Per-invocation customization of the underlying transport (HTTP) while still using ServiceCall abstraction

[artem-v]

Subj.

Current problems:

• HTTP POST is hardcoded in HttpGatewayClientTransport.
• Headers/query params are controlled globally by the transport, not per invocation.

Goals:

• per-invocation customization of the underlying transport (HTTP) while still using ServiceCall abstraction, without breaking the correlation/encoding/decoding machinery.
• Pass metadata inside ServiceMessage.headers.
• real RESTful semantics (GET/POST/PUT) + per-call HTTP headers/query params while keeping ServiceCall architecture intact.

Something like:

serviceCall
  .requestOne(
     ServiceMessage.builder()
        .header("http.method", "GET")
        .header("http.header.X-Custom", "abc")
        .header("http.query", "limit=10")
        .data(payload)
        .build()
  );

[artem-v]

Client:

    @Test
    void testAttributesPropagation() {
      StepVerifier.create(
              serviceCall.requestOne(
                  ServiceMessage.builder()
                      .header("http.method", "GET")
                      .header("http.header.X-String-Header", "abc")
                      .header("http.header.X-Int-Header", "123456789")
                      .header("http.query.debug", "true")
                      .header("http.query.x", "1")
                      .header("http.query.y", "2")
                      .qualifier("v1/restService/propagate/123/bar456/baz789")
                      .build(),
                  SomeResponse.class))
          .assertNext(
              message -> {
                final var someResponse = message.<SomeResponse>data();
                assertNotNull(someResponse, "data");
                assertNotNull(someResponse.name(), "someResponse.name");
              })
          .verifyComplete();
    }
  }

Backend:

@Override
public Mono<SomeResponse> propagateRequestAttributes() {
  return RequestContext.deferContextual()
      .map(
          context -> {
            final var pathParams = context.pathParams();
            assertEquals(123, pathParams.intValue("foo"), "foo");
            assertEquals("bar456", pathParams.stringValue("bar"), "bar");
            assertEquals("baz789", pathParams.stringValue("baz"), "baz");

            final var headers = context.headers();
            assertEquals("GET", headers.get("http.method"));
            assertEquals("abc", headers.get("http.header.X-String-Header"));
            assertEquals("123456789", headers.get("http.header.X-Int-Header"));
            assertEquals("true", headers.get("http.query.debug"));
            assertEquals("1", headers.get("http.query.x"));
            assertEquals("2", headers.get("http.query.y"));

            final var httpHeaders = context.headerParams("http.header");
            assertEquals("abc", httpHeaders.stringValue("X-String-Header"));
            assertEquals(123456789, httpHeaders.intValue("X-Int-Header"));

            final var queryParams = context.headerParams("http.query");
            assertEquals(true, queryParams.booleanValue("debug"));
            assertEquals(1, queryParams.intValue("x"));
            assertEquals(2, queryParams.intValue("y"));

            return new SomeResponse().name(UUID.randomUUID().toString());
          });
}