Boot Errors can only occur when your container is still in its starting state, before entering the running state of its lifespan:

1. building Source fetched
2. building Technology detected
3. building Application image built
4. starting Container started
5. starting Postdeploy hook run
6. running  Application running

These errors are thrown by the platform when it detects that your application doesn’t behave as expected.

There are 3 kinds of Boot Errors: Start Errors, Timeout Errors and Hook Errors.

Start Errors

The Start Error is the default kind of Boot Error. It is thrown as soon as an unmanaged error is detected and caught by the platform. It can occur at any moment, as long as your app isn’t running yet.

A Start Error makes the deployment fails instantly. Note that the former version of your application (if any), keeps running.

In most cases, a Start Error is caused by a misconfiguration of your application, or by some unmanaged error/exception in your application’s code.

Fixing Start Errors

It’s very likely that an action on your side is required to fix the issue. The deployment logs should help you identify the issue.

Timeout Errors

When your application has a web or a tcp process type, the process started in the corresponding container(s) MUST bind to the provided network port (PORT environment variable) within a delay of 60 seconds. After this deadline, the platform considers the application has been unreachable and throws a Timeout Error, causing the deployment to fail.

• If this situation arises after a restart or a scale operation, the platform automatically retries to start your container(s). After 20 unsuccessful attempts (with an exponential backoff strategy), the platform gives up. Note that your existing containers keep running during this time and after.

• If you are trying to deploy a new version of an already running application, this new deployment is considered a failure but the previous version of your application keeps running.

• Conversely, if this is a very first deployment, the platform does not make any attempt to recover from the error. The deployment fails with a timeout-error status, letting you know that your application didn’t bind to PORT soon enough.

Fixing Timeout Errors

To fix a Timeout Error, make sure:

• to bind to the provided network port, by using the PORT environment variable.
• to listen on 0.0.0.0 and not on 127.0.0.1.
• that your application is starting quickly enough.

You may need to edit your Procfile to fulfill these requirements.

If your application doesn’t need a web or tcp process type, make sure to scale the unnecessary process type to zero.

Hook Errors

If your application has a postdeploy process type, the platform can throw a Hook Error if the post-deployment process fails.

• In such a case, the deployment fails with a hook-error status.
• If your application is already running, its code isn’t updated and the former version keeps running.

Fixing Hook Errors

Hook Errors are generally caused by an error in your codebase or by some misconfiguration. To recover from it, we first advise to investigate your application logs, not the deployment logs, to understand the root cause. After fixing it, trigger a new deployment by pushing your updated code to Scalingo.