protected Deque<PooledConnection> getPoolForEventLoop(EventLoop eventLoop) {
        // We don't want to block under any circumstances, so can't use CHM.computeIfAbsent().
        // Instead we accept the slight inefficiency of an unnecessary instantiation of a ConcurrentLinkedDeque.

        Deque<PooledConnection> pool = connectionsPerEventLoop.get(eventLoop);
        if (pool == null) {
            pool = new ConcurrentLinkedDeque<>();
            connectionsPerEventLoop.putIfAbsent(eventLoop, pool);
        }
        return pool;
    }
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