… engine ## 🎯 Overview v0.2.0 is a major release that transforms d-engine from a Raft implementation into a **production-ready distributed coordination engine** with workspace structure, developer-friendly APIs, and comprehensive features. --- ## 🚀 Key Features ### 🏗️ **Workspace Structure (#167)** - Modular design: `d-engine-core`, `d-engine-server`, `d-engine-client`, `d-engine-proto` - Clean separation of concerns for library users - Profile-optimized build configuration ### ⏱️ **TTL & Lease Support (#172)** - Crash-safe TTL with absolute expiration timestamps - WAL format change: relative `ttl_secs` → absolute `expire_at_secs` - Piggyback cleanup mechanism (minimal overhead) - **Breaking Change**: See MIGRATION_GUIDE.md for WAL upgrade ### 👁️ **Watch API (#174, #196)** - Lock-free event notification with crossbeam-channel - gRPC streaming support for real-time key monitoring - <10ns apply-path overhead, <100μs end-to-end latency - Service discovery examples included ### 🚀 **EmbeddedEngine API (#182)** - Zero-config single-node quick start - `wait_leader()` and `leader_notifier()` for event-driven apps - Dynamic cluster expansion (1→3 nodes without downtime) - In-process `LocalKvClient` with explicit consistency levels ### 🎯 **Single-Node Support (#179)** - Configuration-based single-node detection - Automatic election/replication optimization - Production-ready for low-traffic scenarios ### 📖 **Read Consistency Policies (#142)** - LinearizableRead (strong consistency) - LeaseRead (optimized with leader lease) - EventualConsistency (fast local reads) ### 🔧 **Go Client Support (#170, #219)** - Pre-generated Go protobuf code (zero-config) - Comprehensive error handling guide - Service discovery pattern examples --- ## 🐛 Critical Fixes - **#212**: Fix learner promotion stuck (voter count + role transition) - **#218**: Fix leader next_index initialization for new learners - **#222**: Return NOT_LEADER with leader metadata for client redirection - **#209**: Fix node restart wait_ready() timeout (leader notification race) - **#211**: Remove Arc::get_mut anti-pattern from lease injection - **#145**: Fix undefined behavior in mmap zero-copy path - **#197**: Fix integration test failures (Arc ownership, timing issues) --- ## ⚡ Performance Improvements - **#138**: Long-lived peer tasks for append entries (100K+ throughput target) - **#140**: Optimize proto bytes fields (use `bytes::Bytes`) - **#141**: Optimize RocksDB write path for lower latency - **#143**: Refactor gRPC compression configuration - **#194**: Skip protobuf decoding when no active watchers - **#208**: Eliminate redundant async calls in leader write hot path (+2-3% throughput) - **#223**: Optimize check_learner_progress() lock contention ### Benchmark Results - **LeaseRead**: 99,418 ops/s (+7.8% vs v0.1.4) - **EventualConsistency**: 126,095 ops/s (+9.2% vs v0.1.4) - **Linearizable p99**: 23.01ms (-8% vs v0.1.4) --- ## 🔧 Refactoring & Architecture - **#210**: Simplify watch architecture (tokio::broadcast, 90% code sharing) - **#217**: Refactor Node::run() with strategy pattern - **#209**: Consolidate Raft unit tests (27 tests migrated from server to core) - **#201**: Separate static membership from dynamic leader state - **#223**: Extract 5 helper methods in learner promotion (SRP + 11 unit tests) --- ## 📚 Documentation - Restructure quick-start docs (embedded + standalone examples) - Add integration-modes.md and use-cases.md - Comprehensive error handling guide - Service discovery pattern documentation - Delete 911 lines of internal architecture docs (20/80 principle) --- ## 🧪 Testing - **430** core tests + **305** server tests + **292** integration tests passing - Fix flaky tests and timing issues - Add 14+ new unit tests across components - Optimize test suite with nextest --- ## ⚠️ Breaking Changes 1. **WAL Format**: Relative `ttl_secs` → absolute `expire_at_secs` (requires data migration) 2. **Config**: `raft.watch.enabled` removed (Watch always available) 3. **API**: `StateMachine::start()` changed to async 4. **NodeStatus**: Refactored enum (PROMOTABLE/READ_ONLY/ACTIVE) See **MIGRATION_GUIDE.md** for detailed upgrade instructions. --- ## 📦 Dependency Updates - tokio-stream: Fix net feature requirement - astral-tokio-tar: Replace tokio-tar (CVE-2025-62518 fix) - simd-adler32: 0.3.7 → 0.3.8 - rustls-pemfile: unmaintained warning suppressed (tonic 0.12 dep) --- ## 🎯 Closes #43, #45, #59, #66, #70, #71, #79, #89, #90, #101, #102, #106, #107, #109, #110, #119, #120, #121, #122, #123, #125, #133, #135, #138, #139, #140, #141, #142, #143, #145, #146, #147, #148, #150, #151, #152, #153, #154, #155, #156, #157, #158, #159, #161, #164, #167, #170, #172, #174, #175, #176, #178, #185, #186, #187, #192, #193, #194, #195, #196, #197, #200, #201, #203, #204, #205, #208, #209, #210, #211, #212, #213, #217, #218, #219, #222, #223, #224 --- **Files Changed:** 100+ files, ~5,000 insertions, ~1,500 deletions

Refactor check_learner_progress() visibility and fix error handling #223

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