When Randomization Fights Back
Experimentation Platforms at Scale
Flags are the substrate
At scale, experimentation infrastructure is deployment infrastructure: feature flags decouple 'code is shipped' from 'experience is live'. An experiment becomes a flag + a config + a bucketing rule — ramp 1%, hold, ramp 50%, kill — without redeploying.
Deterministic bucketing is the quiet foundation: hash(user_id + experiment_salt) → bucket. Deterministic means the same user lands in the same arm across devices, sessions, and services — reproducible assignment without a central assignment service in the hot path. Change the salt per experiment and experiments stay independent.
insight
Deterministic hashing is why a billion-user system can run consistent experiments with no assignment database: the assignment is a pure function, computable identically everywhere.