Writing to Several Subjects

A command acts on a single subject, since it is a decision on the state of that subject. Some events are no such decision, for example a summary derived from many subjects, which has to be written together with a note on each of them, either all or none. To write events to several subjects at once, call the Write function with a context, the store, the events together with their subjects, and the preconditions:

written, err := architecturekit.Write(context.TODO(), store, []architecturekit.EventOn{
  {Subject: "/inventories/2026", Event: InventoryTaken{BookCount: 2}},
  {Subject: "/books/42", Event: BookCounted{Inventory: "2026"}},
  {Subject: "/books/23", Event: BookCounted{Inventory: "2026"}},
}, architecturekit.Require(eventsourcingdb.NewIsSubjectPristinePrecondition("/inventories/2026")))
if errors.Is(err, architecturekit.ErrConflict) {
  // Somebody else has taken the inventory already.
}

Write writes the events with the source of the store, and returns them as the database recorded them. If a precondition does not hold, nothing is written, and the error belongs to the category ErrConflict. Other failures belong to the same categories as for Execute (see Handling Errors).

Like a command, a write declares at least one precondition, made with Require, or Unconditionally to write without any. OnStateRead has nothing to guard, since Write reads no state.

Note that Write never decides again, since there is nothing to decide. If nothing is to be written, call it with no events, which writes nothing.