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Task manager is a tool for tracking long-running background operations.
Structure overview
Task manager is divided into modules, e.g. repair or compaction module, which keep track of operations of similar nature. Operations are tracked with tasks.
Each task covers a logical part of the operation, e.g repair of a keyspace or a table. Each operation is covered by a tree of tasks, e.g. global repair task is a parent of tasks covering a single keyspace, which are parents of table tasks.
Time to live of a task
Root tasks are kept in task manager for task_ttl time after they are
finished. task_ttl value can be set in node configuration with
--task-ttl-in-seconds option or changed with task manager API
(/task_manager/ttl).
A task which isn't a root is unregistered immediately after it is finished and its status is folded into its parent. When a task is being folded into its parent, info about each of its children is lost unless the child or any child's descendant failed.
Internal
Tasks can be marked as internal, which means they are not listed
by default. A task should be marked as internal if it has a parent
or if it's supposed to be unregistered immediately after it's finished.
Abortable
A flag which determines if a task can be aborted through API.
Type vs scope
type of a task describes what operation is covered by a task,
e.g. "major compaction".
scope of a task describes for which part of the operation
the task is responsible, e.g. "shard".
API
Documentation for task manager API is available under api/api-doc/task_manager.json.
Briefly:
/task_manager/list_modules- lists module supported by task manager;/task_manager/list_module_tasks/{module}- lists (by default non-internal) tasks in the module;/task_manager/task_status/{task_id}- gets the task's status, unregisters the task if it's finished;/task_manager/abort_task/{task_id}- aborts the task if it's abortable;/task_manager/wait_task/{task_id}- waits for the task and gets its status;/task_manager/task_status_recursive/{task_id}- gets statuses of the task and all its descendants in BFS order, unregisters the task;/task_manager/ttl- sets new ttl, returns old value.