Backpressure
Concurrency: lesson 11 of 15
An unbounded queue is not a buffer, it is a delayed crash.
Lesson 11 of 15 · 5 min
Backpressure
Step 1 of 9
belt0 of 2
A fast producer, a slow consumer, and a belt between them that holds exactly two.
The Idea
Backpressure is the signal that travels backwards when the consumer cannot keep up. A bounded queue creates it for free: once full, the producer blocks, fails, or drops. Unbounded queues delete that signal, so latency and memory grow until something dies.
Real-World Example
A bakery with one wrapper and a two-tray shelf. When both trays are full the baker stops pulling loaves out of the oven. Remove the shelf limit and the kitchen fills with cooling bread nobody can wrap.
The Code
import queue, threading, time
belt = queue.Queue(maxsize=2) # the bound IS the backpressure
placed = shed = 0
def wrap(): # slow consumer
for _ in range(3):
time.sleep(0.08); belt.get()
t = threading.Thread(target=wrap); t.start()
for loaf in range(6): # fast producer
try:
belt.put(loaf, timeout=0.05); placed += 1
except queue.Full:
shed += 1 # refuse work instead of hoarding it
t.join()
print(placed + shed == 6, shed > 0) # True TrueYour turn
What does this print?
import queue
q = queue.Queue(maxsize=2)
q.put("a"); q.put("b")
try:
q.put("c", block=False)
print("accepted", q.qsize())
except queue.Full:
print("Full", q.qsize())Mini quiz
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