Retries with backoff and idempotency keys

import random
from tenacity import retry, retry_if_exception_type, stop_after_attempt, wait_exponential_jitter
 
# RETRIES WITH BACKOFF: retry only transient errors, wait longer each time, add jitter, give up eventually.
class Transient(Exception): ...        # e.g. 429, 503, timeouts
class Permanent(Exception): ...        # e.g. 400 bad request, 401: retrying won't help
 
failures = {"left": 2}
@retry(retry=retry_if_exception_type(Transient), stop=stop_after_attempt(4),
       wait=wait_exponential_jitter(initial=0.01, max=0.1), reraise=True)   # tiny waits for the demo
def call_provider(prompt: str) -> str:
    if failures["left"] > 0:
        failures["left"] -= 1
        print("  attempt failed: 503 overloaded")
        raise Transient()
    return f"answer to {prompt!r}"
 
print(call_provider("hello"), f"(attempts: {call_provider.statistics['attempt_number']})")
 
random.seed(1)
waits = [min(30, 1 * 2 ** n) + random.uniform(0, 1) for n in range(5)]   # typical production schedule
print("backoff schedule (s):", [round(w, 1) for w in waits])
 
# IDEMPOTENCY KEYS: the client sends a unique key; repeats of the same request return the saved result.
SAVED, CHARGES = {}, []
def create_payment(idempotency_key: str, amount: int) -> dict:
    if idempotency_key in SAVED:
        return {**SAVED[idempotency_key], "replayed": True}
    CHARGES.append(amount)                                  # the side effect happens once
    SAVED[idempotency_key] = {"payment_id": f"pay_{len(CHARGES)}", "amount": amount}
    return SAVED[idempotency_key]
 
key = "7f1c2e9a-retry-safe"                                 # e.g. a UUID generated by the client
print(create_payment(key, 40))
print(create_payment(key, 40))                              # network glitch -> client retries
print("customer charged:", CHARGES)

  attempt failed: 503 overloaded
  attempt failed: 503 overloaded
answer to 'hello' (attempts: 3)
backoff schedule (s): [1.1, 2.8, 4.8, 8.3, 16.5]
{'payment_id': 'pay_1', 'amount': 40}
{'payment_id': 'pay_1', 'amount': 40, 'replayed': True}
customer charged: [40]