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PHASE_7_REPLICATION_HA.md

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Phase 7: Replication & High Availability

Overview

Phase 7 introduced data redundancy and automatic failover capabilities to the cloudSQL engine, transforming it into a truly fault-tolerant distributed system.

Key Components

1. Multi-Group Raft Management (distributed/raft_manager.hpp)

Developed a sophisticated manager to handle multiple independent consensus groups.

  • Dynamic Raft Instances: Orchestrates RaftGroup objects for different shards and the global catalog.
  • Leadership Tracking: Real-time monitoring of leader status across the entire cluster.

2. Log-Based Data Replication (distributed/raft_group.cpp)

Implemented high-performance replication for DML operations.

  • Binary Log Entries: Serializes SQL statements into binary logs for replication across nodes.
  • State Machine Application: Automatically applies replicated logs to the underlying StorageManager, ensuring consistency across all nodes.

3. Leader-Aware Routing (distributed/distributed_executor.cpp)

Optimized query execution to leverage the replicated state.

  • Dynamic Shard Location: Resolves which node currently leads a specific shard for write operations.
  • Read-Replica Support: Enabled the engine to optionally route read queries to non-leader nodes for improved throughput.

4. Automatic Failover & Recovery

Engineered robust mechanisms for maintaining system availability.

  • Leader Election: Verified that the Raft consensus protocol correctly handles node failures and elects new leaders.
  • Persistence: Full persistence of Raft logs and state ensures cluster recovery after full restarts.

Lessons Learned

  • Managing multiple Raft groups significantly increases coordination complexity but is essential for scaling distributed data.
  • Leader-aware routing must be highly dynamic to avoid performance bottlenecks during cluster transitions.

Status: 100% Test Pass

Successfully verified the entire replication and failover cycle, including node failures during active workloads and final data consistency checks, via automated integration tests.