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status: accepted author: @tangenti created: 2025-06-16 updated: 2025-06-16

Decouple flag sync sources and flag sets

The goal is to support dynamic flag sets for flagd providers and decouple sources and flag sets.

Background

Flagd daemon syncs flag configurations from multiple sources. A single source provides a single config, which has an optional flag set ID that may or may not change in the following syncs of the same source.

The in-process provider uses selector to specify the desired source. In order to get a desired flag set, a provider has to stick to a source that provides that flag set. In this case, the flagd daemon cannot remove a source without breaking the dependant flagd providers.

Assumptions of the current model

  • flagSetIds must be unique across different sources or the configuration is considered invalid.
  • In-process providers request at most one flag set.

Requirements

  • Flagd daemon can remove a source without breaking in-process providers that depend on the flag set the source provides.
  • In-process providers can select based on flag sets.
  • No breaking changes for the current usage of selector

Proposal

API change

Flag Configuration Schema

Add an optional field flagsetID under flag or flag.metadata. The flag set ID cannot be specified if a flag set ID is specified for the config.

Flagd Sync Selector

Selector will be extended for generic flags selection, starting with checking the equivalence of source and flagsetID of flags.

Example

# Flags from the source `override`
selector: override

# Flags from the source `override`
selector: source=override

# Flags from the flag set `project-42`
selector: flagsetID=project-42

The semantic can later be extended with a more complex design, such as AIP-160 filter or Kubernetes selections. This is out of the scope of this ADR.

Flagd Daemon Storage

  1. Flagd will have separate stores for flags and sources

  2. selector will be removed from the store

  3. flagSetID will be added as part of model.Flag or under model.Flag.Metadata for better consistency with the API.

Flags Sync

Sync server would count the extended syntax of selector and filter the list of flags on-the-fly answering the requests from the providers.

The existing conflict resolving based on sources remains the same. Resyncs on removing flags remains unchanged as well.

Consequences

The good

  • One source can have multiple flag sets.
  • selector works on a more grandular level.
  • No breaking change
  • Sync servers and clients now hold the same understanding of the selector semantic.