# List of Protocols

This section is work in progress; we strive to update the documentation as we make changes to the code.

The most important properties are described on the wiki. The idea is that users take one of the predefined configurations (shipped with JGroups) and make only minor changes to it.

For each protocol define:

- Properties provided

- Required services

- Provided services

- Behavior

## Transport

### UDP

${UDP}

### TCP

${TCP}

### TUNNEL

${TUNNEL}

## Initial membership discovery

The task of the discovery is to find an initial membership, which is used to determine the current coordinator. Once a coordinator is found, the joiner sends a JOIN request to the coord.

### PING

${PING}

### FILE_PING

This uses a shared directory into which all members write their addresses. New joiners read all addresses from this directory (which needs to be shared, e.g. via NFS or SMB) and ping each of the elements of the resulting set of members. When a member leaves, it deletes its corresponding file.

FILE_PING can be used instead of GossipRouter in cases where no external process is desired.

${PING}

### JDBC_PING

This uses a shared Database into which all members write their addresses. New joiners read all addresses from this Database and ping each of the elements of the resulting set of members. When a member leaves, it deletes its corresponding record.

JDBC_PING is an alternative to S3_PING by using Amazon RDS instead of S3.

${JDBC_PING}

### TCPPING

${TCPPING}

### TCPGOSSIP

${TCPGOSSIP}

### MPING

${MPING}

### BPING

BPING uses UDP broadcasts to discover other nodes. The default broadcast address (dest) is 255.255.255.255, and should be replaced with a subnet specific broadcast, e.g. 192.168.1.255.

${BPING}

### S3_PING

This uses an Amazon S3 bucket into which all members write their addresses. New joiners read all addresses from this bucket and ping each of the elements of the resulting set of members. When a member leaves, it deletes its corresponding file.

S3_PING is primarily meant to be used on Amazon EC2 where multicast traffic is not allowed and no external process (GossipRouter) is desired. When Amazon RDS is preferred over S3, or if a shared database is used, an alternative is to use JDBC_PING.

${S3_PING}

## Merging after a network partition

### MERGE2

${MERGE2}

## Failure Detection

The task of failure detection is to probe members of a group and see whether they are alive. When a member is suspected (= deemed dead), then a SUSPECT message is sent to all nodes of the cluster. It is not the task of the failure detection layer to exclude a crashed member (this is done by the group membership protocol, GMS), but simply to notify everyone that a node in the cluster is suspected of having crashed.

### FD

Failure detection based on heartbeat messages. If reply is not received without timeout ms, max_tries times, a member is declared suspected, and will be excluded by GMS

Each member send a message containing a "FD" - HEARTBEAT header to its neighbor to the right (identified by the ping_dest address). The heartbeats are sent by the inner class Monitor. When the neighbor receives the HEARTBEAT, it replies with a message containing a "FD" - HEARTBEAT_ACK header. The first member watches for "FD" - HEARTBEAT_ACK replies from its neigbor. For each received reply, it resets the last_ack timestamp (sets it to current time) and num_tries counter (sets it to 0). The same Monitor instance that sends heartbeats whatches the difference between current time and last_ack. If this difference grows over timeout, the Monitor cycles several more times (until max_tries) is reached) and then sends a SUSPECT message for the neighbor's address. The SUSPECT message is sent down the stack, is addressed to all members, and is as a regular message with a FdHeader.SUSPECT header.

${FD}

### FD_ALL

Failure detection based on simple heartbeat protocol. Every member periodically multicasts a heartbeat. Every member also maintains a table of all members (minus itself). When data or a heartbeat from P are received, we reset the timestamp for P to the current time. Periodically, we check for expired members, and suspect those.

Example: <FD_ALL interval="3000" timeout="10000"/>

In the example above, we send a heartbeat every 3 seconds and suspect members if we haven't received a heartbeat (or traffic) for more than 10 seconds. Note that since we check the timestamps every 'interval' milliseconds, we will suspect a member after roughly 4 * 3s == 12 seconds. If we set the timeout to 8500, then we would suspect a member after 3 * 3 secs == 9 seconds.

${FD_ALL}

### FD_SIMPLE

### FD_PING

FD_PING uses a script or command that is run with 1 argument (the host to be pinged) and needs to return 0 (success) or 1 (failure). The default command is /sbin/ping (ping.exe on Windows), but this is user configurable and can be replaced with any user-provided script or executable. ${FD_PING}

### FD_ICMP

Uses InetAddress.isReachable() to determine whether a host is up or not. Note that this is only available in JDK 5, so reflection is used to determine whether InetAddress provides such a method. If not, an exception will be thrown at protocol initialization time.

The problem with InetAddress.isReachable() is that it may or may not use ICMP in its implementation ! For example, an implementation might try to establish a TCP connection to port 9 (echo service), and - if the echo service is not running - the host would be suspected, although a real ICMP packet would not have suspected the host ! Please check your JDK/OS combo before running this protocol.

Properties

Name | Description
--- | ---
bind_addr | The network interface to be used for sending ICMP packets, e.g. `bind_addr="192.16.8.0.2"`

### FD_SOCK

Failure detection protocol based on a ring of TCP sockets created between group members. Each member in a group connects to its neighbor (last member connects to first) thus forming a ring. Member B is suspected when its neighbor A detects abnormally closed TCP socket (presumably due to a node B crash). However, if a member B is about to leave gracefully, it lets its neighbor A know, so that it does not become suspected.

If you are using a multi NIC machine note that JGroups versions prior to 2.2.8 have FD_SOCK implementation that does not assume this possibility. Therefore JVM can possibly select NIC unreachable to its neighbor and setup FD_SOCK server socket on it. Neighbor would be unable to connect to that server socket thus resulting in immediate suspecting of a member. Suspected member is kicked out of the group, tries to rejoin, and thus goes into join/leave loop. JGroups version 2.2.8 introduces srv_sock_bind_addr property so you can specify network interface where FD_SOCK TCP server socket should be bound. This network interface is most likely the same interface used for other JGroups traffic. JGroups versions 2.2.9 and newer consult bind.address system property or you can specify network interface directly as FD_SOCK bind_addr property.

${FD_SOCK}

### VERIFY_SUSPECT

${VERIFY_SUSPECT}

## Reliable message transmission

### pbcast.NAKACK

NAKACK provides reliable delivery and FIFO (= First In First Out) properties for messages sent to all nodes in a cluster.

Reliable delivery means that no message sent by a sender will ever be lost, as all messages are numbered with sequence numbers (by sender) and retransmission requests are sent to the sender of a message

Note that NAKACK can also be configured to send retransmission requests for M to anyone in the cluster, rather than only to the sender of M.

if that sequence number is not received.

FIFO order means that all messages from a given sender are received in exactly the order in which they were sent.

${NAKACK}

### UNICAST

UNICAST provides reliable delivery and FIFO (= First In First Out) properties for point-to-point messages between one sender and one receiver.

Reliable delivery means that no message sent by a sender will ever be lost, as all messages are numbered with sequence numbers (by sender) and retransmission requests are sent to the sender of a message

if that sequence number is not received.

FIFO order means that all messages from a given sender are received in exactly the order in which they were sent.

On top of a reliable transport, such as TCP, UNICAST is not really needed. However, concurrent delivery of messages from the same sender is prevented by UNICAST by acquiring a lock on the sender's retransmission table, so unless concurrent delivery is desired, UNICAST should not be removed from the stack even if TCP is used.

${UNICAST}

### UNICAST2

UNICAST2 provides lossless, ordered, communication between 2 members. Contrary to UNICAST, it uses negative acks (similar to NAKACK) rather than positive acks. This reduces the communication overhead required for sending an ack for every message.

${UNICAST2}

## Fragmentation

### FRAG and FRAG2

${FRAG}

## Ordering

### SEQUENCER

SEQUENCER provider total order for multicast (=group) messages by forwarding messages to the current coordinator, which then sends the messages to the cluster on behalf of the original sender. Because it is always the same sender (whose messages are delivered in FIFO order), a global (or total) order is established.

Sending members add every forwarded message M to a buffer and remove M when they receive it. Should the current coordinator crash, all buffered messages are forwarded to the new coordinator.

Note that retransmissions go to the original sender, not to the coordinator.

${SEQUENCER}

## Group Membership

Group membership takes care of joining new members, handling leave requests by existing members, and handling SUSPECT messages for crashed members, as emitted by failure detection protocols. The algorithm for joining a new member is essentially:

                    - loop
                    - find initial members (discovery)
                    - if no responses:
                    - become singleton group and break out of the loop
                    - else:
                    - determine the coordinator (oldest member) from the responses
                    - send JOIN request to coordinator
                    - wait for JOIN response
                    - if JOIN response received:
                    - install view and break out of the loop
                    - else
                    - sleep for 5 seconds and continue the loop

### pbcast.GMS

${GMS}

#### Disabling the initial coordinator

Consider the following situation: a new member wants to join a group. The prodedure to do so is:

- Multicast an (unreliable) discovery request (ping)

- Wait for n responses or m milliseconds (whichever is first)

- Every member responds with the address of the coordinator

- If the initial responses are > 0: determine the coordinator and start the JOIN protocolg

- If the initial response are 0: become coordinator, assuming that no one else is out there

However, the problem is that the initial mcast discovery request might get lost, e.g. when multiple members start at the same time, the outgoing network buffer might overflow, and the mcast packet might get dropped. Nobody receives it and thus the sender will not receive any responses, resulting in an initial membership of 0. This could result in multiple coordinators, and multiple subgroups forming. How can we overcome this problem ? There are 3 solutions:

1. Increase the timeout, or number of responses received. This will only help if the reason of the empty membership was a slow host. If the mcast packet was dropped, this solution won't help

2. Add the MERGE(2) protocol. This doesn't actually prevent multiple initial cordinators, but rectifies the problem by merging different subgroups back into one. Note that this involves state merging which needs to be done by the application.

3. (new) Prevent members from becoming coordinator on initial startup. This solution is applicable when we know which member is going to be the initial coordinator of a fresh group. We don't care about afterwards, then coordinatorship can migrate to another member. In this case, we configure the member that is always supposed to be started first with disable_initial_coord=false (the default) and all other members with disable_initial_coord=true.This works as described below.

When the initial membership is received, and is null, and the property disable_initial_coord is true, then we just continue in the loop and retry receving the initial membership (until it is non-null). If the property is false, we are allowed to become coordinator, and will do so. Note that - if a member is started as first member of a group - but its property is set to true, then it will loop until another member whose disable_initial_coord property is set to false, is started.

## Security

### ENCRYPT

${ENCRYPT}

### AUTH

## State Transfer

### pbcast.STATE_TRANSFER

### pbcast.STREAMING_STATE_TRANSFER

#### Overview

In order to transfer application state to a joining member of a group pbcast.STATE_TRANSFER has to load entire state into memory and send it to a joining member. Major limitation of this approach is that the state transfer that is very large (>1Gb) would likely result in OutOfMemoryException. In order to alleviate this problem a new state transfer methodology, based on a streaming state transfer, was introduced in JGroups 2.4

Streaming state transfer supports both partial and full state transfer.

Streaming state transfer provides an InputStream to a state reader and an OutputStream to a state writer. OutputStream and InputStream abstractions enable state transfer in byte chunks thus resulting in smaller memory requirements. For example, if application state consists a huge DOM tree, whose aggregate size is 2GB (and which has partly been passivated to disk), then the state provider (ie. the coordinator) can simply iterate over the DOM tree (activating the parts which have been passivated out to disk), and write to the OutputStream as it traverses the tree. The state receiver will simply read from the InputStream and reconstruct the tree on its side, possibly again passivating parts to disk.

Rather than having to provide a 2GB byte[] buffer, streaming state transfer transfers the state in chunks of N bytes where N is user configurable.

Prior to 2.6.9 and 2.8 releases streaming state transfer relied exclusively on its own tcp sockets to transfer state between members. The downside of tcp socket approach is that it is not firewall friendly. If use_default_transport property of pbcast.STREAMING_STATE_TRANSFER is set to true streaming state transfer will use normal messages to transfer state. This approach besides being completely transparent to application is also firewall friendly. However, as expected, tcp sockets have better performance.

#### API

Streaming state transfer, just as regular byte based state transfer, can be used in both pull and push mode. Similarly to the current getState and setState methods of org.jgroups.MessageListener, application interested in streaming state transfer in a push mode would implement streaming getState method(s) by sending/writing state through a provided OutputStream reference and setState method(s) by receiving/reading state through a provided InputStream reference. In order to use streaming state transfer in a push mode, existing ExtendedMessageListener has been expanded to include additional four methods:

                            public interface ExtendedMessageListener
                            {

                            /*non-streaming callback methods ommitted for clarity*/

                            /**
                            * Allows an application to write a state through a provided OutputStream.
                            * An application is obligated to always close the given OutputStream reference.
                            *
                            * @param ostream the OutputStream
                            * @see OutputStream#close()
                            */
                            public void getState(OutputStream ostream);

                            /**
                            * Allows an application to write a partial state through a provided OutputStream.
                            * An application is obligated to always close the given OutputStream reference.
                            *
                            * @param state_id id of the partial state requested
                            * @param ostream the OutputStream
                            *
                            * @see OutputStream#close()
                            */
                            public void getState(String state_id, OutputStream ostream);

                            /**
                            * Allows an application to read a state through a provided InputStream.
                            * An application is obligated to always close the given InputStream reference.
                            *
                            * @param istream the InputStream
                            * @see InputStream#close()
                            */
                            public void setState(InputStream istream);

                            /**
                            * Allows an application to read a partial state through a provided InputStream.
                            * An application is obligated to always close the given InputStream reference.
                            *
                            * @param state_id id of the partial state requested
                            * @param istream the InputStream
                            *
                            * @see InputStream#close()
                            */
                            public void setState(String state_id, InputStream istream);

                            }

For a pull mode (when application uses channel.receive() to fetch events) two new event classes will be introduced:

- StreamingGetStateEvent

- StreamingSetStateEvent

These two events/classes are very similar to existing GetStateEvent and SetStateEvent but introduce a new field; StreamingGetStateEvent has an OutputStream and StreamingSetStateEvent has an InputStream.

The following code snippet demonstrates how to pull events from a channel, processing StreamingGetStateEvent and sending hypothetical state through a provided OutputStream reference. Handling of StreamingSetStateEvent is analogous to this example:

    ...
                        Object obj=channel.receive(0);
                        if(obj instanceof StreamingGetStateEvent) {
                        StreamingGetStateEvent evt=(StreamingGetStateEvent)obj;
                        OutputStream oos = null;
                        try {
                        oos = new ObjectOutputStream(evt.getArg());
                        oos.writeObject(state);
                        oos.flush();
                        } catch (Exception e) {}
                        finally{
                        try {
                        oos.close();
                        } catch (IOException e) {
                        System.err.println(e);
                        }
                        }
                        }
                        ...

API that initiates state transfer on a JChannel level has the following methods:

    public boolean getState(Address target,long timeout)throws
                            ChannelNotConnectedException,ChannelClosedException;
                            public boolean getState(Address target,String state_id,long timeout)throws
                            ChannelNotConnectedException,ChannelClosedException;

Introduction of STREAMING_STATE_TRANSFER does not change the current API.

#### Configuration

State transfer type choice is static, implicit and mutually exclusive. JChannel cannot use both STREAMING_STATE_TRANSFER and STATE_TRANSFER in one JChannel configuration.

STREAMING_STATE_TRANSFER allows the following confguration parameters:

${STREAMING_STATE_TRANSFER}

#### Other considerations

Threading model used for state writing in a member providing state and state reading in a member receiving a state is tunable. For state provider thread pool is used to spawn threads providing state. Thus member providing state, in a push mode, will be able to concurrently serve N state requests where N is max_threads configuration parameter of the thread pool. If there are no further state transfer requests pool threads will be automatically reaped after configurable "pool_thread_keep_alive" timeout expires. For a channel operating in the push mode state reader channel can read state by piggybacking on jgroups protocol stack thread or optionally use a separate thread. State reader should use a separate thread if state reading is expensive (eg. large state, serialization) thus potentially affecting liveness of jgroups protocol thread. Since most state transfers are very short (<2-3 sec) by default we do not use a separate thread.

## Flow control

Flow control takes care of adjusting the rate of a message sender to the rate of the slowest receiver over time. If a sender continuously sends messages at a rate that is faster than the receiver(s), the receivers will either queue up messages, or the messages will get discarded by the receiver(s), triggering costly retransmissions. In addition, there is spurious traffic on the cluster, causing even more retransmissions.

Flow control throttles the sender so the receivers are not overrun with messages.

### FC

FC uses a credit based system, where each sender has `max_credits` credits and decrements them whenever a message is sent. The sender blocks when the credits fall below 0, and only resumes sending messages when it receives a replenishment message from the receivers.

The receivers maintain a table of credits for all senders and decrement the given sender's credits as well, when a message is received.

When a sender's credits drops below a threshold, the receiver will send a replenishment message to the sender. The threshold is defined by `min_bytes` or `min_threshold`.

${FC}

### SFC

A simplified version of FC. FC can actually still overrun receivers when the transport's latency is very small. SFC is a simple flow control protocol for group (= multipoint) messages.

Every sender has max_credits bytes for sending multicast messages to the group.

Every multicast message (we don't consider unicast messages) decrements max_credits by its size. When max_credits falls below 0, the sender asks all receivers for new credits and blocks until *all* credits have been received from all members.

When the receiver receives a credit request, it checks whether it has received max_credits bytes from the requester since the last credit request. If yes, it sends new credits to the requester and resets the max_credits for the requester. Else, it takes a note of the credit request from P and - when max_credits bytes have finally been received from P - it sends the credits to P and resets max_credits for P.

The maximum amount of memory for received messages is therefore <number of senders> * max_credits.

The relationship with STABLE is as follows: when a member Q is slow, it will prevent STABLE from collecting messages above the ones seen by Q (everybody else has seen more messages). However, because Q will *not* send credits back to the senders until it has processed all messages worth max_credits bytes, the senders will block. This in turn allows STABLE to progress and eventually garbage collect most messages from all senders. Therefore, SFC and STABLE complement each other, with SFC blocking senders so that STABLE can catch up.

SFC is currently experimental, we recommend to use MFC and UFC (see below) instead.

${SFC}

### MFC and UFC

In 2.10, FC was separated into MFC (Multicast Flow Control) and Unicast Flow Control (UFC). The reason was that multicast flow control should not be impeded by unicast flow control, and vice versa. Also, performance for the separate implementations could be increased, plus they can be individually omitted. For example, if no unicast flow control is needed, UFC can be left out of the stack configuration.

#### MFC

${MFC}

#### UFC

${UFC}

## Message stability

To serve potential retransmission requests, a member has to store received messages until it is known that every member in the cluster has received them. Message stability for a given message M means that M has been seen by everyone in the cluster.

The stability protocol periodically (or when a certain number of bytes have been received) initiates a consensus protocol, which multicasts a stable message containing the highest message numbers for a given member. This is called a digest.

When everyone has received everybody else's stable messages, a digest is computed which consists of the minimum sequence numbers of all received digests so far. This is the stability vector, and contain only message sequence numbers that have been seen by everyone.

This stability vector is the broadcast to the group and everyone can remove messages from their retransmission tables whose sequence numbers are smaller than the ones received in the stability vector. These messages can then be garbage collected.

### STABLE

${STABLE}

## Misc

### COMPRESS

${COMPRESS}

### pbcast.FLUSH

Flushing forces group members to send all their pending messages prior to a certain event. The process of flushing acquiesces the cluster so that state transfer or a join can be done. It is also called the stop-the-world model as nobody will be able to send messages while a flush is in process. Flush is used:

- State transfer

  When a member requests state transfer it tells everyone to stop sending messages and waits for everyone's ack. Then it asks the application for its state and ships it back to the requester. After the requester has received and set the state successfully, the requester tells everyone to resume sending messages.

- View changes (e.g.a join). Before installing a new view V2, flushing would ensure that all messages *sent* in the current view V1 are indeed *delivered* in V1, rather than in V2 (in all non-faulty members). This is essentially Virtual Synchrony.

FLUSH is designed as another protocol positioned just below the channel, e.g. above STATE_TRANSFER and FC. STATE_TRANSFER and GMS protocol request flush by sending a SUSPEND event up the stack, where it is handled by the FLUSH protcol. The SUSPEND_OK ack sent back by the FLUSH protocol let's the caller know that the flush has completed. When done (e.g. view was installed or state transferred), the protocol sends up a RESUME event, which will allow everyone in the cluster to resume sending.

Channel can be notified that FLUSH phase has been started by turning channel block option on. By default it is turned off. If channel blocking is turned on FLUSH notifies application layer that channel has been blocked by sending EVENT.BLOCK event. Channel responds by sending EVENT.BLOCK_OK event down to FLUSH protocol. We recommend turning on channel block notification only if channel is used in push mode. In push mode application that uses channel can perform block logic by implementing MembershipListener.block() callback method.

${FLUSH}

### SCOPE

As discussed in Scopes, the SCOPE protocol is used to deliver updates to different scopes concurrently. It has to be placed somewhere above UNICAST and NAKACK.

SCOPE has a separate thread pool. The reason why the default thread pool from the transport wasn't used is that the default thread pool has a different purpose. For example, it can use a queue to which all incoming messages are added, which would defy the purpose of concurrent delivery in SCOPE. As a matter of fact, using a queue would most likely delay messages get sent up into SCOPE !

Also, the default pool's rejection policy might not be "run", so the SCOPE implementation would have to catch rejection exceptions and engage in a retry protocol, which is complex and wastes resources.

The configuration of the thread pool is shown below. If you expect concurrent messages to N different scopes, then the max pool size would ideally be set to N. However, in most cases, this is not necessary as (a) the messages might not be to different scopes or (b) not all N scopes might get messages at the same time. So even if the max pool size is a bit smaller, the cost of this is slight delays, in the sense that a message for scope Y might wait until the thread processing message for scope X is available.

To remove unused scopes, an expiry policy is provided: expiration_time is the number of milliseconds after which an idle scope is removed. An idle scope is a scope which hasn't seen any messages for expiration_time milliseconds. The expiration_interval value defines the number of milliseconds at which the expiry task runs. Setting both values to 0 disables expiration; it would then have to be done manually (see for details).

${SCOPE}

### RELAY

RELAY bridges traffic between seperate clusters, see for details.

${RELAY}

### STOMP

STOMP is a JGroups protocol which implements the STOMP protocol. Currently (as of Nov 2010), transactions and acks are not implemented.

The location of a STOMP protocol in a stack is shown in .

STOMP in a protocol stack

The STOMP protocol should be near the top of the stack.

A STOMP instance listens on a TCP socket for client connections. The port and bind address of the server socket can be defined via properties.

A client can send SUBSCRIBE commands for various destinations. When a SEND for a given destination is received, STOMP adds a header to the message and broadcasts it to all cluster nodes. Every node then in turn forwards the message to all of its connected clients which have subscribed to the same destination. When a destination is not given, STOMP simply forwards the message to all connected clients.

Traffic can be generated by clients and by servers. In the latter case, we could for example have code executing in the address space of a JGroups (server) node. In the former case, clients use the SEND command to send messages to a JGroups server and receive messages via the MESSAGE command. If there is code on the server which generates messages, it is important that both client and server code agree on a marshalling format, e.g. JSON, so that they understand each other's messages.

Clients can be written in any language, as long as they understand the STOMP protocol. Note that the JGroups STOMP protocol implementation sends additional information (e.g. INFO) to clients; non-JGroups STOMP clients should simply ignore them.

JGroups comes with a STOMP client (org.jgroups.client.StompConnection) and a demo (StompDraw). Both need to be started with the address and port of a JGroups cluster node. Once they have been started, the JGroups STOMP protocol will notify clients of cluster changes, which is needed so client can failover to another JGroups server node when a node is shut down. E.g. when a client connects to C, after connection, it'll get a list of endpoints (e.g. A,B,C,D). When C is terminated, or crashes, the client automatically reconnects to any of the remaining nodes, e.g. A, B, or D. When this happens, a client is also re-subscribed to the destinations it registered for.

The JGroups STOMP protocol can be used when we have clients, which are either not in the same network segment as the JGroups server nodes, or which don't want to become full-blown JGroups server nodes. shows a typical setup.

STOMP architecture

There are 4 nodes in a cluster. Say the cluster is in a LAN, and communication is via IP multicasting (UDP as transport). We now have clients which do not want to be part of the cluster themselves, e.g. because they're in a different geographic location (and we don't want to switch the main cluster to TCP), or because clients are frequently started and stopped, and therefore the cost of startup and joining wouldn't be amortized over the lifetime of a client. Another reason could be that clients are written in a different language, or perhaps, we don't want a large cluster, which could be the case if we for example have 10 JGroups server nodes and 1000 clients connected to them.

In the example, we see 9 clients connected to every JGroups cluster node. If a client connected to node A sends a message to destination /topics/chat, then the message is multicast from node A to all other nodes (B, C and D). Every node then forwards the message to those clients which have previously subscribed to /topics/chat.

When node A crashes (or leaves) the JGroups STOMP clients (org.jgroups.client.StompConnection) simply pick another server node and connect to it.

The properties for STOMP are shown below:

${STOMP}

### DAISYCHAIN

The DAISYCHAIN protocol is discussed in .

${DAISYCHAIN}

### RATE_LIMITER

RATE_LIMITER can be used to set a limit on the data sent per time unit. When sending data, only max_bytes can be sent per time_period milliseconds. E.g. if max_bytes="50M" and time_period="1000", then a sender can only send 50MBytes / sec max.

${RATE_LIMITER}

### Locking protocols

There are currently 2 locking protocols: org.jgroups.protocols.CENTRAL_LOCK and org.jgroups.protocols.PEER_LOCK.

#### CENTRAL_LOCK

CENTRAL_LOCK has the current coordinator of a cluster grants locks, so every node has to communicate with the coordinator to acquire or release a lock. Lock requests by different nodes for the same lock are processed in the order in which they are received.

A coordinator maintains a lock table. To prevent losing the knowledge of who holds which locks, the coordinator can push lock information to a number of backups defined by num_backups. If num_backups is 0, no replication of lock information happens. If num_backups is greater than 0, then the coordinator pushes information about acquired and released locks to all backup nodes. Topology changes might create new backup nodes, and lock information is pushed to those on becoming a new backup node.

The advantage of CENTRAL_LOCK is that all lock requests are granted in the same order across the cluster, which is not the case with PEER_LOCK.

${CENTRAL_LOCK}

#### PEER_LOCK

PEER_LOCK acquires a lock by contacting all cluster nodes, and lock acquisition is only successful if all non-faulty cluster nodes (peers) grant it.

Unless a total order configuration is used (e.g. org.jgroups.protocols.SEQUENCER based), lock requests for the same resource from different senders may be received in different order, so deadlocks can occur. Example:

- Nodes A and B
- A and B call lock(X) at the same time
- A receives L(X,A) followed by L(X,B): locks X(A), queues L(X,B)
- B receives L(X,B) followed by L(X,A): locks X(B), queues L(X,A)

To acquire a lock, we need lock grants from both A and B, but this will never happen here. To fix this, either add SEQUENCER to the configuration, so that all lock requests are received in the same global order at both A and B, or use java.util.concurrent.locks.Lock.tryLock(long,javaTimeUnit) with retries if a lock cannot be acquired.

${PEER_LOCK}

### CENTRAL_EXECUTOR

CENTRAL_EXECUTOR is an implementation of Executing which is needed by the ExecutionService.

${CENTRAL_EXECUTOR}