History task

A history task is an item in one of the History service's internal per-shard queues. They are how the engine schedules deferred and outbound work; together with the events and mutable state, they are the third pillar of every workflow's persistent state.

Queue families

Queue	Purpose	Source
Transfer	Push a Workflow / Activity task to Matching, update visibility, kick off child workflows. Immediate.	service/history/transfer_queue_active_task_executor.go
Timer	Fire deferred work — workflow sleeps, schedule-to-start timeouts, retry backoffs.	service/history/timer_queue_active_task_executor.go
Visibility	Update the visibility store on workflow status changes.	service/history/visibility_queue_task_executor.go
Outbound	Deliver Nexus operations, callbacks, and other external HTTP/gRPC requests.	service/history/outbound_queue_active_task_executor.go
Archival	Push completed history into the configured archive backend.	service/history/archival_queue_task_executor.go
Replication	Push events to peer clusters in a multi-cluster setup.	service/history/replication/

Each family has both an active and a standby executor. The active runs in the cluster that owns the namespace; the standby runs in passive replicas and verifies replicated state without advancing the workflow.

Persistence

Tasks are written into per-shard task tables in the same DB transaction as the corresponding mutable state and history events — this is the transactional outbox pattern that keeps the engine and Matching consistent. Schema: each backend's transfer_tasks, timer_tasks, visibility_tasks, etc., tables.

Code references

• Task type registry: service/history/tasks/.
• Queue framework: service/history/queues/.
• Per-queue factory: service/history/queue_factory_base.go.

Where to read more

• History service
• docs/architecture/history-service.md