Domain events on the stream.

XADD key field value … appends to the stream named key, creating it on first append. Choose the key by the event type and the key is the channel: every room-opened event lands on the room-opened stream, every guess-scored on the guess-scored stream. A reader of one feed reads one stream; it does not filter a combined log to find its events.

A new event type costs a new name and nothing else — no shared table to migrate, no enum to extend. The cost of one stream per type is a key per type; the buy is a focused log per feed, so a reader's range read returns only its own events in order.

Each entry carries a time-ordered id, and a later append always carries a larger id than an earlier one — within one stream. The order holds because the entries are minted by a single writer in increasing order; the id sort, the entry order, and the write order are the same sequence. Across two streams the ids are independent — room-opened and guess-scored each start their own sequence — so a reader folds one stream at a time, in that stream's order.

A domain event is a fact that happened: a room was opened, a guess scored 480, a game settled to a winner. It is appended once, never edited, never deleted as a correction — a correction is a new event. The streams grow forward only, and any view over them is a fold of the facts. Custom events change what the facts are, not the rule that they are immutable and append-only.

EchoMQ.Stream.append(conn, queue, name, fields) is the writer, and the name is the domain-event channel. It mints an EVT-branded record id host-side, derives the explicit XADD id from it, and issues XADD emq:{q}:stream:<name> <xadd_id> id <branded> <fields…>, returning {:ok, branded} — the branded id as the receipt. The key is emq:{q}:stream:<name> via EchoMQ.Keyspace.queue_key/2, so each named stream of one queue shares that queue's {q} hash-tag slot.

One brand per stream is enforced at the door. EchoMQ.Stream.Id.evt?/1 is the kind predicate, and EchoMQ.Stream.append_id/5 raises before any wire if a caller supplies a record id of the wrong namespace. That single-brand rule is what keeps the order theorem sound: base-62 byte order equals integer order only within one namespace, so a stream that admits one brand (EVT) keeps its byte-ordered ids in mint order. append/4 mints its own EVT id, so a wrong kind there cannot occur.

The worked consumer is codemojex (echo/apps/codemojex), a Telegram emoji-guessing game on this stack. Its lifecycle produces the domain events: Codemojex.Rooms.create_room/3 opens a room (a ROM) and the first joiner starts a game (a GAM); Codemojex.Guesses.submit/3 enqueues a guess as a JOB on the player's PLR lane, which the consumer scores with Codemojex.Scoring.score/2; the game settles to a winner. Each of those — room opened, guess scored, game settled — is one domain event appended to its own named stream, so the activity feed and the results view each fold their own channel.