Using Server-Side-Events (SSE) with Camel acting as a reverse-proxy

[David Kumar <da...@e-asolutions.com>]

I want to be able to use Server-Side-Events (SSE) in a web service to send a stream of messages to the browser and I have a Camel application acting as a reverse-proxy between them. However the Camel route is buffering the messages sent using SSE and messages are not delivered to the browser until the request is completed. My question is: which Camel components support SSE and what configuration is required to achieve streaming of messages using SSE.

Here are the details of what I did. I have configured Camel as a reverse-proxy between a browser and a web service, using the bridgeEndpoint setting in the http component. The route works as expected and HTTP requests are served back to the browser via Camel. Here is my route:

from("servlet:app//"
    + "?matchOnUriPrefix=true")
.to("http://{{ctg.edm.base-url}}"
    + "?bridgeEndpoint=true"
    + "&preserveHostHeader=true"
    + "&throwExceptionOnFailure=false");


As a test, I have implemented SSE in the web service using Spring's SseEmitter class. When a request is made to a /notifications/subscribe end-point, the service will send 4 messages at 5 second intervals using the SseEmitter send() method and then we explicitly close the connection using the SseEmitter complete() method.

If I configure the browser to make the request directly to the web service, it works as expected and the browser receives messages at 5 second intervals. If I direct the browser request via the Camel route I have configured to be the reverse-proxy, I receive all the messages at the same time and only when the request has been completed by the server. How do I configure the Camel route to stream the messages to the browser instead of buffering them?

I'm using Camel 3.6.0 in a Spring Boot application to implement the reverse-proxy.

Here are the response headers for one of the messages, as logged by my Camel application. You can see that the headers are set correctly for SSE, i.e. Content-Type: text/event-stream, Transfer-Encoding: chunked, Cache-Control: no-cache, Connection: keep-alive. The Camel logs are showing the messages being received by the route at 5 second intervals but they are being buffered by Camel until after the response has been closed.
2020-10-22 11:20:32.825 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "HTTP/1.1 200 [\r][\n]"
2020-10-22 11:20:32.825 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Pragma: no-cache[\r][\n]"
2020-10-22 11:20:32.825 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Expires: Thu, 01 Jan 1970 00:00:00 GMT[\r][\n]"
2020-10-22 11:20:32.825 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Cache-Control: no-cache[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Cache-Control: no-store[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "X-XSS-Protection: 1; mode=block[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "X-Frame-Options: SAMEORIGIN[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "X-Content-Type-Options: nosniff[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Content-Type: text/event-stream;charset=UTF-8[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Transfer-Encoding: chunked[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Date: Thu, 22 Oct 2020 11:20:32 GMT[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Keep-Alive: timeout=20[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "Connection: keep-alive[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "5[\r][\n]"
2020-10-22 11:20:32.826 DEBUG 21995 --- [io-8082-exec-35] org.apache.http.wire                     : http-outgoing-33 << "data:[\r][\n]"

Many thanks for your help,
David



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