Canary, blue-green and feature flags

import hashlib
 
def bucket(user_id: str, salt: str) -> float:
    """Stable 0-100 number per user: the same user always lands in the same bucket."""
    return int(hashlib.sha256(f"{salt}:{user_id}".encode()).hexdigest()[:8], 16) / 0xFFFFFFFF * 100
 
# CANARY: send a small, stable percentage of users to the new version.
def version_for(user_id: str, canary_percent: float) -> str:
    return "v2 (canary)" if bucket(user_id, "release-v2") < canary_percent else "v1 (stable)"
 
users = [f"user{i}" for i in range(1000)]
for pct in [5, 25, 100]:
    on_canary = sum(version_for(u, pct).startswith("v2") for u in users)
    print(f"canary {pct:3}%: {on_canary:4} of 1000 users on v2; user42 gets {version_for('user42', pct)}")
 
# FEATURE FLAGS: turn features on per user group without a deploy, and off instantly if something breaks.
FLAGS = {"new-reranker": {"enabled": True, "percent": 20, "allow": {"staff"}},
         "agent-mode":   {"enabled": False, "percent": 100, "allow": set()}}      # kill switch: enabled=False
def is_on(flag: str, user_id: str, groups: set) -> bool:
    f = FLAGS[flag]
    return f["enabled"] and (bool(groups & f["allow"]) or bucket(user_id, flag) < f["percent"])
print("new-reranker for staff user7:", is_on("new-reranker", "user7", {"staff"}))
print("agent-mode for anyone       :", is_on("agent-mode", "user7", {"staff"}))

canary   5%:   57 of 1000 users on v2; user42 gets v1 (stable)
canary  25%:  254 of 1000 users on v2; user42 gets v1 (stable)
canary 100%: 1000 of 1000 users on v2; user42 gets v2 (canary)
new-reranker for staff user7: True
agent-mode for anyone       : False