Consuming Server-Sent Events (SSE) with Indy TIdHTTP and TIdEventStream

A new Indy HTTP client / JAX-RS server example is now available on GitHub. The server side generates Server-sent events. Server-sent events (SSE) is a technology enabling a browser to receive automatic updates from a server via HTTP connection.

The example code uses TIdHTTP and TIdEventStream to connect to the server, and writes the incoming events to the console window.

Requirements

• Delphi or Lazarus IDE
• Indy 10.6.2
• Java IDE
• Java EE 8 application server

Client

The SSE client setup in the TIndySSEClient.Create creates a TIdHTTP instance and a TIdEventStream instance.
As recommended, the Accept header is set to 'text/event-stream'. It also sets the Cache-Control header to 'no-store' to prevent proxy servers from caching the result.
Our event handler TIndySSEClient.MyOnWrite is assigned to the TIdEventStreamOnWrite property.

constructor TIndySSEClient.Create;
begin
  inherited Create;

  SSE_URL := URL;

  EventStream := TIdEventStream.Create;
  EventStream.OnWrite := MyOnWrite;

  IdHTTP := TIdHTTP.Create;
  IdHTTP.Request.Accept := 'text/event-stream';
  IdHTTP.Request.CacheControl := 'no-store';
end;

The OnWrite handler decodes the UTF-8 encoded event data and writes it to the console:

procedure TIndySSEClient.MyOnWrite;
begin
  WriteLn('Received ' + IntToStr(Length(ABuffer)) + ' bytes');
  WriteLn;
  WriteLn(IndyTextEncoding_UTF8.GetString(ABuffer));
  ...
end;

Server (main REST method)

The server generates the server-sent event and places a Stock instance in its data part:

  @GET
  @Path("prices")
  @Produces(MediaType.SERVER_SENT_EVENTS)
  public void getStockPrices(@Context SseEventSink sseEventSink) {
    int lastEventId = 1;

    while (running) {
      Stock stock = getNextTransaction();

      System.out.println("Send event ...");
      OutboundSseEvent sseEvent = this.eventBuilder
              .name("stock")
              .id(String.valueOf(lastEventId))
              .mediaType(MediaType.APPLICATION_JSON_TYPE)
              .data(Stock.class, stock)
              .reconnectDelay(3000)
              .comment("price change")
              .build();

      sseEventSink.send(sseEvent);
      lastEventId++;
    }
    sseEventSink.close();
  }

Additional notes

• this is the first draft of a SSE client, which does not support features such as reconnect
• the example assumes that every call of MyOnWrite contains exactly one event, teminated by a sequence of two line separators (cr lf / cr / lf) so that no extra code is required to do proper event stream parsing

Full source code

https://github.com/michaelJustin/indy-sse-jaxrs

References

• https://www.indyproject.org/2016/01/10/new-tidhttp-flags-and-onchunkreceived-event/
• https://html.spec.whatwg.org/multipage/server-sent-events.html
• https://www.html5rocks.com/en/tutorials/eventsource/basics/
• https://www.baeldung.com/java-ee-jax-rs-sse
• https://www.jcp.org/en/jsr/detail?id=370