docker image: support configuration with environment variables

[t-8ch]

Is your feature request related to a problem? Please describe.

The APM docker container can not be configured with environment variables. Environment variables have various advantages over the other available configuration mechanisms. (Especially on Kubernetes)

Describe the solution you'd like

It would be nice if the apm container could be configured with environment variables, similar to the other elastic products.

Describe alternatives you've considered

• Volume mounting a custom configuration that uses custom configuration environment variables

  • Has to be done by each user again.
  • Has to be adapted for each option that the user wants to configure
  • Makes discovery harder because for it to work users have to perform multiple steps before

• Volume-mounting a custom configuration file into the container

  • A volume-mount is not as transparent as environment variables for the administrator
  • The volume-mount contains credentials which should not be part of that
  • All configuration options have to be managed as one entity

• Secret mounting a custom configuration file into the container

  • A secret-mount is even less transparent
  • Non-secret options are unnecessarily made less accessible
  • All configuration options have to be managed as one entity

• Image with custom configuration

  • Large overhead
  • Even less transparent then the other options

[axw]

Thanks for opening the issue @t-8ch, and for all the details/thought you put into it.

I agree that being able to configure apm-server using environment variables would be a nice. I don't think this is an easy task given the config framework we use, but it's worth looking into.

For Kubernetes specifically: in case you're not aware, there is an operator ("Elastic Cloud on Kubernetes") which you can use to orchestrate the setup of Elasticsearch, Kibana, Elastic APM Server, and more besides. The operator provides CRDs which can define configuration directly in the manifest: https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-apm-advanced-configuration.html

We're currently in the process of rearchitecting how APM Server is deployed, to build on top of Fleet: https://www.elastic.co/guide/en/fleet/current/index.html. That will mean APM Server is not deployed as a single standalone service, but will be delivered as an "integration" to Elastic Agent via Fleet. I'm not yet sure how declarative configuration will look in this new world -- we'll update this issue when things become clearer.

[t-8ch]

@axw Thanks for your response. The operator looks indeed very useful for my usecase. I will take a look at it.