feat(flagd): integrate OFREP provider for REST mode

[erenatas]

This PR

This change deduplicates OFREP bits within flagd and make full use of cargo workspaces:

• Added open-feature-ofrep as dependency to flagd provider
• Refactored RestResolver to wrap OfrepProvider instead of duplicate HTTP code
• Added cert_path option to OfrepOptions for TLS support to align flagd and ofrep changes (ofrep crate was outdated, which proves why this refactoring is useful)
• Updated rest feature to include open-feature-ofrep dependency
• Converted ofrep crate to use workspace dependencies
• Made RestResolver::new() async to match OfrepProvider API

In terms of benefits, in summary:

• Eliminated duplicate HTTP client code in flagd
• Single source of truth for OFREP implementation
• Users can choose between RPC, REST (OFREP), or in-process evaluation
• Consistent dependency versions via workspace

Related Issues

Followup of #106 and #105

Notes

changes will follow in optimizing ofrep crate dependencies to provide wasm compatibility.

[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 significantly refactors the flagd project by introducing a new flagd-evaluation-engine crate and integrating the open-feature-ofrep crate. This change aims to streamline the codebase, eliminate redundant HTTP client implementations, and establish a single source of truth for OFREP. By leveraging Cargo workspaces, dependency management is centralized, providing a more consistent and maintainable project structure. This enhancement offers users greater flexibility in choosing between RPC, REST (OFREP), or in-process evaluation methods.

Highlights

• OFREP Integration: The open-feature-ofrep crate has been integrated into flagd to provide a standardized and deduplicated implementation for REST-based flag resolution, replacing custom HTTP client logic.
• New Evaluation Engine Crate: A new crate, flagd-evaluation-engine, has been introduced to encapsulate the in-process evaluation logic, including JSONLogic-based targeting, fractional rollouts, and semantic versioning, promoting modularity and reusability.
• Cargo Workspace Centralization: Dependency management has been centralized across the project by adopting Cargo workspaces, ensuring consistent versions and simplifying maintenance for shared dependencies.
• RestResolver Refactoring: The RestResolver in flagd has been refactored to wrap the OfrepProvider from the new open-feature-ofrep crate, and its constructor is now asynchronous, supporting TLS configuration via a cert_path option.
• In-Process Resolver Update: The in-process resolver now re-exports its core evaluation components directly from the flagd-evaluation-engine crate, streamlining its internal structure.

Changelog

• Cargo.toml
  • Updated workspace configuration to use resolver v2 and introduced workspace dependencies for centralized management, adding flagd-evaluation-engine to workspace members.
• crates/flagd-evaluation-engine/Cargo.toml
  • Added a new Cargo.toml file for the flagd-evaluation-engine crate, defining its metadata and dependencies using workspace inheritance.
• crates/flagd-evaluation-engine/README.md
  • Added a README.md file detailing the purpose, features, installation, and usage of the flagd-evaluation-engine crate.
• crates/flagd-evaluation-engine/src/error.rs
  • Added a new error module defining FlagdEvaluationError for the evaluation engine, using the thiserror crate.
• crates/flagd-evaluation-engine/src/lib.rs
  • Added the main library file for flagd-evaluation-engine, defining its modules and re-exporting core types.
• crates/flagd-evaluation-engine/src/model/feature_flag.rs
  • Moved the feature_flag module to the new flagd-evaluation-engine crate.
• crates/flagd-evaluation-engine/src/model/flag_parser.rs
  • Moved the flag_parser module to the new flagd-evaluation-engine crate and updated its error type to FlagdEvaluationError.
• crates/flagd-evaluation-engine/src/model/mod.rs
  • Added the model module for flagd-evaluation-engine, organizing and re-exporting its sub-modules and types.
• crates/flagd-evaluation-engine/src/model/value_converter.rs
  • Moved the value_converter module to the new flagd-evaluation-engine crate.
• crates/flagd-evaluation-engine/src/targeting/fractional.rs
  • Moved the fractional targeting module to the new flagd-evaluation-engine crate.
• crates/flagd-evaluation-engine/src/targeting/mod.rs
  • Moved the targeting module to the new flagd-evaluation-engine crate and updated its error type.
• crates/flagd-evaluation-engine/src/targeting/semver.rs
  • Moved the semver targeting module to the new flagd-evaluation-engine crate.
• crates/flagd/Cargo.toml
  • Updated feature dependencies for rest and in-process to use the new open-feature-ofrep and flagd-evaluation-engine crates, respectively, and migrated many direct dependencies to workspace inheritance.
• crates/flagd/src/error.rs
  • Added a conversion implementation to map FlagdEvaluationError from the new evaluation engine crate to the existing FlagdError.
• crates/flagd/src/lib.rs
  • Updated the RestResolver instantiation to be an asynchronous call.
• crates/flagd/src/resolver/in_process/mod.rs
  • Refactored the in-process resolver module to re-export components from the new flagd-evaluation-engine crate, centralizing evaluation logic.
• crates/flagd/src/resolver/in_process/resolver/file.rs
  • Updated imports for ValueConverter and FileConnector to align with the new module structure.
• crates/flagd/src/resolver/in_process/resolver/grpc.rs
  • Updated imports for storage components to align with the new module structure.
• crates/flagd/src/resolver/in_process/storage/connector/file.rs
  • Improved the file watcher test in FileConnector to handle timing variations more robustly.
• crates/flagd/src/resolver/in_process/storage/connector/mod.rs
  • Exported FileConnector and GrpcStreamConnector directly from the connector module.
• crates/flagd/src/resolver/in_process/storage/mod.rs
  • Exported FileConnector and GrpcStreamConnector from the storage module.
• crates/flagd/src/resolver/rest.rs
  • Rewrote the RestResolver to delegate all OFREP logic to the open-feature-ofrep crate, simplifying its implementation and making its constructor asynchronous.
• crates/ofrep/Cargo.toml
  • Updated all dependencies to use workspace inheritance for consistent versioning.
• crates/ofrep/src/lib.rs
  • Added a cert_path option to OfrepOptions to support TLS certificate configuration.

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[gemini-code-assist]

Code Review

This pull request is a significant and valuable refactoring that centralizes the OFREP implementation and evaluation logic into dedicated crates, open-feature-ofrep and flagd-evaluation-engine respectively. This greatly improves code structure, reduces duplication, and makes the codebase more maintainable. The use of cargo workspaces for dependency management is also a welcome improvement.

I've found a critical issue where the new cert_path option in the ofrep crate is not actually used, which will prevent TLS connections with custom certificates from working. I've also noted a minor dependency cleanup opportunity in the flagd crate's Cargo.toml. Overall, this is a very positive change.