@Override
    public void drain() {
        if (draining) {
            throw new IllegalStateException("Already draining");
        }

        draining = true;
        connectionsPerEventLoop.forEach((eventLoop, v) -> drainIdleConnectionsOnEventLoop(eventLoop));
    }
function to run when a connection is acquired before returning it to caller.
zuul-core/src/main/java/com/netflix/zuul/netty/connectionpool/PerServerConnectionPool.java