Reconnecting

A client such as EventSource reconnects when a stream ends or its connection drops: it waits the retry time the stream set, and sends the id of the last event it got as the Last-Event-ID header. A mock serves that reconnection like any request: resume the stream where it was, answer with another mock, or tell the client to stop.

Go

mockhttp.OK().EventStream(mockhttp.NewEventStream(
	mockhttp.NewEvent().ID("1").Data("a"),
	mockhttp.NewEvent().ID("2").Data("b"),
	mockhttp.NewEvent().ID("3").Data("c"),
).Retry(3 * time.Second).ResumeFromLastEventID())

Mock file

response:
  status: 200
  event_stream:
    retry: 3s     # sent first: how long clients wait before they reconnect
    resume: true  # a Last-Event-ID gets only the events after that id
    events:
      - {id: "1", data: a}
      - {id: "2", data: b}
      - {id: "3", data: c}

Retry

EventStream.Retry(d), retry beside events, sends a retry field before the first event: how long the client waits before it reconnects. An event's own retry changes it from that event on. Clients read it in whole milliseconds, so 1.5s goes as retry: 1500.

Resuming

With ResumeFromLastEventID(), resume: true, a request with a Last-Event-ID header gets only the events after the one with that id: above, Last-Event-ID: 1 gets b and c. An id no event has gets them all, and so does a request without the header. The stream's retry is sent either way.

Only ids set with ID, or id in a mock file, count: an id line inside a raw event is sent, but resuming doesn't know it.

Answering a reconnection with another mock

Match the Last-Event-ID header to give a reconnection another answer, such as the rest of a stream, or a different one:

Go

m.MustMock(mockhttp.Getf("/jobs/7/progress").
	Header(mockhttp.HeaderLastEventID, match.Equal("3")).
	Reply(mockhttp.NoContent()))

Mock file

request:
  method: GET
  path: /jobs/7/progress
  header: {last-event-id: "3"}
response:
  status: 204

Telling clients to stop

A stream that ends makes EventSource reconnect, even when the server is done. Answer that reconnection with 204 No Content, a reply without event_stream, as above, and the client stops.

An answer that isn't a stream also stops it: a status other than 200 after redirects, or a Content-Type other than text/event-stream. A mock that answers 401 to a request without credentials, before any stream starts, makes EventSource give up rather than retry.

Testing a reconnection

Cut the first stream with fault.LimitData, so the connection drops after the events a client should keep, and resume the stream on the reconnection:

Go

stream := mockhttp.NewEventStream(
	mockhttp.NewEvent().ID("1").Data("a"),
	mockhttp.NewEvent().ID("2").Data("b"),
	mockhttp.NewEvent().ID("3").Data("c"),
).Retry(1500 * time.Millisecond).ResumeFromLastEventID()

// The first connection drops after the retry field and event 1.
m.MustMock(mockhttp.Getf("/events").Once().
	Pipe(fault.LimitData(len("retry: 1500\n\nid: 1\ndata: a\n\n"))).
	Reply(mockhttp.OK().EventStream(stream)))
// The reconnection gets events 2 and 3: nothing is lost, nothing sent twice.
m.MustMock(mockhttp.Getf("/events").
	Header(mockhttp.HeaderLastEventID, match.Equal("1")).
	Reply(mockhttp.OK().EventStream(stream)))

drop.yaml

# The first connection drops after the retry field and event 1.
request: {method: GET, path: /events}
repeat: 1
pipes: [{type: limit_data, bytes: 28}]
response:
  status: 200
  event_stream:
    retry: 1500ms
    resume: true
    events:
      - {id: "1", data: a}
      - {id: "2", data: b}
      - {id: "3", data: c}

resume.yaml

# The reconnection gets events 2 and 3.
request:
  method: GET
  path: /events
  header: {last-event-id: "1"}
response:
  status: 200
  event_stream:
    retry: 1500ms
    resume: true
    events:
      - {id: "1", data: a}
      - {id: "2", data: b}
      - {id: "3", data: c}

The client must reconnect after the drop with Last-Event-ID: 1, and end with a, b and c, once each.