public boolean release() {
        if (released) {
            return true;
        }

        if (isActive()) {
            if (connectionState != ConnectionState.WRITE_READY) {
                closeWrtBusyConnCounter.increment();
            }
        }

        if (!isShouldClose() && connectionState != ConnectionState.WRITE_READY) {
            CurrentPassport passport = CurrentPassport.fromChannel(channel);
            LOG.info(
                    "Error - Attempt to put busy connection into the pool = {}, {}",
                    this.toString(),
                    String.valueOf(passport));
            this.shouldClose = true;
        }

        // reset the connectionState
        connectionState = ConnectionState.WRITE_READY;
        released = true;
        return channelManager.release(this);
    }
Can not be put in pool
zuul-core/src/main/java/com/netflix/zuul/netty/connectionpool/PooledConnection.java