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0046-pack-pull-policy.md

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  • Name: pack Image Pull Policy
  • Start Date: 2020-05-21
  • Author(s): Javier Romero
  • Status: Implemented
  • RFC Pull Request: rfcs#80
  • CNB Pull Request: (leave blank)
  • CNB Issue: (leave blank)
  • Supersedes: N/A

Summary

Replace the flag --no-pull with --pull-policy <option> and introduce if-not-present.

Motivation

  1. Enable the use case where the user may locally create an alternative to an image that would otherwise be pulled without having to manually pull all other images if using --no-pull.
  2. Provide support for an expected pull policy without polluting the number of flags available to commands.
  3. Allow for additional, potentially more complex pull policies such as periodic=1d or lifecycle=never,buildpack=always

What it is

A new flag --pull-policy that takes the following possible options:

  • if-not-present - only attempt to pull the images if it's not present
  • always - attempt to always pull images
  • never- don't attempt to pull images

How it Works

In the following commands, the images listed will adhere to the mentioned Pull Policy.

pack build

  • Builder - Used as the build environment
  • Buildpacks - When refered to in --buildpacks or project.toml -> [[build.buildpacks]]
  • Run (for stack) - Used as the base for the built app image
  • Lifecycle - Used in cases where Builder is not trusted

pack create-builder

  • Builder - Used as the build environment
  • Buildpacks - When refered to in builder.toml

pack rebase

  • Run (for stack) - Used as the base for the built app image

pack package-buildpack

  • Buildpacks - When refered to in package.toml

Backwards Compatibility

For a period of time, --no-pull flag will map to --pull-policy never and print a deprecation warning.

Drawbacks

  • Longer CLI arguments (--no-pull -> --pull-policy never)
    • This might be a superfluous mention but does impact UX.

Alternatives

Boolean flags

Instead of a single flag --pull-policy with multiple options, we could add to our existing boolean flags such as:

  • --no-pull: don't attempt to pull images
  • --pull-missing: only attempt to pull the images if it's not present
  • --always-pull: attempt to always pull images
Drawbacks
  • Discovery: harder to discover related flags

Prior Art

  • Docker CLI: docker build works as if-not-present with an additional flag that changes it to always (docker build --pull).
  • Docker Compose: docker-compose build works as if-not-present with an additional flag that changes it to always (docker-compose build --pull).
  • Kubernetes:

    The default pull policy is IfNotPresent which causes the Kubelet to skip pulling an image if it already exists

Unresolved Questions

Spec. Changes

None