Enforce uniqueness with DCB

Make usernames or emails unique across all streams

You want a rule like “this username can only exist once”. That rule spans every user stream, so DCB is the right tool.

Goal

Enforce a unique username across all users using tags + append conditions.

Step 1: Tag the event

Pick a tag format and stick to it:

const tag = `username:${username.toLowerCase()}`;

When you append the event, include the tag:

const event = {
  type: 'UserRegistered',
  data: { userId, username },
  tags: [tag],
};

Step 2: Read by query

Use a query that matches any existing user with the same username:

const query = {
  items: [
    {
      types: ['UserRegistered'],
      tags: [tag],
    },
  ],
};

const { events, lastPosition } = await eventStore.readByQuery({
  query,
  after: 0,
});

Step 3: Decide and append with condition

if (events.length > 0) {
  throw new Error('Username already taken');
}

await eventStore.append([event], {
  failIfEventsMatch: query,
  after: lastPosition,
});

If someone else registers the same username between your read and append, the append fails with a 409.

Step 4: Handle conflicts

try {
  await eventStore.append([event], {
    failIfEventsMatch: query,
    after: lastPosition,
  });
} catch (error) {
  if (isAppendConditionError(error)) {
    throw new Error('Username already taken');
  }
  throw error;
}

What’s next

Use multiple tags for compound uniqueness (e.g. tenant:acme, email:...)
Switch to DCB for other global constraints (invoice numbers, limits)
Keep stream-based logic where it already works