The current shutdown logic of the server shuts down the io threads first, followed by acceptor and finally processor threads. The shutdown API of io threads enqueues a special AllDone command into the common request queue. It shuts down the io thread when it dequeues this special all done command. What can happen is that while this shutdown command processing is happening on the io threads, the network/processor threads can still accept new connections and requests and will add those new requests to the request queue. That means, more requests can be enqueued after the AllDone command. What happens is that after the io threads have shutdown, there is no thread available to dequeue from the request queue. So the processor threads can hang while adding new requests to a full request queue, thereby blocking the server from shutting down.