precondition-violated

• Route: write-events
• Description: /preconditions/<index>: precondition <type> does not hold, with the place of the precondition in its list, counting from 0, and its type
• Details: precondition, the place of the precondition in its list, counting from 0.

Why It Happens#

The state of the database is not the one the write expects: a subject that is meant to be pristine has events, one that is meant to be populated has none, the latest event of a subject is not the one the write names, or an isEventQlQueryTrue query returns false. Usually, another client wrote in the meantime, after your code read the state it decided on.

If several preconditions do not hold, the error names the first of them. The preconditions on subjects are checked in the order of the request, and every isEventQlQueryTrue precondition after them, since its query may read every event.

How to Fix It#

Read the state again, decide again, and write with the preconditions that the new state calls for. Do not resend the same request unchanged: it succeeds only once the precondition holds again, which for isSubjectOnEventId it never does.

Example#

For a write whose second precondition expects /books/42 to be on the event 23, while another event has been written to it since, EventSourcingDB answers with this error, as a line of the stream, and then ends the stream with its end item:

{
  "type": "error",
  "data": {
    "code": "https://docs.eventsourcingdb.io/errors/precondition-violated",
    "description": "/preconditions/1: precondition isSubjectOnEventId does not hold",
    "details": {
      "precondition": 1
    }
  }
}