How to return Server-Sent Events

Problem

Your client uses the EventSource API (or any RFC 8895 compatible consumer) and expects text/event-stream framing.

Solution

events(_Req) ->
    livery_resp:sse(200, fun(Emit) ->
        Emit(#{event => <<"tick">>, data => <<"1">>}),
        Emit(#{event => <<"tick">>, data => <<"2">>}),
        ok
    end).

livery_resp:sse/2 sets content-type: text/event-stream and cache-control: no-cache. The producer fun runs in the per-request process and drives Emit with one event at a time.

Event shape

Emit accepts:

A map #{event, id, retry, data} where any field is optional except data. Livery formats it into the standard SSE frame.
Plain iodata(), framed as a data: line only.

Emit(#{event => <<"ping">>, data => <<"pong">>}).
%% event: ping
%% data: pong
%%
%% (blank line terminates the frame)

Emit(#{event => <<"update">>, id => <<"42">>,
       retry => 1500, data => <<"v">>}).
%% event: update
%% id: 42
%% retry: 1500
%% data: v
%%

Emit(<<"plain">>).
%% data: plain
%%

Multi-line data is fine: pass an iolist whose bytes contain \n and Livery emits each line with its own data: prefix when you use the helper. For multi-line, use iolist of lines:

Emit(#{data => [<<"line 1">>, <<"\nline 2">>]}).

Heartbeats

To keep idle connections alive through proxies, emit a comment line periodically (a line beginning with :):

loop(Emit) ->
    receive
        {event, E} -> Emit(E), loop(Emit)
    after 15_000 ->
        Emit(<<":heartbeat">>),
        loop(Emit)
    end.

Disconnect detection

Emit returns {error, closed} once the client disconnects (the H1/H2/H3 adapters surface this; the test adapter always returns ok).

loop(Emit) ->
    receive {event, E} ->
        case Emit(E) of
            ok          -> loop(Emit);
            {error, _}  -> ok
        end
    end.