SSE

Info

SSE connections are inherently limited across browser tabs on the same domain.
KEML does everything it can to minimize them while remaining spec compliant.
But you are still advised to limit them as much as possible.
Reuse the same endpoint for SSE communication and only change the event type.

SSE, or Server-sent Events, is a method of sending results from the server without first being prompted by a request. The server decides entirely on its own when and what to send.

Think of it as a lighter and simpler alternative to WebSockets that can only send data in one direction — from server to client. And given that we have always been able to send data from client to server using normal requests, this already satisfies the need for full bi-directional communication.

Polling also works, but it is like spamming your server with constant "Are we there yet?" queries. You have to balance between sending them too often and getting a "No!" every time, or sending them too infrequently and accepting that you might be displaying stale data sometimes.

  • the attribute takes an event type sent by the server as a value, and if omitted, defaults to message
  • endpoint resolution and override rules are the same as with on
  • the HTTP method is always GET and cannot be overridden
  • the credentials attribute behaves the same
  • result actions and events behave the same, except there is no concept of an invalid status code
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<h4 class="mt0">Inception clock</h4>

<p>Counting time elapsed since the initial KEML release.</p>

<p>Fully controlled and updated by the server. No polling.</p>

<div
  class="clock"
  sse="inception-clock"
  src="/sse"
  result="sseResult"
  render="sseResult"
></div>

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{{
  const segment = ([value, label]) =>
    "<span>" + value + "</span><span>" + label + "</span>";

  const clock = () => server.timeSince(1716062717000).map(segment).join("");

  const tick = () => server.dispatchEvent("inception-clock", clock());

  server.addIntervalId(setInterval(tick, 1000));
}}

Inception clock

Counting time elapsed since the initial KEML release.

Fully controlled and updated by the server. No polling.