# Event Sourcing Pattern

> Event sourcing as a runnable model: state is derived from an append-only event stream, checked so replaying the events always rebuilds the same state.

Canonical: https://fizzbee.io/design/patterns/event-sourcing/
Source: https://github.com/fizzbee-io/fizzbee/tree/main/examples/references/13-07-02-event-sourcing


State derived from event stream

**Demonstrates:** event sourcing, event replay, state reconstruction

```fizz
# Example 0076: Event Sourcing Pattern
# State derived from event stream
# Demonstrates: event sourcing, event replay, state reconstruction

EventType = enum('CREATED', 'INCREMENTED', 'DECREMENTED')

action Init:
    events = []
    current_state = 0
    replayed_state = None

atomic action AppendCreateEvent:
    require len(events) == 0

    event = record(type=EventType.CREATED, value=0)
    events.append(event)
    current_state = 0

atomic action AppendIncrementEvent:
    require len(events) > 0
    require len(events) < 3

    event = record(type=EventType.INCREMENTED, value=1)
    events.append(event)
    current_state = current_state + 1
    # Invalidate replay - needs to be redone
    replayed_state = None

atomic action AppendDecrementEvent:
    require len(events) > 0
    require len(events) < 3

    event = record(type=EventType.DECREMENTED, value=1)
    events.append(event)
    current_state = current_state - 1
    # Invalidate replay - needs to be redone
    replayed_state = None

atomic action ReplayEvents:
    require replayed_state == None
    require len(events) > 0

    # Rebuild state from events
    state = 0
    for event in events:
        if event.type == EventType.INCREMENTED:
            state = state + event.value
        elif event.type == EventType.DECREMENTED:
            state = state - event.value

    replayed_state = state

# Safety: replayed state matches current state
always assertion ReplayConsistency:
    if replayed_state != None:
        return replayed_state == current_state
    return True

atomic action NoOp:
    if len(events) >= 3:
        pass
```
Run it in the FizzBee playground: https://fizzbee.io/play


**Expected result** (from `fizz`):

```
StateSpaceOptions: options:{max_actions:100 max_concurrent_actions:2}
Nodes: 15, queued: 0
Valid Nodes: 15 Unique states: 15
IsLive: true
PASSED: Model checker completed successfully
```

[Source on GitHub](https://github.com/fizzbee-io/fizzbee/tree/main/examples/references/13-07-02-event-sourcing)

## Language features used

[Assertions](/reference/language/assertions/), [Flow Modifiers](/reference/language/flow-modifiers/), [Data Structures](/reference/language/data-structures/)