What are distributed Java systems?

Distributed Java systems are interconnected components that work together to achieve a common goal. These components can be located on different machines or even in different geographical locations. Distributed Java systems allow developers to create large-scale applications that can handle high user loads and provide high availability. The main features of distributed Java systems include:

• Scalability: the ability to handle increased user load and data processing requirements;
• Fault tolerance: the ability to recover from errors or failures, ensuring that the application remains available to users;
• Concurrency: the ability to process multiple tasks or requests at the same time.

Building distributed Java systems

Building a distributed Java system involves several steps, including designing the system architecture, selecting the appropriate technologies, and implementing the components. Here are some best practices to follow when building distributed Java systems:

1. Design the system architecture
   A well-designed architecture is the foundation of a successful distributed system. When designing your system architecture, consider the following factors:

Decompose the application into smaller manageable components, often called microservices. This allows each component to be developed, deployed, and scaled independently.
Design for scalability with horizontal scaling techniques such as data partitioning and load balancing.
Ensure fault tolerance by implementing redundancy and monitoring mechanisms such as health checks and replication.

2. Choose the right technologies

Choosing the right technologies is critical to the success of your distributed system. Some popular Java technologies for building distributed systems include:

Spring Boot: A widely used framework for building microservices and other distributed systems.
Apache Kafka: A high-performance, fault-tolerant message broker used for event-driven architectures and streaming data.
Apache Cassandra: a highly scalable distributed NoSQL database designed to process large amounts of data on many standard servers.

3. Implementation of components

After designing your architecture and selecting the appropriate technologies, implement the components of your distributed system.

Managing distributed Java systems

Managing a distributed Java system involves deploying, monitoring, and maintaining components. Some of the best practices for managing Java distributed systems are:

1. Use containerization and orchestration
   Containerization simplifies deployment by packaging an application and its dependencies into a single, portable unit. Docker is a popular containerization platform that allows you to create and manage containers. Kubernetes is an orchestration platform that automates the deployment, scaling, and management of containerized applications.
2. Implement monitoring and logging
   Monitoring and logging are essential to maintaining the health and performance of your distributed system. Use tools like Prometheus for monitoring and the Elasticsearch, Logstash, and Kibana (ELK) stack for logging and log analysis.
3. Ensure data consistency and integrity
   Data consistency and integrity are critical in distributed systems. Implement strategies such as final consistency, careful data distribution, and transactions to ensure data consistency across your system.

Distributed Java systems allow developers to create and manage large-scale applications that can handle high user loads and provide high availability. By following best practices and using the right tools, you can create a reliable and efficient distributed system. If you’re looking to hire remote Java developers for your project, consider Reintech.io’s dedicated team of Java developers with the knowledge and experience needed to build and manage distributed Java systems.

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At the application level, a distributed program can be broken down into the following parts:

Processes

A typical computer operating system on a computer host can run several processes at the same time. A process is created by describing a sequence of steps in a programming language, compiling the program into an executable form, and running the executable on the operating system. During operation, a process has access to computer resources (such as CPU time and input/output devices) through the operating system. A process can be entirely dedicated to a particular program, or several programs can use a single process to perform tasks.

Threads

Each process has at least one control thread. Some operating systems support the creation of multiple control threads in a single process. Each thread in a process can run independently of the other threads, although there is usually some synchronization between them. For example, one thread may monitor input through a socket connection, while another may listen for user events (keystrokes, mouse movements, etc.) and provide feedback to the user through output devices (monitor, speakers, etc.). At some point, the input from the input stream may require feedback from the user. At this point, the two streams will need to coordinate the transfer of input to the user’s attention.

Objects

Programs written in object-oriented languages consist of interacting objects. One simple definition of an object is a group of related data with available methods for querying or modifying the data (getName() , set-Name() ) or for performing certain actions based on the data (sendName(Out-putStream o ) ). A process can consist of one or more objects, and these objects can be accessed by one or more threads in the process. And with the introduction of distributed object technology such as RMI and CORBA, an object can also be logically distributed across multiple processes on multiple computers.

Agents

For the purpose of this book, we will use the term “agent” as a generic way of referring to the important functional elements of a distributed application.
While process, flow, and object are fairly well-defined entities, an agent (at least the definition we will use for this book) is a higher-level system component defined around a particular function, or utility, or role in the overall system. For example, an e-banking program can be broken down into a customer agent, a transactional agent, and an information brokerage agent. Agents can be distributed across multiple processes and consist of multiple objects and flows in those processes. Our client agent can consist of an object in a process running on the client’s desktop that listens for data and updates the local display, and an object in a process running on the bank server that makes requests and sends data back to the client. There are two objects running in different processes on separate machines, but together we can think of them as one client agent with client-side elements and server-side elements.

Thus, a distributed program can be viewed as a coordinated group of agents working to achieve a specific goal. Each of these agents can be distributed across multiple processes on remote hosts and can consist of multiple objects or control threads. Agents can also belong to multiple programs at the same time. For example, you might be developing an ATM application that consists of an account database server with client request agents distributed across the network that send requests. The account server agent and the customer request agents are agents in the ATM program, but they can also serve agents residing at the financial institution’s headquarters as part of the administrative program.

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The Java RMI Registry is a key component of the Java RMI system and provides a centralized directory for servers to register services and for clients to search for those services.

Server Interface Announcement

To learn the intricacies of the Java RMI system, let’s implement a simple server object that provides a method to accept a name and return a greeting. Here is the definition of the object’s interface:

import java.rmi.Remote; import java.rmi.RemoteException; public interface Greeting extends Remote { public String greet(String name) throws RemoteException; }

The name of the interface is called Greeting . It provides a single method called greet () that takes a name and returns a suitable greeting.

To mark this interface as exportable, it must extend the interface java.rmi.Remote . In addition, the method must declare a throws clause listing java.rmi.RemoteException in addition to any application-specific exceptions. Exceptions This is to allow client code to handle (or propagate) remote method invocation errors such as host-not-found , connection failure, etc. Д.

Server object implementation

After declaring the interface (which is used by clients), we implement the server-side object and provide the greet () method as shown. A simple format string is used to format the greeting.

public class GreetingObject implements Greeting { private String fmtString = "Hello, %s"; public String greet(String name) { return String.format(this.fmtString, name); } }

The main server method

Let’s now put all these pieces together and implement the main () server method. Let’s walk through each of the relevant steps.

The first step is to create an implementation of the server object.

Greeting greeting = new GreetingObject();

Next, we get a stub for the server object from the RMI runtime environment. The stub implements the same interface as the server object. However, the method implements the necessary communication with the remote server object. This stub is used by the client to transparently call the method on the server object.

Greeting stub = (Greeting)UnicastRemoteObject.exportObject(greeting, 0);

Once a stub is obtained, we pass that stub to the RMI registry to bind to the specified named service. When a client requests an implementation of that service, it receives a stub that knows how to contact the server object. The static LocateRegistry.getRegistry () method is then used to obtain a reference to the local registry. The rebind () method is then used to bind a name to the stub.

String name = "Greeting"; Registry registry = LocateRegistry.getRegistry(port); registry.rebind(name, stub);

The complete main method.

import java.rmi.registry.LocateRegistry; import java.rmi.registry.Registry; import java.rmi.registry.Registry; import java.rmi.server.UnicastRemoteObject; public class Main { static public void main(String[] args) throws Exception { if ( args.length == 0 ) { System.err.println("usage: java Main port#"); System.exit(1); } int index = 0; int port = Integer.parseInt(args[index++]); String name = "Greeting"; Greeting greeting = new GreetingObject(); Greeting stub = (Greeting)UnicastRemoteObject. exportObject(greeting, 0); Registry registry = LocateRegistry.getRegistry(port); registry.rebind(name, stub); System.out.println("Greeting bound to \"" + name + "\""); } }

Building the server

Let’s now look at building the server. For simplicity, we build using the command line in Linux rather than a build tool such as Maven.

The following compiles the source files into class files in the target directory.

rm -rf target mkdir target javac -d target src/server/*.java

Compile the class files into a JAR file for execution.

jar cvf target/rmi-server.jar -C target server

We also assemble the interface files needed to compile the client into a JAR library.

jar cvf target/rmi-lib.jar -C target server/Greeting.class

Client implementation

Let’s now look at the client implementation used to call the methods of the server object.

As with the server, get a reference to the registry by specifying the hostname where the registry is running and the port number.

Registry registry = LocateRegistry.getRegistry(host, port);

Next, we look for the service in the registry. The lookup () method returns a stub that can be used to call the services.

Greeting greeting = (Greeting) registry.lookup(name);

And call the method, passing the necessary arguments. Here we get the greeting by passing the name and printing it.

System.out.println(name + " reported: " + greeting.greet(myName));

The complete client code:

package client; import java.rmi.registry.LocateRegistry; import java.rmi.registry.Registry.Registry; import server.Greeting; public class Client { static public void main(String[] args) throws Exception { if ( args.length != 3 ) { System.err.println("usage: java Client client host port myName"); System.exit(1); } int index = 0; String host = args[index++]; int port = Integer.parseInt(args[index++]); String myName = args[index++]; String name = "Greeting"; Registry registry = LocateRegistry.getRegistry(host, port); Greeting greeting = (Greeting) registry.lookup(name); System.out.println(name + " reported: " + greeting.greet(myName)); } }

RMI Registry

Now let’s start the server program so it can start serving requests.

java -cp target/rmi-server.jar server.Main 1099 # throws Exception in thread "main" java.rmi.ConnectException: Connection refused to host: xxx; nested exception is: java.net.ConnectException: Connection refused

What is this exception? Connection refused.

The reason you get this exception is this: note that from the server code, it is trying to connect to the local registry on port 1099. In the event of a failure, you will get this exception.

The solution is to run the RMI Registry. The RMI Registry is a program that comes with the Java Virtual Machine and is called rmiregistry . It should be located in the bin directory of the Java Virtual Machine installation. Running it is as simple as:

/usr/lib/jvm/jdk1.8.0_71/bin/rmiregistry

By default, the registry listens on port 1099. To listen on a different port, specify the port number as follows:

/usr/lib/jvm/jdk1.8.0_71/bin/rmiregistry 1100

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