What exactly does --use-compose-depends-on do?

[zypA13510]

I wonder if --use-compose-depends-on respects depends_on.restart = true?

For example, given the compose.yaml:

services:
  app1:
    depends_on:
      app2:
        restart: true
      db:
  app2:
    depends_on:
      - db
  db:

If app2 and db both received updates in the same run, I understand that with --use-compose-depends-on, db will be restarted before app2, right? What about app1? Will it be restarted or not?

Given the wording of the current docs, I'm under the impression that app1 will not be automatically restarted. But I would like a more definitive answer. I believe the docs can be improved to be more explicit and detailed on this behavior and possibly other depends_on attributes.

[nicholas-fedor]

The long-form syntax is not used. The semantic understanding of using the Compose depends_on configuration option is that linked dependencies should be restarted in the proper order. The --use-compose-depends-on configuration option was added to allow users to disable the implicit restarting of services linked via the Compose depends_on.

Watchtower's interpretation of Compose's depends_on feature is more opinionated, as the Compose spec and Docker's implemementation defaults to requiring the explicit long-form syntax of depends_on.restart=true to enable restarts when linked services are restarted.

If there's a gap in Watchtower's documentation, then please feel free to let me know, but I disagree that it needs to be more explicit, given that there's already a full page dedicated to the topic of how Watchtower is expected to handle linked containers.

References:

• Configuration documentation: https://watchtower.nickfedor.com/latest/configuration/arguments/#label_precedence
• Feature documentation: https://watchtower.nickfedor.com/latest/advanced-features/linked-containers/#watchtower_depends-on_label
• Follow-up regarding handling the long-form syntax: Honour missing restart: true in depends_on for Linked Containers #1413 (comment)

[zypA13510]

To clarify, in the case of the compose file given above, whenever db receives an update, app1 and app2 will both be restarted, regardless of the depends_on.restart value, right?
As for the gap in docs I mentioned, I think it may be better if we add a note with a link from Arguments - Use Docker Compose Depends-On to the Advanced Features - Linked Containers page. I can open a PR.

[nicholas-fedor]

Yes, app1 and app2 will be both restarted.

• app1 depends upon app2 and the db.
• app2 depends upon the db.

If the db needs to be updated, then the stop-start chain is (stop) app1 -> app2 -> db -> (recreate/start with updated image) db -> (start) app2 -> app1

Adding a link over to the advanced documentation from the configuration section would be good. If you're interested in the PR, then an info admonition would be probably the best approach.

[nicholas-fedor]

I went ahead and added the the reference #1532