Database Replication

-- Master-Slave Replication Setup (PostgreSQL)

-- Primary database (master)
-- Håndterer ALL writes
CREATE TABLE products (id SERIAL PRIMARY KEY, name TEXT);
INSERT INTO products (name) VALUES ('Laptop');
-- Dette skrives til Write-Ahead Log (WAL)

-- Replica 1, 2, 3 (slaves/read replicas)
-- Modtager og replayer WAL fra primary
-- Dataene kommer automatisk til replicas

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-- Application Logic
class DatabaseConnection {
  constructor() {
    this.primary = new Client({ host: 'primary.db.com' });
    this.replicas = [
      new Client({ host: 'replica1.db.com' }),
      new Client({ host: 'replica2.db.com' }),
      new Client({ host: 'replica3.db.com' })
    ];
  }

  // Write operations GÅR TIL PRIMARY
  async write(query) {
    return await this.primary.query(query);
  }

  // Read operations LOAD BALANCES over replicas
  async read(query) {
    const replica = this.getRandomReplica();
    return await replica.query(query);
  }

  getRandomReplica() {
    const index = Math.floor(Math.random() * this.replicas.length);
    return this.replicas[index];
  }
}

// Usage
const db = new DatabaseConnection();

// Write går til primary
await db.write('INSERT INTO products (name) VALUES ($1)', ['Mouse']);

// Reads distribueres til replicas (1/3 chance for hver)
await db.read('SELECT * FROM products WHERE category = $1', ['electronics']);

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-- Synchronous vs Asynchronous

-- Synchronous: Primary venter på replica confirmation
BEGIN;
  INSERT INTO orders VALUES (...);
COMMIT; -- Blocks until ALL replicas confirm
-- Pro: Guaranteed consistency
-- Con: Higher latency

-- Asynchronous: Primary commits immediately  
BEGIN;
  INSERT INTO orders VALUES (...);
COMMIT; -- Returns immediately
-- Replicas får data 10-100ms senere
-- Pro: Low latency
-- Con: Temporary inconsistency (eventual consistency)