Event Sourcing Pattern

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61

# 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

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

gdoc_arrow_left_alt Write-Ahead Log Pattern Model-Based Testing gdoc_arrow_right_alt