JGroups is a library for reliable one-to-one or one-to-many communication written in the Java language. It can be used to create groups of processes whose members send messages to each other. JGroups enables developers to create reliable multipoint multicast applications where reliability is a deployment issue. JGroups also relieves the application developer from implementing this logic themselves. This saves significant development time and allows for the application to be deployed in different environments without having to change code.
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The canonical reference for building a production grade API with Spring. It features a simple interface that provides:. In this tutorial, we'll create a simple application for exchanging String messages between applications and supplying shared state to new applications as they join the network.
The latest version of the library can be checked on Maven Central. JGroups will try to use IPV6 by default. Depending on our system configuration, this may result in applications not being able to communicate.
To avoid this, we'll set the java. Our connection to a JGroups network is a JChannel. The channel joins a cluster and sends and receives messages, as well as information about the state of the network. We create a JChannel with a path to a configuration file. If we omit the file name, it will look for udp. We've included the file for UDP in our code and will use it for this tutorial. For more information on configuring the transport see the JGroups manual here.
After we've created our channel, we need to join a cluster. A cluster is a group of nodes that exchange messages. The first node that attempts to join a cluster will create it if it doesn't exist. We'll see this process in action below. Nodes are identified by a name so that peers can send directed messages and receive notifications about who is entering and leaving the cluster.
JGroups will assign a name automatically, or we can set our own:. Channel cleanup is essential if we want peers to receive timely notification that we have exited. With a JChannel created we're now ready to see the state of peers in the cluster and exchange messages with them. JGroups maintains cluster state inside the View class.
Each channel has a single View of the network. When the view changes, it's delivered via the viewAccepted callback. For this tutorial, we'll extend the ReceiverAdaptor API class that implements all of the interface methods required for an application. Each View contains a List of Address objects, representing each member of the cluster.
JGroups offers convenience methods for comparing one view to another, which we use to detect new or exited members of the cluster. Message handling in JGroups is straightforward. A Message contains a byte array and Address objects corresponding to the sender and the receiver. For this tutorial we're using Strings read from the command line, but it's easy to see how an application could exchange other data types. A Message is created with a destination and a byte array; JChannel sets the sender for us.
If the target is null , the entire cluster will receive the message. If we run multiple instances of our program and send this message after we implement the receive method below , all of them would receive it, including the sender. Sending a direct message requires a valid Address. If we're referring to nodes by name, we need a way to look up an Address. Fortunately, we have the View for that. Address names are available via the class toString method, so we merely search the List of cluster members for the name we want.
So we can accept a name on from the console, find the associated destination, and send a direct message:. Since we know the message contains a String , we can safely pass getObject to System. When a node enters the network, it may need to retrieve state information about the cluster. JGroups provides a state transfer mechanism for this.
When a node joins the cluster, it simply calls getState. The cluster usually retrieves the state from the oldest member in the group — the coordinator. Let's add a broadcast message count to our application.
We'll add a new member variable and increment it inside receive :. We check for a null destination because if we count direct messages, each node will have a different number. JGroups supplies an InputStream to the coordinator to write the state to, and an OutputStream for the new node to read. The API provides convenience classes for serializing and deserializing the data.
A null destination indicates the coordinator and 0 means do not timeout. When we run this app with a pair of nodes and exchange broadcast messages, we see the message count increment. Then if we add a third client or stop and start one of them, we'll see the newly connected node print the correct message count.
Enhanced JGroups configuration
This is a short tutorial on how to install JGroups and write a simple application. The goal is to show how to configure JGroups and how to write a simple application showing the major methods of the API. JGroups can be downloaded here. Add jgroups If you use the log4j logging system, you also have to add log4j. If you started them simultaneously, they could initially show a membership of 1 in their title bars.
A Simple Clustered Task Distribution System
Infinispan uses JGroups as its underlying clustering layer. In order to configure the finer details of clustering discovery, flow control, cross-site, etc you have to provide a separate XML file with the desired configuration and reference it from your Infinispan XML file as follows:. For simple configurations this is usually fine, but configuring complex setups, such as cross-site replication, means juggling multiple files one for the local stack, one for the cross-site stack and one for the relay configuration. Starting with Infinispan 10 Alpha2 we have introduced a number of changes to make your life with JGroups configurations a lot easier. Infinispan now comes with two pre-declared stacks: tcp and udp. Inlining a stack means you can put the JGroups configuration inside the Infinispan one as follows:.
The JGroups tutorial
Comment 8. This article will introduce the basic concepts of JGroups and then implement a task distribution system on top of JGroups , where tasks can be placed into the cluster and are executed by worker nodes. I'll show that worker nodes can be added at run time to add more processing power, or taken down when we don't have much load. Plus, tasks assigned to workers who subsequently crash are automatically reassigned to live nodes.
Reliable Messaging with JGroups