Ordering with depends_on

Services roll out concurrently, which is right until one of them reads from another. depends_on says which way round:

services:
  discover:
    version: abc1234
    type: container-app
    name: evolve-tst-discover

  purchase:
    version: abc1234
    type: container-app

  site:
    version: abc1234
    depends_on: [discover, purchase]
    type: container-app
    name: evolve-tst-site

site now starts only once both backends have finished. Everything else still runs at once — this is an ordering constraint, not a queue, so each service waits for exactly what it named and nothing else.

Why bother

Deploying a frontend and its backend at the same time makes a window where the new frontend talks to the old backend. That window is real, it is usually short, and it is entirely invisible in the output. depends_on closes it for the cases where it matters.

Two things it deliberately does not do

It does not pull a service into a release. A dependency that is not part of this run — filtered out by --only, or already at its version — is simply satisfied. That is what makes it usable in CI, where the pipeline deploys only what it rebuilt:

evolve-deploy apply deploy/tst.yaml --only site --set site=abc1234

site depends on discover, discover is not in this run, and the run proceeds. Anything else would mean a one-service deploy quietly becoming a whole-environment one.

It does not deploy a service whose dependency failed. That one is reported as not deployed rather than counted as a failure of its own, because nothing was written for it:

site        not deployed (depends_on: discover failed)

Checked while reading the config

A depends_on naming a service that is not in the file, and a cycle between services, both fail while reading the config — before anything is deployed, and before any credentials are even used.

Not with blue-green

depends_on between two blue-green services is refused. Staging carries no traffic, and a staged side reaches its own side by label URL whatever the weights say, so there is nothing left to order — the whole release stages, then gates, then switches, as one thing.

A blue-green service may still depend on a direct one, and the other way round.