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

Field	Purpose
`sequence`	Ordering within aggregate (0, 1, 2, …)
`timestamp`	When event was persisted
`event`	Serialized 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