Timers in Deployment Groups
Timer facilities provided by EJB APIs
@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
Programmatic timers are created via calls to the
TimerService.createTimer method and its equivalents.
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
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