---
layout: default
navsection: api
title: "Resource: JobTasks"
navorder: 16
---

h1. JobTasks

A Job Task is a well defined independently-computable portion of a "Job":Jobs.html.

Job tasks are created two ways:
* When a job starts, it is seeded with a job task with @sequence=0@ and an empty @parameters{}@ list.
* Job task A can create additional job tasks B, C, D, which will belong to the same job. Tasks B, C, D will not be performed until job task A is complete. If job task A fails, tasks B, C, D will be deleted.

Job tasks have particular update semantics:
* Progress reporting: A job task should only be <code>PATCH</code>ed by a worker process which has been dispatched to work on that task and is reporting progress or completion status &mdash; and by the job manager itself.
* Completion: When a job task process terminates, the task is considered complete only if its most recent @PATCH@ transaction had @progress=1.0@ and @success=true@.
* Temporary failure: If a job task process terminates without updating @success@ to @true@ or @false@, it is assumed that the task failed but is worth re-attempting (at a different time, on a different node, etc).


h2. Methods

See "REST methods for working with Arvados resources":methods.html

API endpoint base: @https://{{ site.arvados_api_host }}/arvados/v1/job_tasks@

h2. Resource

Each job has, in addition to the usual "attributes of Arvados resources":resources.html:

table(table table-bordered table-condensed).
|*Attribute*|*Type*|*Description*|
|sequence|integer|Execution sequence.
A step cannot be run until all steps with lower sequence numbers have completed.
Job steps with the same sequence number can be run in any order.|
|parameters{}|list||
|runtime_constraints{}|list|Constraints that must be satisfied by the job/task scheduler in order to run the task (see "Jobs":Jobs.html).|
|output|string||
|progress|float||
|success|boolean|Is null if the task has neither completed successfully nor failed permanently.|

The following attributes should not be updated by anyone other than the job manager:

table(table table-bordered table-condensed).
|*Attribute*|*Type*|*Description*|*Notes*|
|qsequence|integer|Order of arrival|0-based|
|failed_count|number|Times this step has failed||
|failed_at|number|Time of last failure|Is null if failed_count=0|
|failed_log|string|stderr from last failure|Is null if failed_count=0|
|log|string|stderr||


h3. Resource limits

table(table table-bordered table-condensed).
|*Key*|*Type*|*Description*|*Implemented*|
|min_nodes|integer|||
|max_nodes|integer|||
|max_tasks_per_node|integer|Maximum simultaneous tasks on a single node|&#x2713;|
|min_ram_per_task|integer|Minimum real memory (KiB) per task||
|min_ram_per_node|integer|Minimum real memory (KiB) per node||