Atomic Operations

Concurrency: lesson 8 of 10

Indivisible: nobody sees it half-done.

Lesson 8 of 10 · 4 min

Step 1 of 11

counter += 1 looks like one step. The interpreter sees three: read, add, store.

The Idea

An atomic operation cannot be split. Other threads see the world before it or after it, never in between, so no lock is needed. The catch is scope: atomicity covers one step, and most real invariants span several.

Real-World Example

The ticket dispenser at a deli counter. Pulling the tab tears your number and advances the roll in a single motion, so two customers reaching at once still leave with different numbers. Nobody can grab a half-torn 47.

The Code

import itertools, threading

dispenser = itertools.count(1)          # next() is one indivisible step
tickets = []

def take(n):
    for _ in range(n):
        tickets.append(next(dispenser))  # append is atomic too

crowd = [threading.Thread(target=take, args=(50,)) for _ in range(4)]
for person in crowd: person.start()
for person in crowd: person.join()
print(len(tickets), len(set(tickets)))   # 200 200 — no number issued twice

Your turn

What does this print?

import itertools

dispenser = itertools.count(1)
served = [next(dispenser) for _ in range(3)]
served.append(next(dispenser))

print(served, len(set(served)))

Mini quiz

1 / 3

Atomic Operations