Sequence Number

The ordered position of an event within an aggregate's timeline. Sequence numbers start at 0 and increment by 1.

Purpose

1. Ordering: Guarantees events are applied in correct order
2. Idempotency: Prevents double-application of the same event
3. Concurrency control: Detects conflicting concurrent writes

In Commands

Commands include an expected sequence number. If the aggregate's current sequence doesn't match:

Merge Strategy	Behavior
MERGE_STRICT	Reject command
MERGE_COMMUTATIVE	Allow if mutations don't overlap
MERGE_AGGREGATE_HANDLES	Let aggregate decide
MERGE_MANUAL	Route to DLQ

Example

Aggregate: order-123
Current sequence: 5

Command arrives expecting sequence 5
  → Valid, produces event with sequence 6

Command arrives expecting sequence 4
  → Conflict! Handle per merge strategy

Sequence vs Timestamp

• Sequence: Logical ordering within aggregate (gaps not allowed)
• Timestamp: Physical time (may have gaps, clock skew)

Use sequence for state reconstruction; use timestamp for temporal queries.

Fact Sequences

Not all events come from command processing. External facts (payment confirmations, delivery updates) use a FactSequence marker instead of an integer:

message EventPage {
  oneof sequence_type {
    uint32 sequence = 1;        // Normal: position in stream
    FactSequence fact = 5;      // External fact marker
  }
}

message FactSequence {
  string source = 1;            // Origin system (e.g., "stripe")
  string description = 2;       // Human-readable description
}

The idempotency key lives on Cover.external_id, not in FactSequence. This ensures:

• System-wide propagation through sagas, PMs, and projectors
• Consistent deduplication at every coordinator
• Full traceability from external source to all effects

FactSequence Elimination

The FactSequence marker is replaced with a real sequence number at persistence time. When the coordinator receives a fact event:

1. Routes to aggregate (if route_facts_to_aggregate = true, the default)
2. Aggregate updates state and returns events
3. Coordinator assigns the next sequence number
4. Persists with sequence instead of fact
5. Publishes event with valid sequence

Downstream consumers (sagas, projectors, process managers) always receive events with proper sequence numbers—they never see FactSequence markers.

See Commands vs Facts for details.