Anatomy of a Fact


FactCast is centered around Facts. We say Facts instead of Events, because Event has become a blurry term that could mean any number of things from a simple onWhatNot() call handled by an Event-Loop to a LegalContractCreated with any flavor of semantics.

We decided to use the term Fact over Domain-Event because we want to highlight the notion of an Event being an immutable thing that, once it is published, became an observable Fact.

Obviously, a Fact is history and cannot be changed, after it happened. This is one of the cornerstones of EventSourcing and provides us with Facts being immutable, which plays an important role when it comes to caching.

Facts consist of two JSON documents: Header and Payload.

The Header

consists of:

  • a required Fact-Id ‘id’ of type UUID
  • a required namespace ‘ns’ of type String
  • an optional set of aggregateIds ‘aggId’ of type array of UUIDs
  • an optional (but mostly used) Fact-Type ‘type’ of type String
  • an optional Object ‘meta’ any number of key-value pairs, where the values are Strings
  • any additional information you want to put in a Fact Header


    "$schema": "",
    "definitions": {},
    "id": "",
    "properties": {
        "id": {
            "id": "/properties/id",
            "type": "string",
            "pattern": "^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}$",
        "aggIds": {
            "id": "/properties/aggIds",
            "type": "array",
            	"id": "/properties/aggIds/items",
            	"type": "string",
                "pattern": "^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}$"
        "ns": {
            "id": "/properties/ns",
            "type": "string",
        "type": {
            "id": "/properties/type",
            "type": "string"
        "meta": {
            "id": "/properties/meta",
            "type": "object",
            "additionalProperties": {
		        "type": "string",
		        "description": "Some string values"
    "type": "object",
    "additionalProperties": {
        "type": "object"
    required: ["id","ns"]

The Metadata Object

The Meta-Data Object is optional and consist of key:value pairs. The reason for it is that implementations can filter facts on certain attributes efficiently (without indexing the whole Fact payload). When a fact is read from FactCast, it is guaranteed to have two field set in the Meta-Data object of the header:

Attribute Type Semantics
_ser long / int64 unique serial number for the fact, that determines a before/after relationship between facts
_ts long / int64 timestamp in milliseconds, when this fact was published to factcast.

As you can see, all meta-data attributes prefixed with “” are supposed to be server created, so please do not use an “” prefix yourself.

The Payload

The payload has no constraints other than being a valid JSON document.

please, see GRPC docs for further details.