What is jgroups in java

This is needed so that all servers have the same state. However, what happens when a new server is started? That server has to somehow get the state e. This is called state transfer. Note that, in order to be able to use state transfer in an application, the protocol stack has to have a state transfer protocol the default stack used by the demo app does. If the state cannot be transferred within this time, then an exception will be thrown.

ReceiverAdapter defines a callback getState which is called on an existing instance usually the coordinator to fetch the cluster state. In our demo application, we define the state to be the chat conversation. This is a simple list, to the tail of which we add every message we receive.

Note that this is probably not the best example for state, as this state always grows. As a workaround, we could have a bounded list, which is not done here though. The getState method is called in the state provider , ie. It is passed an output stream to which the state has to be written.

Since access to state may be concurrent, we synchronize it. Then we call Util. The setState method is called on the state requester , ie. Its task is to read the state from the input stream and set it accordingly:. We again call a JGroups utility method Util. We also print the number of messages in the received chat history to stdout.

Note that this is not feasible with a large chat history, but - again - we could have a bounded chat history list. In this tutorial, we showed how to create a channel, join and leave a cluster, send and receive messages, get notified of view changes and implement state transfer.

Building blocks are classes residing on top of a JChannel that provide a higher abstraction level, e. The protocol stack allows for complete customization of JGroups: protocols can be configured, removed, replaced, enhanced, or new protocols can be written and added to the stack.

Copyright Red Hat - Installation 1. Download JGroups can be downloaded here. JGroups core, demo and selected test classes Sample configuration files, e. JGroups 4. Final for example.

Configuration Add jgroups Testing your Setup To see whether your system can find the JGroups classes, execute the following command:. You should see the following output more or less if the class is found:. Version Version: 4. Running a Demo Program To test whether JGroups works okay on your machine, run the following command twice:.

Using JGroups without a network You may skip this section if the 2 instances found each other correctly in the previous section. Writing a simple application The goal of this chapter is to write a simple text-based chat application SimpleChat , with the following features:.

All instances of SimpleChat find each other and form a cluster. A chat message is sent to all instances of the cluster. JGroups overview JGroups uses a JChannel as the main API to connect to a cluster, send and receive messages, and to register listeners that are called when things such as member joins happen. The main event loop and sending chat messages We now run an event loop, which reads input from stdin a message and sends it to all instances currently in the cluster.

Extra credits: maintaining shared cluster state One of the use cases of JGroups is to maintain state that is replicated across a cluster. One example is like numbering all the datagrams we've sent in application layer. And the receiver will need to check the data is not lost and then sort the packets into correct order. For example:. If sender sent packet 1, 2 to multicast group, and in this group there are two receivers called A and B.

ReceiverA received packet 1, 2 successfully, but receiver B only get packet 1. Then receiverB will ask sender to retransmit packet2. Please note this time the sender don't have to send packet 1 to multicast address, because other receivers in the group didn't ask for it because they have gotten the packet 2 successfully.

Sender just need to send packet 1 to receiverB again with an unicast to reduce bandwidth cost:. If sender sent packet 1, packet 2, and receiverA received packet 2, packet 1, it will reorder the packet to packet 1, packet 2 by itself. JGroup is certainly much more complex than above example but the design concept is basically the same. In JGroup, the minimal group unit is called 'Channel'.

Members in a channel can talk with each other. Here is the use case:. Generally speaking, JGroup is designed as a 'layered protocol stack'. In the bottom of the stack it's the 'UDP' protocol. Other protocols are built on top of it to meet different requirements of a cluster. You can even guess some protocol's purpose by its name.

The complete description of each protocol could be found here:. JGroup - List of Protocols. I wish you have caught the general design concepts of JGroup with above description. It can be used to create clusters whose nodes can send messages to each other. The main features include Cluster creation and deletion.

The most powerful feature of JGroups is its flexible protocol stack, which allows developers to adapt it to exactly match their application requirements and network characteristics. The benefit of this is that you only pay for what you use.