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Trapperkeeper Scheduler Service

A Trapperkeeper service that provides a simple API for scheduling background tasks.

Other Trapperkeeper services may specify a dependency on the Scheduler service, and then use its functions to schedule and cancel jobs to be run on background worker threads.

What Does This Service Do?

The SchedulerService provides some simple API for scheduling (potentially recurring) background tasks. The service manages the lifecycle of the underlying scheduling subsystem so that other services don't need to (and avoids potential issues around multiple services attempting to initialize the same scheduling subystem in a single JVM process).

The functions that are currently available are as follows:

• interspaced [interval-ms f]: schedules a job that will call f, block until that call completes, sleep for interval-ms milliseconds, and then repeat. Returns an identifier that can be used to reference this scheduled job (e.g., for cancellation) later.
• interspaced [interval-ms f group-id]: schedules a job that will call f, block until that call completes, sleep for interval-ms milliseconds, and then repeat. Returns an identifier that can be used to reference this scheduled job (e.g., for cancellation) later. A group identifier group-id can be provided that allows jobs in the same group to be stopped at the same time.
• interval [interval-ms f]: schedules a job that will call f, block until that call completes, and then run again at the next logical interval based on interval-ms and the original start time. In other words, f will get called every interval-ms unless the execution time for f exceeds interval-ms in which case that execution is skipped. Returns an identifier that can be used to reference this scheduled job (e.g., for cancellation) later.
• interval [interval-ms f group-id]: schedules a job that will call f, block until that call completes, and then run again at the next logical interval based on interval-ms and the original start time. In other words, f will get called every interval-ms unless the execution time for f exceeds interval-ms in which case that execution is skipped. If there are insufficient threads in the thread pool to run the interval job at the time of execution, the job will be skipped. Returns an identifier that can be used to reference this scheduled job (e.g., for cancellation) later. A group identifier group-id can be provided that allows jobs in the same group to be stopped at the same time.
• cron [cron-string f]: schedules a job that will call f in accordance with the cron schedule indicated by the 'cron-string'. Returns an identifier that can be used to reference this scheduled job (e.g. for cancellation) later. More information on a valid cron string can be found here.
• cron [cron-string f group-id]: schedules a job that will call f in accordance with the cron schedule indicated by the 'cron-string'. Returns an identifier that can be used to reference this scheduled job (e.g. for cancellation) later. A group identifier group-id can be provided that allows jobs in the same group to be stopped at the same time. More information on a valid cron string can be found here.
• cron-next-valid-time [cron-string date]: Given a cron specification and a date, returns a date that corresponds to the next execution of the timer based on that cron value.
• after [interval-ms f]: schedules a job that will call f a single time, after a delay of interval-ms milliseconds. Returns an identifier that can be used to reference this scheduled job (e.g. for cancellation) later.
• after [interval-ms f group-id]: schedules a job that will call f a single time, after a delay of interval-ms milliseconds. Returns an identifier that can be used to reference this scheduled job (e.g. for cancellation) later. A group identifier group-id can be provided that allows jobs in the same group to be stopped at the same time.
• interval-after [initial-delay-ms interval-ms f]: Similar to interval but delays initial execution until initial-delay-ms has occurred.
• interval-after [initial-delay-ms interval-ms f group-id]:Similar to interval but delays initial execution until initial-delay-ms has occurred A group identifier group-id can be provided that allows jobs in the same group to be stopped at the same time.
• stop-job [job-id]: Given a job-id returned by one of the previous functions, cancels the job. If the job is currently executing it will be allowed to complete, but will not be invoked again afterward. Returns true if the job was successfully stopped, false otherwise.
• stop-jobs [group-id]: Given a group-id identifier, cancel all the jobs associated with that group-id. If any of the jobs are currently executing they will be allowed to complete, but will not be invoked again afterward. Returns a sequence of maps, one for each job in the group, with each map containing the job and a boolean stopped? key indiciating if the job was stopped successfully or not.
• count-jobs []: Return a count of the total number of scheduled jobs known to to the scheduling service. after jobs that have completed won't be included in the total.
• count-jobs [group-id]: Return a count of the total number of scheduled jobs with the associated group-id known to to the scheduling service. after jobs that have completed won't be included in the total.
• get-jobs []: return a list of the current job identifiers
• get-jobs [group-id]: return a list of the current job identifiers associated with the specified group identifier

Implementation Details

A configuration value is available under scheduler->thread-count that controls the number of threads used internally by the quartz library for job scheduling. If not specified, it defaults to 10, which is the quartz internal default.

The current implementation of the SchedulerService is a wrapper around the org.quartz-scheduler/quartz library.

What's Next?

• Add additional scheduling API functions with more complicated recurring models.`.
• Add API for introspecting the state of currently scheduled jobs

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