mediumSystem Design

What is database connection pooling?

Q: What is database connection pooling?

## Connection pooling Opening a database connection is **expensive** — TCP handshake, TLS, and auth round trips — and databases cap how many connections they can handle. A **connection pool** keeps a fixed set of connections **open and reusable**: a request **borrows** one, runs its query, and **returns** it. If all are busy, requests **queue** until one frees up. App many reqs Connection pool</tex

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Explain connection pools.

connection-pool

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A pool of N reusable connections

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import threading, queue, time

class Pool:
    def __init__(self, size):
        self.free = queue.Queue()
        for i in range(size):
            self.free.put("conn-" + str(i))   # pre-open N connections
    def acquire(self):
        return self.free.get()                 # blocks/queues if none free
    def release(self, conn):
        self.free.put(conn)

pool = Pool(2)                                  # only 2 connections for everyone

def handle(req):
    conn = pool.acquire()
    print(req, "using", conn)
    time.sleep(0.05)                            # run query
    pool.release(conn)                          # return -> reused

threads = [threading.Thread(target=handle, args=("req" + str(i),)) for i in range(5)]
for t in threads: t.start()
for t in threads: t.join()
print("5 requests served by 2 pooled connections")