Google App Engine application deployment fails despite readiness_check returning a 200 status response

I'm trying to setup a readiness_check for my application. Here's the related section of my app.yaml:

readiness_check:
  path: '/readiness_check'
  check_interval_sec: 30
  timeout_sec: 4
  failure_threshold: 10
  success_threshold: 1
  app_start_timeout_sec: 300

(Full config)

The project I'm developing is a Node.js application working on Express. Here's how I handle the /readiness_check endpoint:

app
  .get(['/readiness_check'], (req, res) => res.sendStatus(200))

Without readiness_check configured, my deployment process succeeds and I can access my application without any problems. However, when I include the readiness_check, the process fails with the following error:
> OperationError: Error Response: 4 Your deployment has failed to become healthy in the allotted time and therefore was rolled back. If you believe this was an error, try adjusting the 'app_start_timeout_sec' setting in the 'readiness_check' section.

I checked the logs and I could see that /readiness_check returned 502 at first (while the application was still starting) and then started to return 200 status codes. Accessing the endpoint with curl manually showed the same results. But still for some reason GCP didn't see my deployment as healthy.

Running gcloud app describe confirms that I have splitHealthChecks feature enabled.

I walked through troubleshooting sections in the documentation and discovered that I didn't have servicecontrol.googleapis.com and endpoints.googleapis.com services enabled, so I enabled them, but that didn't help either.

I also saw the following note in the documentation:
> If you examine the nginx.health_check logs for your application, you might see health check polling happening more frequently than you have configured, due to the redundant health checkers that are also following your settings. These redundant health checkers are created automatically and you cannot configure them.

It might be an unrelated question, but I couldn't find nginx.health_check in my application logs. I tried to search for "nginx" text, but I didn't see anything related to health checking. Though when looking for "readiness_check" it indeed showed me the responses that I've mentioned above.

There could be more possible ways to fix this:

1. You can increase the values in the resources rubric within your app.yaml file. You can check more about this here.

2. You can increase the value of app_start_timeout_sec to the maximum value, which is 1800. That way you can give a bit more time to your app to become healthy.

3. Even though, running gcloud app describe confirms that you have splitHealthChecks feature enabled, did you do the all the proper steps in migrating the health check from the legacy version? Is it applied to all the versions of your app, even the old ones?
   You can check carefully all the steps that needed to be taken to convert the health checks here. Applying the command gcloud app update --split-health-checks --project [YOUR_PROJECT_ID] may not be enough.

EDIT:
Even though, theoretically, if you did not split your traffic across different versions, this should not be a problem ( I cannot think for a reason why it would be ). In the documentation about migration, in the step 2, it says that :

> Convert legacy health check options for each version in your application.

In order to do that, you should write and update the app.yaml accordingly for each version and, then deploy the service, as assigned to a certain version ID. eg : gcloud app deploy --project PROJECT_ID --version VERSION_ID --no-promote

4. As a workaround, you could "fake" the readiness_check response, to give a 200 status response after a certain time. You would have to add a custom handler in this section. This way the deployment won't time out, and will keep working in the background. However, this misses the purpose of the readiness checks, as your instance might receive traffic when it's not ready to do so. If you have this in mind, and can handle this in your application, it would be an option to consider.

Finally, I suppose you are using App Engine Flex, as for the Standard version the healthchecks are not available and there will be errors. You can check this discussion here.