Semaphore

TechniqueConcurrencyJuniorMidUpdated Nov 10, 2026

Counter with atomic acquire/release: lets at most N permits be held simultaneously. Generalizes the mutex (N=1).

When to Use Semaphore

Bounding concurrent access to a finite resource pool: DB connections, outbound HTTP slots, GPU memory. Also for classic signaling patterns.

In Practice

Java's Semaphore, Python's threading.Semaphore, Linux kernel struct semaphore. Most JDBC connection pools (HikariCP) use a semaphore-like counter to cap in-flight connections.

Sketch

Java

Semaphore dbSlots = new Semaphore(10);

public Row query(String sql) throws InterruptedException {
    dbSlots.acquire();
    try {
        return runQuery(sql);
    } finally {
        dbSlots.release();
    }
}

How Semaphore Comes Up in Interviews

Scenario

Your service can open at most 20 connections to a downstream DB or it will be throttled. Hundreds of request threads need to share those slots, blocking briefly when none are free. Which primitive is the direct fit?

A semaphore initialized with 20 permits is exactly 'N concurrent holders allowed.' A mutex would serialize all DB calls (N=1); rwlock is the wrong axis (read vs. write); CAS is the wrong tool for resource capacity.