The chart defines what an instance means. Run & Observe connects that model to an engine and a typed client, then teaches the ordinary application reads and controlled rehearsal surfaces around the running instance.
Start with the failure model, cross the durable commit, read current truth without confusing it with retained evidence, then rehearse behavior under explicit events, time, and outcomes.
Run one chart
An engine surrounds the same deterministic chart with an operational failure model. Its Layer owns instance lifetime, chart registration, concurrency, timers, effects, children, retention, and—when selected—durable storage and distributed ownership.
Recover committed work
A durable engine commits accepted state and planned work before enacting that work. Recovery reads those committed facts; it does not reconstruct intent from logs or pretend a failed pre-commit attempt happened.
Observe current truth
The typed client addresses one instance and can read or follow its current Snapshot. A Selection projects a typed value only while its owning State is present. These are application-facing reads: they preserve chart types and coordinate changes by sequence rather than by timing.
Keep one current Snapshot, current-first changes, and the retained macrostep diary distinct.
Typed projectionSelections & PerspectivesProject values from exact active geometry while preserving honest absence.
Rehearse without pretending
The Simulator folds chart meaning as immutable values. It can control events, time, Timer occurrences, and declared Activity or child outcomes. It does not execute effects, prove a storage transaction, recover a process, or establish distributed ownership.
Produce the next world and retain the semantic route that reached it.
Controlled timeVirtual TimeAdvance the due window or fire one exact mounted Timer occurrence.
Controlled completionActivity & Child OutcomesDeliver decoded results through exact mounted citizens without claiming their effects ran.
Where this boundary ends
Ordinary application interaction is not the whole operational system. Once the question becomes “what exactly happened?”, “what should an operator change?”, or “how does this instance advance to a new chart?”, follow the authority that owns that decision.
Join committed facts, provenance, causation, correlation, spans, and incidents.
Change operational stateOperationsReplay, interrupt, or redrive against exact retained operator authority.
Change chart meaningVersioning & MigrationsMove retained instances between exact chart identities under restoration laws.
Next, Engines begins with the failures an instance must survive while holding the chart itself fixed.