Advanced Modeling Pattern

Event-to-State Projection

Event-to-State Projection derives valid state intervals from ordered business events.

Problem

Source systems store events, but reporting often needs state.

Many systems store changes as events. A contract is activated, suspended, reactivated and cancelled as a sequence of business events.

For reporting, users usually need to know the state of the contract at each reporting date. Event-to-State Projection converts event sequences into valid-time intervals.

Incorrect orderingMissing transitionsInvalid state sequencesWrong interval boundaries

Example

A contract event stream becomes valid state intervals.

Contract events

Jan 05Created

Feb 01Activated

Jun 10Suspended

Jul 03Reactivated

Nov 20Cancelled

Projected state intervals

DraftJan 05 – Jan 31

ActiveFeb 01 – Jun 09

SuspendedJun 10 – Jul 02

ActiveJul 03 – Nov 19

CancelledNov 20 onward

Projection rule

Each event creates the next state. The following event closes the previous state interval.

Why it happens

Events describe changes, while reports ask for conditions over time.

An event tells you that something changed at a point in time. A state table tells you what was true during an interval. Projection bridges those two representations.

Event-sourced historyStatus transitionsCDC journalsWorkflow systemsSnapshot reportingInterval joins

Common modeling approaches

Turn ordered events into stable valid-time intervals.

Order events

Sort events by entity, event time and a deterministic tie-breaker.

Map events to states

Define which resulting state each event type creates.

Close with next event

Use the next event time as the valid_to boundary of the current state.

Validate transitions

Check that the projected sequence follows allowed business state changes.

Validation checks

Validate the projection before using it for snapshots.

✓ Validate event ordering per entity✓ Detect duplicate transition events✓ Check missing terminal or initial events✓ Validate state coverage after projection✓ Compare projected state against known snapshots

Why it matters

Projection creates a stable bridge between event history and interval reporting.

Event-to-State Projection is essential when source systems provide history as events but analytics require interval-based state.

Without projection, reporting logic often repeatedly reinterprets raw events in inconsistent ways.

Related Patterns

Event Modeling State Modeling State Reduction Historical Backfill Snapshot Reproducibility

Try it

Explore event-to-state behavior in the Workbench.

Use the Historical Modeling Workbench to reason about event streams, projected state intervals, snapshot joins and historical validation checks.

Open Historical Modeling Workbench →