable in another App Service we are running with the same configuration. We don't write too the root (/), only to /home/. Shouldn't that be enough?

Although, somehow the App Service came back to live without us changing anything...
brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2023-06-07T02:11:47.8266667+00:00

@Frans Koning Regarding the read-only file system error, it's important to note that the --read-only flag enforces the container's root filesystem to be mounted as read-only. This means that any write operations to the root filesystem will fail. However, write operations to other directories, such as /home, should still be possible.

If you are only writing to the /home directory and not to the root filesystem, then the --read-only flag should not be causing the issue. In this case, it's possible that there was a different configuration or setting that was causing the issue.

If the issue occurs again, you can try to SSH into the container and check the permissions of the /home directory. You can use the ls -l command to check the permissions of the directory. If the directory is read-only, you can try to change the permissions of the directory using the chmod command.

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App Service Linux WebApp Java 11/TomCat 9 readonly filesystem error

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