What time semantics forbids, and what each way out costs
What this mechanism forbids
- Completeness is unobservable. Watermarks declare it. They never prove it.
- You cannot have low latency, completeness and a single immutable answer at the same time.
- You cannot recover two independent time axes from one stored timestamp. The collapse happens at write time.
- You cannot order an unboundedly disordered stream in bounded memory.
- You cannot make a computation reproducible if it reads the ambient clock.
- You cannot expire state without silently redefining the guarantee that state was giving.
Escapes and what each costs
| Escape | Buys | Price |
|---|---|---|
| Wait longer (larger δ, longer lateness) | Completeness | Latency and state grow with the horizon; each extra nine costs multiplicatively on a heavy tail |
| Emit early | Latency | Visibly wrong intermediate answers; needs a consumer who understands that results are provisional |
| Revise (retractions, upserts) | Apparently both | The sink must be keyed and mutable, or the operator must have an inverse; complexity moves downstream |
| Use commutative operators | No buffer, no ordering cost | Restricts the computation: no first/last, no sessionization, no exact percentiles |
| Source-declared progress | Removes the guess completely | The producer must accept a contract; the obstacle is organizational, not technical |
| Ingestion time instead of event time | Monotone, replayable, simple | Outages become permanent facts about the world; every query pays the skew tail |
| Bounded horizon, then freeze | Finality, prunability, cheap maintenance | Corrections after the horizon become separate, out-of-band restatements |