Dynamic Consistency Boundary (DCB)

This is the reference for DCB in DeltaBase. No fluff, just facts.

Endpoints

POST /event-stores/{eventStoreName}/events

Streamless append with an optional append condition.

Request:

{
  "events": [
    {
      "type": "UserRegistered",
      "data": { "userId": "u1", "username": "alice" },
      "tags": ["username:alice"]
    }
  ],
  "condition": {
    "failIfEventsMatch": {
      "items": [
        { "types": ["UserRegistered"], "tags": ["username:alice"] }
      ]
    },
    "after": 123
  }
}

Response:

{
  "lastPosition": 456
}

POST /event-stores/{eventStoreName}/query

Read events by query (types and/or tags).

Request:

{
  "query": {
    "items": [
      { "types": ["UserRegistered"], "tags": ["username:alice"] }
    ]
  },
  "after": 0,
  "limit": 1000
}

Response:

{
  "events": [
    {
      "eventId": "...",
      "type": "UserRegistered",
      "data": { "userId": "u1", "username": "alice" },
      "tags": ["username:alice"],
      "streamId": "__dcb__",
      "streamPosition": 1,
      "globalPosition": 456,
      "schemaVersion": "0.0.1",
      "transactionId": "0",
      "createdAt": "2026-02-04T20:00:00Z"
    }
  ],
  "lastPosition": 456
}

streamId is included for compatibility. For DCB events it is a reserved internal value. Treat it as an implementation detail.

Types

Tag

Opaque string. Commas are forbidden.

Query

{ "items": [ { "types": ["A"], "tags": ["t1", "t2"] } ] }

• Types are OR within a query item
• Tags are AND within a query item
• Query items are OR across the query

AppendCondition

{
  "failIfEventsMatch": { "items": [ ... ] },
  "after": 123
}

Errors

AppendConditionError (409)

{
  "error": "Append condition failed",
  "details": "...",
  "conflictingPositions": [456],
  "afterPosition": 123
}

Constraints

• Tags must not contain commas.
• The reserved stream id __dcb__ is not allowed in stream endpoints.