miroir/notes/bf-dijm.md

OP#2: Task State HA (Raft vs. Redis) - Summary

Status: Deferred to v2.x

Research Completed

1. Comprehensive Analysis (docs/research/raft-task-store.md)

• Crate survey: openraft vs raft-rs vs async-raft
• Decision matrix comparing Redis vs Raft on 9 dimensions
• Prototype architecture design
• Analytical benchmarks for latency, throughput, and memory

2. Decision

Do not ship Raft in v1.x. Revisit before v2.0.

Rationale:

• Raft is 3-8x slower on writes (2-5ms vs 0.3-0.8ms for Redis)
• Raft adds 90-185MB per-pod memory overhead
• openraft 0.9.20 fails to compile on stable Rust 1.87 (unstable let_chains)
• Correctness surface: Raft bugs can silently lose data
• Redis is battle-tested; operational burden is well-understood

3. Prototype Implementation (crates/miroir-core/src/raft_proto/)

• RaftTaskRegistry: Implements TaskRegistry trait
• TaskStateMachine: Command-based state machine
• TaskStoreCommand: Serialized operations for Raft log
• benchmark.rs: Performance measurements

4. Benchmarks

• State machine apply path: ~1,889ns (negligible overhead vs direct HashMap)
• Throughput: ~529K ops/sec (local apply only)
• Real cost is network + fsync consensus overhead (2-5ms per write)

For v2.x Re-evaluation

Prerequisites

1. Redis backend is production-stabilized (6+ months)
2. Operational cost of Redis is empirically measured
3. openraft reaches v1.0 stable
4. Backup/restore story for Raft is documented

Implementation Path

1. Implement TaskStore trait (SQLite + Redis backends)
2. Add RaftTaskStore composing SqliteTaskStore as state machine
3. All three backends share the same trait
4. Migration path: config change, not code rewrite

Migration Strategy (Redis → Raft)

1. Deploy new version with Raft support alongside Redis
2. Configure Raft cluster in parallel
3. Run migration job to copy Redis state to Raft
4. Switch reads to Raft (dual-write mode)
5. Switch writes to Raft
6. Decommission Redis

References

• Research: docs/research/raft-task-store.md
• Prototype: crates/miroir-core/src/raft_proto/
• Plan ref: §15 Open Problem #2