docs: add ADR 0009 for static-context cache-first local persistence

[jonathannorris]

Summary

• add ADR 0009 proposing local persistence with cache-first initialization for static-context OFREP providers
• on startup, providers load cached evaluations immediately so initial flag evaluations never return defaults, then refresh from the network in parallel
• define cache-hit and cache-miss initialization paths mapped to the OpenFeature provider lifecycle (PROVIDER_READY, PROVIDER_CONFIGURATION_CHANGED)
• cache key uses hash(targetingKey), no auth token dependency

Motivation

Every major vendor SDK (LaunchDarkly, Statsig, DevCycle, Eppo) uses cache-first initialization by default. Current OFREP static-context providers keep their cache in memory only, losing all state on restart. See vendor mobile SDK caching research for a detailed comparison.

Related

• Closes OFREP Static-Context Provider Local Persistence #65

Test plan

• not applicable; documentation-only change

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces an Architectural Decision Record (ADR) proposing a standard approach for web and mobile OFREP providers to enhance offline functionality. It aims to improve user experience by enabling the persistence of feature flag evaluations in local storage, ensuring that applications can operate and maintain consistent feature states even with intermittent network connectivity or after restarts.

Highlights

• Offline Resilience for OFREP Static-Context Providers: Proposes that web and mobile OFREP providers should persist their last successful bulk evaluation in local storage by default. This enables offline application startup and preserves feature state across restarts, addressing intermittent connectivity issues.
• Persistence Mechanism Details: Specifies that the persisted entry should include the bulk evaluation payload, ETag, metadata for matching the current provider instance (like OFREP endpoint and evaluation context), and the write time. It also outlines the initialization flow for loading from local storage.
• Consequences and Considerations: Discusses positive impacts like improved resilience and consistent offline behavior, as well as negative aspects such as increased provider complexity, potential for stale data, and security/privacy concerns with persisted data.

Changelog

• service/adrs/0009-localStorageForStaticContextProviders.md
  • Added a new Architectural Decision Record (ADR) proposing the default use of local storage for static-context OFREP providers to enable offline evaluation caching.

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[gemini-code-assist]

Code Review

This pull request introduces ADR 0009, which proposes persisting static-context evaluations in local storage for web and mobile providers to improve offline capabilities. The ADR is well-structured and covers the main aspects of the proposal. My review includes a few suggestions to enhance the clarity and robustness of the proposed design, specifically regarding the handling of server errors during initialization, clarifying the purpose of the persisted timestamp, and making the initialization flow more explicit and unambiguous.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[lukas-reining]

Thanks @jonathannorris, left some feedback.
As you, I think this is a very important addition to the OFREP providers.

I have some thoughts regarding the cache key, which could also influence the whole concept due to the static context possibly also not being available as you said in open question no. 2.

[gemini-code-assist]

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[jonathannorris]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces ADR 0009, which proposes a cache-first local persistence strategy for static-context OFREP providers. The ADR is well-structured and detailed, covering the motivation, design, consequences, and implementation considerations. The proposed cache-first initialization aligns with industry best practices and addresses the 'flash-of-defaults' problem effectively. I have a couple of suggestions to improve the clarity and security aspects of the proposal.