Read replicas · fullstack.org.ua

fullstack.org.ua

Read replicas · fullstack.org.ua

javascript

class DatabaseRouter {
  constructor(primary, replicas) {
    this.primary = primary;
    this.replicas = replicas;
    this.nextReplica = 0;
  }

  route(operation) {
    if (operation.type === "write") {
      return this.primary;
    }

    const availableReplicas = this.replicas.filter(
      (replica) => replica.healthy && replica.lagMs <= operation.maxLagMs
    );

    if (availableReplicas.length === 0) {
      return this.primary;
    }

    const replica = availableReplicas[this.nextReplica % availableReplicas.length];
    this.nextReplica += 1;

    return replica;
  }
}

const router = new DatabaseRouter(
  { name: "primary" },
  [
    { name: "replica-eu", healthy: true, lagMs: 80 },
    { name: "replica-us", healthy: true, lagMs: 120 },
    { name: "replica-asia", healthy: false, lagMs: 0 }
  ]
);

console.log(router.route({
  type: "read",
  maxLagMs: 200
}));
// { name: "replica-eu", healthy: true, lagMs: 80 }

console.log(router.route({
  type: "read",
  maxLagMs: 50
}));
// { name: "primary" }

console.log(router.route({
  type: "write"
}));
// { name: "primary" }

javascript

class Primary {
  constructor() {
    this.users = new Map();
    this.version = 0;
  }

  writeUser(id, name) {
    this.version += 1;

    const user = {
      id,
      name,
      version: this.version
    };

    this.users.set(id, user);

    return {
      ...user,
      version: this.version
    };
  }

  readUser(id) {
    return this.users.get(id) ?? null;
  }
}

class ReadReplica {
  constructor(primary, lagMs) {
    this.primary = primary;
    this.lagMs = lagMs;
    this.users = new Map();
    this.appliedVersion = 0;
  }

  startReplication() {
    this.timer = setInterval(() => {
      if (this.appliedVersion >= this.primary.version) {
        return;
      }

      setTimeout(() => {
        for (const [id, user] of this.primary.users) {
          if (user.version > this.appliedVersion) {
            this.users.set(id, { ...user });
          }
        }

        this.appliedVersion = this.primary.version;
      }, this.lagMs);
    }, 100);
  }

  stopReplication() {
    clearInterval(this.timer);
  }

  readUser(id) {
    return this.users.get(id) ?? null;
  }
}

async function main() {
  const primary = new Primary();
  const replica = new ReadReplica(primary, 500);

  replica.startReplication();

  primary.writeUser("user-1", "Олена");

  console.log("Одразу після запису:");
  console.log("Primary:", primary.readUser("user-1"));
  console.log("Replica:", replica.readUser("user-1"));

  await new Promise((resolve) => setTimeout(resolve, 800));

  console.log("\nПісля синхронізації репліки:");
  console.log("Replica:", replica.readUser("user-1"));

  replica.stopReplication();
}

main();