Throttling in Camel - apache-camel

Question

I am trying to figure out throttling concepts in Camel. I have already seen Camel's route policy, but this works for number of inflight exchanges.
My route is following:
routeBuilders.add(new RouteBuilder() {
#Override
public void configure() throws Exception {
from("rabbitmq://127.0.0.1:5672/abc?queue=a&username=XXX&password=XXX&autoAck=false&durable=true&exchangeType=direct&autoDelete=false")
.to("rabbitmq://127.0.0.1:5672/abc?queue=b&username=XXX&password=XXX&autoAck=false&durable=true&exchangeType=direct&autoDelete=false");
}
});
Now my use case is that I want to transfer say 2000 messages between these routes, and I know that it can be done via .throttle(2000). But I am stuck at the point where I have to decide that how would I control that when the next 2000 messages should be routed. I want to route next 2000 messages only when the receiver queue becomes empty.
For example, messages are getting routed from queue a to b. Say 2k messages have been routed successfully, now I want to suspend my route so that it won't transfer more messages until the queue b becomes empty (assume that there is a consumer which is pulling messages from queue b)
Any help/direction on this is appreciated.

Answer 1

You can use a route policy for that. The you implement logic in that route policy to suspend/resume the route to throttle the route.
We have a out of the box policy that is using the number of inflight messages in Camel as its metrics. But you should add logic to check that queue if its empty or not.
The documentation for route policy is here
http://camel.apache.org/routepolicy
And the code for the inflight throttler
https://github.com/apache/camel/blob/master/camel-core/src/main/java/org/apache/camel/impl/ThrottlingInflightRoutePolicy.java