Enhancement Description

• One-line enhancement description (can be used as a release note): kflag

• Kubernetes Enhancement Proposal:

Create a legacyflags package, which wraps pflag and addresses the pain-points of backwards-compatible
ComponentConfig migration, and selective registration of third-party library flags.

Make it easier for component owners to implement
Versioned Component Configuration Files
(a.k.a. ComponentConfig).

While a number of problems with existing machinery are highlighted in the above doc, this
proposal is focused on the following two use-cases:

• Enforcing flag precedence between values
  specified on the command line and values from a config file, which is required to incrementally
  migrate flags to a config file with backwards compatibility.
• Preventing third-party code from
  implicitly registering flags
  against a component's command-line interface, which is required for components to maintain
  explicit control over their command-line interface.

Rather than require other components to copy and customize the Kubelet's relatively complex
solutions to these problems, we should put the common functionality in a library, legacyflags, to reduce
the burden on component owners.

• Discussion Link:

This proposal recommends the approach in this example PR:

• WIP Kflag example kubernetes#73494

See also @sttts's prototype for an alternative approach that predates this KEP:

• WIP: component: prototyping custom flag semantics kubernetes#72037

• Primary contact (assignee): @mtaufen
• Responsible SIGs: sig-api-machinery
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y):
  • Beta release target (x.y):
  • Stable release target (x.y):
• Alpha
  • KEP (k/enhancements) update PR(s):
  • Code (k/k) update PR(s):
  • Docs (k/website) update PR(s):

Implementation History

• 2019-01-29: Initial KEP PR.

Please keep this description up to date. This will help the Enhancement Team to track the evolution of the enhancement efficiently.