الـ Replication والـ Read Replicas: نسخ حية من الـ database

import { primary, replica, sleep, one } from "./db.js";

await primary.query("CREATE TABLE posts (id serial PRIMARY KEY, title text)");
await primary.query("INSERT INTO posts (title) VALUES ('أول بوست')");
await sleep(200);
console.log("الـ replica شايفة:", await one(replica, "SELECT count(*)::int AS n FROM posts"));

try {
  await replica.query("INSERT INTO posts (title) VALUES ('من الـ replica')");
} catch (err) {
  console.log("كتابة على الـ replica:", err.message);
}

console.log(await one(primary, "SELECT application_name, state, sync_state FROM pg_stat_replication"));

// نوقف الـ replay على الـ replica، كأنها متأخرة (حمل تقيل أو شبكة بطيئة)
await replica.query("SELECT pg_wal_replay_pause()");
await primary.query("INSERT INTO posts (title) VALUES ('بوست جديد')");
await sleep(300);
console.log("primary:", await one(primary, "SELECT count(*)::int AS n FROM posts"));
console.log("replica:", await one(replica, "SELECT count(*)::int AS n FROM posts"), "← stale");
console.log(await one(replica, `SELECT pg_last_wal_receive_lsn() > pg_last_wal_replay_lsn() AS behind,
  round(extract(epoch FROM now() - pg_last_xact_replay_timestamp()) * 1000) > 200 AS lag_over_200ms`));
await replica.query("SELECT pg_wal_replay_resume()");
await primary.end();
await replica.end();

$ bash lab.sh && node main.js; bash lab.sh down
primary و replica شغالين
الـ replica شايفة: { n: 1 }
كتابة على الـ replica: cannot execute INSERT in a read-only transaction
{
  application_name: 'walreceiver',
  state: 'streaming',
  sync_state: 'async'
}
primary: { n: 2 }
replica: { n: 1 } ← stale
{ behind: true, lag_over_200ms: true }
lab.sh: line 4: pg_config: command not found

import { primary, replica, sleep, one } from "./db.js";

await primary.query("CREATE TABLE profiles (id int PRIMARY KEY, bio text)");
await primary.query("INSERT INTO profiles VALUES (7, 'قديم')");
await sleep(200);


async function write(sql, params) {
  await primary.query(sql, params);
  return (await one(primary, "SELECT pg_current_wal_lsn()::text AS lsn")).lsn;
}

async function read(sql, params, afterLsn) {
  if (afterLsn) {
    for (let i = 0; i < 4; i++) {
      const { ok } = await one(replica, "SELECT pg_last_wal_replay_lsn() >= $1::pg_lsn AS ok", [afterLsn]);
      if (ok) break;
      if (i === 3) return { rows: (await primary.query(sql, params)).rows, from: "primary" };
      await sleep(50);
    }
  }
  return { rows: (await replica.query(sql, params)).rows, from: "replica" };
}

const show = ({ rows, from }) => console.log(from, rows[0].bio);
show(await read("SELECT bio FROM profiles WHERE id = 7"));

await replica.query("SELECT pg_wal_replay_pause()");
const lsn = await write("UPDATE profiles SET bio = 'جديد' WHERE id = 7");
show(await read("SELECT bio FROM profiles WHERE id = 7", [], lsn));

await replica.query("SELECT pg_wal_replay_resume()");
show(await read("SELECT bio FROM profiles WHERE id = 7", [], lsn));
await primary.end();
await replica.end();

$ bash lab.sh && node main.js; bash lab.sh down
primary و replica شغالين
replica قديم
primary جديد
replica جديد
lab.sh: line 4: pg_config: command not found