Timers in Deployment Groups

Timer facilities provided by EJB APIs TimerService and @Scheduled need special configuration in order to work intuitively for applications deployed to a deployment group.

There are four types of timers distinguished by their lifecycle (persistent, non-persistent) and the mean of their definition (declarative, programmatic).

Declarative timers are defined in EJB deployment descriptors or with annotation @Scheduled. Programmatic timers are created via calls to the TimerService.createTimer method and its equivalents.

Non-Persistent Timers

Non-Persistent timers exist in memory only for the duration in which the application runs on the instance. Declarative non-persistent timers run on every instance, whereas programmatic ones only on the instance that created them. When the instance shuts down, the timer stops firing. Instances do not see each other’s non-persistent timers, so calling TimerService.getTimers() only returns local non-persistent timers.

Non-Persistent timers have a simpler lifecycle but are not the default option To make a timer non-persistent, the @Schedule annotation needs the attribute persistent=false, or the TimerConfig objects need to have their property persistent set to false explicitly.

Persistent Timers

Persistent timers exist in the persistent store for the entire time for which application is deployed to a domain. Contrary to non-persistent ones, each persistent timer is only executed on a single instance of a deployment group. When the instance shuts down, each persistent timer migrates to another instance in the application’s deployment group and continues firing there. Since persistent timers are shared, TimerService.getTimers() returns all persistent timers in the deployment group, regardless of which instance created them.

Persistent timers are the default kind of timer.

Preconditions for using Persistent Timers in Deployment Groups

Because deployment groups do not need to share configurations amongst instances, there are several configuration and operational constraints that are not enforced by the server.

To ensure correct behaviour of persistent timers within a deployment group, the following conditions need to be met:

  • EJB Timer Service needs to use shared storage — external database or DataGrid

  • All instances of deployment groups and the Domain Administration Server share the same configuration of EJB Timer Service

  • Applications with persistent timers are deployed to a deployment group and not managed on instance level afterwards

  • Instances are members of only a single deployment group