Cache-aside with cache stampede protection

# ======================================================================
# Q: Implement cache-aside with protection from a stampede.
#
# Alternatives: stale-while-revalidate (serve the stale value and refresh in the
# background with Celery), or refresh before expiry.
# ======================================================================

def get_or_compute(key, compute, ttl=300):
    value = cache.get(key)
    if value is not None:
        return value
    # only one computes. cache.add is atomic (SET NX)
    if cache.add(f'{key}:lock', 1, timeout=30):
        try:
            value = compute()
            cache.set(key, value, ttl + random.randint(0, ttl // 10))   # jitter
            return value
        finally:
            cache.delete(f'{key}:lock')
    time.sleep(0.05)                  # the others wait a little
    return cache.get(key) or compute()