How to convert a vert.x ReactiveReadStream<Document> to ReactiveWriteStream<Buffer> - spring

Question

I have a straightforward use case. This is to make a rest call, query mongo and then return an arbitrarily large stream of data back to the client, all with reactive streams type back pressure management.
This was quite easy to achieve using Spring WebFlux and Reactor. I am now trying to achieve the same goal using vert.x, as a comparison of ease of implementation.
Having found the vert.x mongo client to be lacking any support for managing back pressure, I am now attempting to use the WebFlux mongo client and then pump the data back through the vert.x HttpResponse, as shown in the following code:
public class MyMongoVerticle extends AbstractVerticle {
ReactiveMongoOperations operations;
public void start() throws Exception {
final Router router = Router.router(vertx);
router.route().handler(BodyHandler.create());
router.get("/myUrl").handler(ctx -> {
// WebFlux mongo operations returns a ReactiveStreams compatible entity
Flux<Document> mongoStream = operations.findAll(Document.class, "myCollection");
ReactiveReadStream rrs = ReactiveReadStream.readStream();
// rrs is ReactiveStream streams subscriber
mongoStream.subscribe(rrs);
// Pump pumps the rrs (ReactiveReadStream) to the HttpServerResponse (ReactiveWriteStream)
Pump pump = Pump.pump(rrs, ctx.response());
pump.start();
});
vertx.createHttpServer().requestHandler(router::accept).listen(8777);
}
}
The issue I have encountered is that the HttpServerResponse implements ReactiveWriteStream<Buffer> so is expecting a Buffer rather than a stream of Document's. The result is a ClassCaseException.
The question I have is how can I convert this stream of Documents into a into a ReactiveWriteStream<Buffer>? There may be another better way to do this, so I'm open to other suggestions on how to achieve this.

Answer 1

Pump won't work for you, as it doesn't support transformations currently. You'll have to implement pump by yourself. Luckily, this shouldn't be too hard:
Flux<Document> mongoStream = operations.findAll(Document.class, "myCollection");
ReactiveReadStream<Document> rrs = ReactiveReadStream.readStream();
mongoStream.subscribe(rrs);
HttpServerResponse outStream = ctx.response();
// Changes start here
rrs.handler(d -> {
if (outStream.writeQueueFull()) {
outStream.drainHandler((s) -> {
rrs.resume();
});
rrs.pause();
}
else {
outStream.write(d.toJson());
}
}).endHandler(h -> {
outStream.end();
});
Note that I wouldn't expect this to be more effective than "native" WebFlux implementation.
Also, JSON in this example will be mangled, as I don't wrap it in proper JSON Array