Semaphore

A counting semaphore for async functions that manages available permits. Semaphores are mainly used to limit the number of concurrent async tasks.

• Each acquire operation takes a permit or waits until one is available.
• Each release operation adds a permit, potentially allowing a waiting task to proceed.

The semaphore ensures fairness by maintaining a FIFO (First In, First Out) order for acquirers.

Signature

class Semaphore {
  public capacity: number;
  public available: number;

  constructor(capacity: number);

  acquire(): Promise<void>;
  release(): void;
}

Properties

• capacity (number): The maximum number of concurrent operations allowed.
• available (number): The number of available permits.

Methods

• acquire (() => Promise<void>): Acquires a semaphore, blocking if necessary until one is available.
• release (() => void): Releases a semaphore, allowing one more operation to proceed.

Examples

const sema = new Semaphore(2);

async function task() {
  await sema.acquire();
  try {
    // This code can only be executed by two tasks at the same time
  } finally {
    sema.release();
  }
}

task();
task();
task(); // This task will wait until one of the previous tasks releases the semaphore.