org.apache.zookeeper.server.quorum
Class QuorumCnxManager

java.lang.Object
  org.apache.zookeeper.server.quorum.QuorumCnxManager

public class QuorumCnxManager
extends Object

This class implements a connection manager for leader election using TCP. It maintains one connection for every pair of servers. The tricky part is to guarantee that there is exactly one connection for every pair of servers that are operating correctly and that can communicate over the network. If two servers try to start a connection concurrently, then the connection manager uses a very simple tie-breaking mechanism to decide which connection to drop based on the IP addressed of the two parties. For every peer, the manager maintains a queue of messages to send. If the connection to any particular peer drops, then the sender thread puts the message back on the list. As this implementation currently uses a queue implementation to maintain messages to send to another peer, we add the message to the tail of the queue, thus changing the order of messages. Although this is not a problem for the leader election, it could be a problem when consolidating peer communication. This is to be verified, though.

Nested Class Summary

class	QuorumCnxManager.Listener Thread to listen on some port
static class	QuorumCnxManager.Message

Field Summary

QuorumCnxManager.Listener	listener
ArrayBlockingQueue<QuorumCnxManager.Message>	recvQueue

Constructor Summary

QuorumCnxManager(QuorumPeer self)

Method Summary

void	addToRecvQueue(QuorumCnxManager.Message msg) Inserts an element in the recvQueue.
void	connectAll() Try to establish a connection with each server if one doesn't exist.
QuorumPeer	getQuorumPeer() Return reference to QuorumPeer
long	getThreadCount() Return number of worker threads
void	halt() Flag that it is time to wrap up all activities and interrupt the listener.
boolean	initiateConnection(Socket sock, Long sid) If this server has initiated the connection, then it gives up on the connection if it loses challenge.
QuorumCnxManager.Message	pollRecvQueue(long timeout, TimeUnit unit) Retrieves and removes a message at the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
boolean	receiveConnection(Socket sock) If this server receives a connection request, then it gives up on the new connection if it wins.
void	softHalt() A soft halt simply finishes workers.
void	testInitiateConnection(long sid) Invokes initiateConnection for testing purposes
void	toSend(Long sid, ByteBuffer b) Processes invoke this message to queue a message to send.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

recvQueue

public final ArrayBlockingQueue<QuorumCnxManager.Message> recvQueue

listener

public final QuorumCnxManager.Listener listener

Constructor Detail

QuorumCnxManager

public QuorumCnxManager(QuorumPeer self)

Method Detail

testInitiateConnection

public void testInitiateConnection(long sid)
                            throws Exception

Invokes initiateConnection for testing purposes

Parameters:

sid -

Throws:

Exception

initiateConnection

public boolean initiateConnection(Socket sock,
                                  Long sid)

If this server has initiated the connection, then it gives up on the connection if it loses challenge. Otherwise, it keeps the connection.

receiveConnection

public boolean receiveConnection(Socket sock)

If this server receives a connection request, then it gives up on the new connection if it wins. Notice that it checks whether it has a connection to this server already or not. If it does, then it sends the smallest possible long value to lose the challenge.

toSend

public void toSend(Long sid,
                   ByteBuffer b)

Processes invoke this message to queue a message to send. Currently, only leader election uses it.

connectAll

public void connectAll()

Try to establish a connection with each server if one doesn't exist.

halt

public void halt()

Flag that it is time to wrap up all activities and interrupt the listener.

softHalt

public void softHalt()

A soft halt simply finishes workers.

getThreadCount

public long getThreadCount()

Return number of worker threads

getQuorumPeer

public QuorumPeer getQuorumPeer()

Return reference to QuorumPeer

addToRecvQueue

public void addToRecvQueue(QuorumCnxManager.Message msg)

Inserts an element in the recvQueue. If the Queue is full, this methods removes an element from the head of the Queue and then inserts the element at the tail of the queue. This method is synchronized to achieve fairness between two threads that are trying to insert an element in the queue. Each thread checks if the queue is full, then removes the element at the head of the queue, and then inserts an element at the tail. This three-step process is done to prevent a thread from blocking while inserting an element in the queue. If we do not synchronize the call to this method, then a thread can grab a slot in the queue created by the second thread. This can cause the call to insert by the second thread to fail. Note that synchronizing this method does not block another thread from polling the queue since that synchronization is provided by the queue itself.

Parameters:

msg - Reference to the message to be inserted in the queue

pollRecvQueue

public QuorumCnxManager.Message pollRecvQueue(long timeout,
                                              TimeUnit unit)
                                       throws InterruptedException

Retrieves and removes a message at the head of this queue, waiting up to the specified wait time if necessary for an element to become available. ArrayBlockingQueue.poll(long, java.util.concurrent.TimeUnit)

Throws:

InterruptedException