Vertx event bus slow consuming issue - vert.x

Question

We have a non clustered vertx application, and we use the event bus to internally communicate between verticles.
Verticle A consumes from the bus, performs a HTTP request, and sends the response back through the bus.
Verticle B just request to perform that HTTP request.
The problem appears when a "high" request volume is performed by Verticle B. Then, the consumer starts receiving the events slower and slower (presumably because they are getting queued in the event bus). For 8 requests/second the bus takes up to 3-4 seconds to consume the event. When the requests/second are elevated, it can take more than 30 seconds to consume it, so the bus timeout is triggered.
The thing is, Verticle A is really fast performing the HTTP operation (~200ms) so I don't really understand why the requests get stuck in the bus.
We've tried many solutions but none ot then worked:
Deploy multiple instances of Verticle A as workers
Use vertx.executeBlocking() to perform the HTTP request
The only thing that worked was commenting the HTTP request and returning a mock object through the bus. But again, the HTTP request doesn't take more than 200ms, so it shouldn't be blocking the bus.
Additional information: We use an autogenerated rest client that uses Retrofit + OkHttpClient. Due to company policy, we cannot use Vertx WebClient, so I didn't try this solution.
EXAMPLE
This is a really simplified version of our code so you can check if I'm missing something.
VERTICLE A
// Instantiated in Verticle A
public class EmailSender {
private final Vertx vertx;
private final EmailApiClient emailApiClient;
public EmailSender(Vertx vertx) {
this.vertx = vertx;
emailApiClient = ClientFactory.createEmailApiClient();
}
public void start() {
vertx.eventBus().consumer("sendEmail", this::sendEmail);
}
public void sendEmail(Message<EmailRequest> message) {
EmailRequest emailRequest = message.body();
emailApiClient.sendEmail(emailRequest).subscribe(
response -> {
if (response.code() == 200) {
EmailResponse emailResponse = response.body();
message.reply(emailResponse);
} else {
message.fail(500, "Error sending email");
}
});
}
}
VERTICLE B
// Instantiated in Verticle B
public class EmailCommunications {
private final Vertx vertx;
public EmailCommunications(Vertx vertx) {
this.vertx = vertx;
}
public Single<EmailResponse> sendEmail(EmailRequest emailRequest) {
SingleSubject<EmailResponse> emailSent = SingleSubject.create();
vertx.eventBus().request(
"sendEmail",
emailRequest,
busResult -> {
if (busResult.succeded()) {
emailSent.onSuccess(busResult.result().body())
} else {
emailSent.onError(busResult.cause())
}
}
);
return emailSent;
}
}

Answer 1

We fixed the issue changing our OkHttpClient configuration so HTTP requests won't get stuck
default void configureOkHttpClient(OkHttpClient.Builder okHttpClientBuilder) {
ConnectionPool connectionPool = new ConnectionPool(40, 5, TimeUnit.MINUTES);
Dispatcher dispatcher = new Dispatcher();
dispatcher.setMaxRequestsPerHost(200);
dispatcher.setMaxRequests(200);
okHttpClientBuilder
.readTimeout(60, TimeUnit.SECONDS)
.retryOnConnectionFailure(true)
.connectionPool(connectionPool)
.dispatcher(dispatcher);
}