The append-only log.

A Redis Stream is an ordered sequence of entries, each a small set of field-value pairs under an id. The only way to add to it is XADD, which appends to the end and returns the new entry's id. There is no in-place update of a written entry: a stream grows forward only. That single property — append, never overwrite — is what makes it the right structure for event sourcing.

The id is <milliseconds>-<sequence>: the millisecond the entry was added, then a sequence number that breaks ties within the same millisecond. A later append always carries a larger id than an earlier one, so the id orders the log. The * asks the server to mint the id; an explicit id may be supplied instead, and must exceed the current top of the stream or XADD rejects it.

Event sourcing records each change as its own entry. An order placed, an order filled, an order cancelled — three events, appended in the order they happened, never collapsed into one mutable "order" row. The log holds the whole history; the present is whatever the history adds up to. Because nothing is overwritten, the log is an audit trail for free: it says not only what is true now but how it came to be true.

A stream's entry id must strictly exceed its current top. XADD with an explicit id at or below the top is refused — the server answers an error rather than break the order. This is the property that keeps the log a log: the id sequence is strictly increasing, so reading by id reads in append order. A writer that mints ids in order never trips it; one that does trips it loudly, and the error is not swallowed.

EchoMQ.Stream is the writer of EchoMQ's Stream Tier. EchoMQ.Stream.append(conn, queue, name, fields) does the append in four steps, in real code in echo/apps/echo_mq: it mints an EVT-branded record id host-side, derives the explicit XADD id from it, issues XADD emq:{q}:stream:<name> <xadd_id> id <branded> <fields…>, and returns {:ok, branded} — the branded id is the receipt.

The writer owns the mint, so there is nothing to spoof — the id comes from the host, not a client. The 14-byte branded id is stored as the stream entry's id field, so a reader in any language gets the canonical id without re-encoding it. The append returns {:ok, branded}; a Valkey rejection of an id at or below the top is mapped to {:error, :nonmonotonic} — never swallowed, never retried with *, because that error is the wire telling the truth that an upstream mint-order violation happened.

The order theorem holds because a single writer per stream mints strictly increasing snowflakes: the next id always exceeds the stream top, so no XADD rejection is possible. Stream order equals id sort equals mint order — the log is ordered by append the way the property store is ordered by mint.