构建 WAL 复制管理器

项目概述

本项目实现一个面向 write-ahead log replication 的完整系统，核心要解决的问题是 shipping an ordered log to followers so they converge without gaps after a leader failover。整体由 Log appender、Follower shipper、Checkpoint store、Gap detector、Catch-up replay 五个模块组成，每个模块单一职责、可独立测试，最后通过集成测试验证端到端正确性。设计上把 a follower that falls behind the log retention window and must request a full snapshot instead of replaying entries 作为第一类需求先行处理，而非事后再补。

架构与关键决策

数据模型为 a monotonically increasing sequence of log entries replicated to followers, with a checkpoint marking the durable tail。Log appender 定义其余阶段共同依赖的数据形状并在边界处校验输入；Follower shipper 承载 write-ahead log replication 的正确性逻辑；Checkpoint store 专门处理 a follower that falls behind the log retention window and must request a full snapshot instead of replaying entries，在失败路径上给出明确的拒绝、重试或降级策略；Gap detector 提供可观测性，每次操作输出带关联 id 的结构化事件；Catch-up replay 在过载时按既定方式降级，避免过载演变为数据损坏。配置与代码分离，使风险变更可经 diff 审阅。

实施与测试

按五个阶段依次实现 Log appender、Follower shipper、Checkpoint store、Gap detector、Catch-up replay，每阶段配套聚焦单测。集成测试组合各模块并驱动真实 write-ahead log replication 场景：先验证正常路径，再刻意触发 a follower that falls behind the log retention window and must request a full snapshot instead of replaying entries 并断言系统降级而非损坏。所有依赖均在进程内或被桩替换，用例在 CI 中确定可复现。

限制与后续

• 单区域设计，多区域复制超出范围。
• Log appender 默认进程内，未水平共享。
• 无鉴权层，假定部署在强制身份的边缘之后。

代码示例

# setup for wal-replication-manager
pnpm init
pnpm add fastify lmdb
pnpm add -D typescript tsx vitest

wal_replication_manager/
├── src/index.ts
├── src/log-appender.ts
├── src/follower-shipper.ts
├── tests/wal_replication_manager.test.ts
└── package.json

// Log appender: write-ahead log replication core
export function logappender(input: LogappenderInput): LogappenderResult {
  // Build the log appender that anchors write-ahead log replication and defines the core data shape.
  return processLogappender(input);
}

// Follower shipper — handles write-ahead log replication Add the follower shipper so
export class Followershipper {
  constructor(private opts: FollowershipperOptions) {}
  async run(ctx: Context): Promise<FollowershipperOutput> {
    return this.execute(ctx);
  }
}

// Checkpoint store: write-ahead log replication stage
export const checkpointstore = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Gap detector for write-ahead log replication
export async function gapdetector(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Catch-up replay: write-ahead log replication finalization
export function catchupreplay(state: State): Result {
  validate(state);
  return finalize(state);
}

# run the write-ahead log replication test suite
pnpm vitest run