How PDO Connection Pool Works

3 coroutines, 2 pooled connections — automatic management via $stmt and transactions

                    Code — one PDO with pool, 3 coroutines
                
// Single PDO object with a pool of 2 connections
$pdo = new PDO($dsn, $user, $pass, [
  PDO::ATTR_POOL_ENABLED => true,
  PDO::ATTR_POOL_MAX     => 2,
]);
for ($i = 1; $i <= 3; $i++) {
  spawn(function() use ($pdo, $i) {
    // prepare() auto-acquires conn from pool
    $stmt = $pdo->prepare("SELECT * FROM users WHERE id=?");
    $stmt->execute([$i]);  // waits for DB
    $user = $stmt->fetch();
    processUser($user);
    // $stmt goes out of scope -> refcount=0
    // conn auto-returns to pool!
  });
}

PDO Pool (ATTR_POOL_MAX: 2)

🔌

Connection #1

available

🔌

Connection #2

available

Wait queue:

empty

Coroutine 1

ready

Coroutine 2

ready

Coroutine 3

ready

Execution Timeline 0 ms

Coroutine 1

Coroutine 2

Coroutine 3

015304555ms

Speed:

Loop

CPU working

Waiting for DB

Waiting for pool

0 ms

Total time

0 ms

DB wait

0 ms

Pool wait

0 ms

Time saved

Click "Play" to start

This visualization shows how the PDO connection pool automatically manages resources. A connection is pinned while $stmt exists or a transaction is active. When $stmt is destroyed and there's no transaction, the connection automatically returns to the pool.