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javajakarta-eeejbcluster-computing

Deploying Java EE application on clustered app server

How does an application will be deployed when the server is clustered. I am not talking about docker. I mean Domain mode in WildFly/JBoss or Cluster in Glassfish or Websphere.

Is it occur to be like one standalone application on each node/instance of a cluster? Like if I have a @Singleton instance will it be the one on the whole cluster or one for each node and so one for each running app.

Or it will be treated like the one application just spreaded on a cluster under the hood? like the @Singleton is somewhere on the node X and if I call it from node Y it will call it by tcp or something (if not using EJB client)?

Solution

  • Is it occur to be like one standalone application on each node/instance of a cluster? Like if I have a @Singleton instance will it be the one on the whole cluster or one for each node and so one for each running app.

    Assuming that the cluster has been set up with two nodes/hosts (i.e., two JVMs), there will two copies (i.e., one copy in each JVM host) of the @Singleton objects.

    In general, singleton = single object (instance) per webapplication per JVM

    In other words, when you are deploying multiple web applications inside the same server (JVM), you can have multiple objects/instances (one per each web application) of the same class within the same JVM.

    Or it will be treated like the one application just spread on a cluster under the hood?

    In general, the cluster will be set up in two or more separate hosts (in order to achieve the resilience, load balancing, etc.. reasons), i.e., the application will be deployed into separate JVMs, so there will be separate copies of the @Singleton objects in each JVM.

    If I have nodes n1 n2 n3. And on the node n3, I''ll create some bean with singleton injected like @Inject private SingleBean singleBean; Will it be the singleton from node n3 only or there is some way that the singleton from n1 may be injected?

    It will be singleton for Node3 (JVM) only.

    If you really want to share (because of any reason) the singleton instance across JVMs, you can refer here for more details.