A group name (in the context of task type) provides for a way to define a "join" on the group - a task that gets triggered when the whole group finished (either successfully or not).
A group task's unique identifier consists of the task type (e.g. "reconcile-set-of-records"), a group name ("reconcile-system-x"), task number in group(10), group total tasks (100). The group task generation/submission should be done in a fashion that re-doing the generation results in the same identifier.
When the total number of task that will be submitted for a group is unknown whilst generating the tasks, only the last record submitted for group has both the task number in group and group total tasks, which must be the same (100/100). This is necessary for the system to detect when the whole group has finished. Previous tasks only have the task number in group set. The generation of tasks within a group should be idempotent, i.e. it should be able to re-do the task generation in a fashion that duplicates are ignored because the taskid is the same again.
A task can submit sub-tasks through the handler provided context. This implies that the spawning task is the parent of these sub-tasks. This allows for an enclosing task to a) ensure only one enclosing task of the same type (controlled through task id) runs at the same time in the cluster, and b) there is a place to handle a "join" (group finished handler) of the overall task, after all the sub-tasks are finished.
Only when the sub-tasks all finished will the parent task's "group finished handler" be triggered. For example, if only one reconciliation for a given system should run at any given time, and it is broken into sub-tasks a Handler for Single task "[recon-parent][recon-system-x] will submit 100 sub-tasks as follows:
-
Submit single task with task type [recon-parent], task name "recon-system-x".
-
Node 1 picks up "[recon-parent][recon-system-x]", submits 100x grouped tasks with task type "recon-record-set", group name "recon-system-x", record numbers 1-100 / 100
-
All nodes start picking up and processing "[recon-record-set][recon-system-x][<batch number>/100]" tasks
-
When all "[recon-record-set][recon-system-x][<batch number>/100]" tasks finished, a task gets inserted to trigger the group finished handler for "recon-batch".
-
A node picks up and handles the group finished handler 'task' for "recon-record-set"
-
Upon completion of the "[recon-record-set][recon-system-x][group-finished-task]" group finished handler, a task gets inserted to trigger the group finished handler on the parent task, "[recon-parent][recon-system-x][group-finished-task]"
-
A node picks up and handles the group finished handler task for "[recon-parent][recon-system-x][group-finished-task]"
