storage = LocalStorage()        # ship the senior's pick
client.save(storage, payload)   # works today
storage = S3Storage()           # swap one line, same call signature
client.save(storage, payload)   # works tomorrow if we were wrong

flag = FeatureFlag("new_checkout", percentage=10)
flag.enabled_for("user-100")  # hash bucket 7 -> True
flag.enabled_for("user-101")  # hash bucket 73 -> False
flag.set_percentage(50)        # crank later without a deploy

producer = QueueProducer.from_config(settings)  # could be Kafka, SQS, or in-memory
producer.publish("order.created", {"id": 42})
# Three callers in the codebase, one wrapper, one config flag.

write_user(old_schema, new_schema, user)   # writes to both stores
read_user(old_schema, user_id)              # reads from old, the source of truth today
# After backfill + verification, flip read_user to new_schema, keep old write live.