Services: Custom Health Checks

[aluzzardi]

Currently, services have a built-in health checker (portHealthChecker) that verifies the container ports defined in withExposedPort are reachable.

Sometimes, this is not enough for services binding ports before they're actually healthy.

Dagger could provide a way to define "custom" healthchecks.

Possible solutions:

• Define a custom command that gets invoked in the service container to check for health
• Same as above, but use the HEALTHCHECK docker/oci directive
  • Dagger uses HEALTHCHECK to check for service readiness (would also work for any container images having that metadata)
  • Dagger provides a WithHealthcheck (also useful for non-services to export the HEALTHCHECK metadata)
• Define a custom container that gets executed to check for health
  • Same concept as above, but the health check container can be an entirely different thing than the actual service container
  • The main advantage is running health checks on 3rd party containers that might lack the toolchain to actually test (e.g. curl, grep etc on a nginx container)

IMHO, the HEALTHCHECK approach feels better:

• It's somewhat "standard"
• A ton of OCI images already come with healthchecks that would work out of the box with dagger
• WithHealthcheck is useful even outside of services to publish OCI images with that metadata

/cc @vito

[vito]

Looks like HEALTHCHECK never became part of the OCI spec, but there is a separate Docker Image spec that we can use?

Context: opencontainers/image-spec#749 (comment)