Chapter 2. Overview of Eclipse Vert.x Eclipse Vert.x is a toolkit used for creating reactive, non-blocking, and asynchronous applications that run on the Java Virtual Machine (JVM ). Eclipse Vert.x is designed to be cloud-native. It allows applications to use very few threads. This avoids the overhead caused when new threads are created. This enables Eclipse Vert.x applications and services to effectively use their memory as well as CPU quotas in cloud environments. Using the Eclipse Vert.x runtime in OpenShift makes it simpler and easier to build reactive systems. The OpenShift platform features, such as, rolling updates, service discovery, and canary deployments, are also available. With OpenShift, you can implement microservice patterns, such as externalized configuration, health check, circuit breaker, and failover, in your applications. 2.1. Key concepts of Eclipse Vert.x This section describes some key concepts associated with the Eclipse Vert.x runtime. It also provides a brief overview of reactive systems. Cloud and Container-Native Applications Cloud-native applications are typically built using microservices. They are designed to form distributed systems of decoupled components. These components usually run inside containers, on top of clusters that contain a large number of nodes. These applications are expected to be resistant to the failure of individual components, and may be updated without requiring any service downtime. Systems based on cloud-native applications rely on automated deployment, scaling, and administrative and maintenance tasks provided by an underlying cloud platform, such as, OpenShift. Management and administration tasks are carried out at the cluster level using off-the-shelf management and orchestration tools, rather than on the level of individual machines. Reactive Systems A reactive system, as defined in the reactive manifesto , is a distributed systems with the following characteristics: Elastic The system remains responsive under varying workload, with individual components scaled and load-balanced as necessary to accommodate the differences in workload. Elastic applications deliver the same quality of service regardless of the number of requests they receive at the same time. Resilient The system remains responsive even if any of its individual components fail. In the system, the components are isolated from each other. This helps individual components to recover quickly in case of failure. Failure of a single component should never affect the functioning of other components. This prevents cascading failure, where the failure of an isolated component causes other components to become blocked and gradually fail. Responsive Responsive systems are designed to always respond to requests in a reasonable amount of time to ensure a consistent quality of service. To maintain responsiveness, the communication channel between the applications must never be blocked. Message-Driven The individual components of an application use asynchronous message-passing to communicate with each other. If an event takes place, such as a mouse click or a search query on a service, the service sends a message on the common channel, that is, the event bus. The messages are in turn caught and handled by the respective component. Reactive Systems are distributed systems. They are designed so that their asynchronous properties can be used for application development. Reactive Programming While the concept of reactive systems describes the architecture of a distributed system, reactive programming refers to practices that make applications reactive at the code level. Reactive programming is a development model to write asynchronous and event-driven applications. In reactive applications, the code reacts to events or messages. There are several implementations of reactive programming. For example, simple implementations using callbacks, complex implementations using Reactive Extensions (Rx), and coroutines. The Reactive Extensions (Rx) is one of the most mature forms of reactive programming in Java. It uses the RxJava library.

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