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EventPage

An individual event within an EventBook. Each page represents one state change.

Structure​

message EventPage {
  uint64 sequence = 1;      // Position in aggregate timeline
  google.protobuf.Timestamp timestamp = 2;
  google.protobuf.Any event = 3;  // Or PayloadReference
}

Fields​

FieldPurpose
sequenceOrdering within aggregate (0, 1, 2, ...)
timestampWhen event was persisted
eventSerialized event payload

Large Payloads​

For events exceeding message bus limits, use PayloadReference:

message PayloadReference {
  PayloadStorageType storage_type = 1;  // GCS, S3, filesystem
  string location = 2;                   // Storage path
}

This implements the Claim Check Pattern - store large payloads externally and reference them.

Immutability​

Once persisted, EventPages cannot be modified. This guarantees:

  • Audit trail integrity
  • Consistent replay
  • Trust in historical data