Support for weighted resource hooks: helm.sh/hook-weight

[jessesuen]

There is a need to perform a phased rollout of multiple deployments where deployment B needs to be deployed only after deployment B is fully ready.

Typically this use case is solved by defining an init container in deployment B, which queries its dependency (deployment A) to be up. However, there are some scenarios where init containers are unable to solve the problem.

For example, one scenario is that deployment B should not start until deployment A is running with same version of deployment B. An init container performing a version check would not help here because deployment A may be running with multiple replicas, and could potentially be returning a mix of versions, when deployment A is performing a rolling update.

Depended on by #355.

[jessesuen]

Reference on helm hook weights:
https://github.com/helm/helm/blob/master/docs/charts_hooks.md#writing-a-hook

When we implement this, we need to additionally honor helm weights

[alexec]

  annotations:
    "argoproj.io/hook-weight": "-5"

[alexec]

@jessesuen I've implemented weight hooks, but I don't think they actually solve your problem 😠 . It does move us towards better Helm support mind you.

You want waves of roll out. I'm going to speak to @dthomson25 about this.

[alexmt]

@alexec , replied in PR

[alexec]

Closing this issue and replacing.

[jessesuen]

Reopening because I don't agree argoproj.io/sync-wave is the right syntax, and I think it's important to be consistent in concept and implementation with helm hook-weights, especially since we plan to map the helm-hook weights to this feature

[alexec]

I think we keep this open for Helm hook support.