Fixed issues with Zone-MTA to resolve dns

[aamirmhussain]

Fixed issues with Zone-MTA regarding dns resolution by minimizing dns resolution time. This will fix connection timed out errors and minimize mailtrain load time

mailtrain    | ERR! Queue/47 Could not initialize database: connection timed out
mailtrain    | ERR! Child/default/47 [] Sender process for default exited with 1

depends_on ensures all other container services are up before mailtrain container starts links ensure proper network connectivity for mailtrain container with rest of the container services

[aamirmhussain]

This change will also solve the following issue https://github.com/Mailtrain-org/mailtrain/issues/977

[bures]

Thank you for the patch. I have doubts about why it is needed. The links section are just aliases and as per the documentation, they are not needed for services to communicate (https://docs.docker.com/compose/compose-file/compose-file-v2/#links). The depends_on only tells in which order services are started. But it does not wait for a service to be up and ready. Waiting for other services to become ready happens in https://github.com/Mailtrain-org/mailtrain/blob/v2/docker-entrypoint.sh . So from this perspective, I don't really understand the patch.

[almereyda]

Yes, links (to other containers) are not needed, when two containers belong to the same docker network. Compose will also provide hostnames that map to a services definition, so they can be easily reached. This is solved via an internal DNS to Docker. Links were an early Docker feature, that have been suitable replaced by depends_on and healthcheck.

The https://docs.docker.com/compose/compose-file/compose-file-v2/#depends_on specification for a v2 Compose file knows a condition: service_healthy that will wait for a dependant service to pass its healtcheck, before attempting to run the container.

See https://docs.docker.com/compose/compose-file/compose-file-v2/#healthcheck

Since the Compose v3 format used for Mailtrain v2 does not know the condition: service_healthy format, it comes to my mind to ask, if this PR could be reworked to introduce a dedicated user network for connecting the database services to the frontend, which is currently not present, and to follow the official guideline for maintaining a startup order, instead?

• Networking in Compose | Docker Documentation
• Control startup and shutdown order in Compose | Docker Documentation