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
Enterprise Architecture Solutions Ltd
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