Arvados

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h1=. Reusable tasks

p>. *"Tom Clegg":mailto:tom@curoverse.com

Last Updated: October 6, 2014*

h2. Overview

h3. Objective

Say jobs A and B, although not identical, have some tasks in common. Job A is complete. Job B starting now. They use the same script, version, docker image, etc. The only difference between A and B is that B's input collection has one more file; the rest of the files are identical. The script processes each input file independently, and it is a pure function (re-computing the same files will produce the same result). This means most of Job B's work has already been done. Task re-use will allow Arvados to recognize this condition and re-use the outputs of Job A's tasks instead of recomputing them.

Task re-use will not attempt to detect equivalence conditions like differently-encoded collection manifests with identical data, differing git commits with identical trees, and differing docker images with functionally equivalent content.

The intended audience for this document is software engineers.

h3. Background

The arvados.v1.jobs.create API offers a find_or_create feature which searches for an existing job which meets criteria specified by the client (e.g., same script, compatible script_version) and additional criteria (e.g., did not fail, is not marked impure/nondeterministic, does not diagree with other jobs passing the same criteria about what the correct output is).

* http://doc.arvados.org/api/methods/jobs.html#create

h3. Alternatives

Always recompute each task (i.e., leave existing behavior).

bq. This makes desirable use cases prohibitively expensive.

Use smaller jobs, and more jobs per pipeline.

bq. We could make the dynamic-structure capabilities of crunch jobs available at the pipeline level, and de-emphasize or stop using the features that encourage long-running jobs. Disadvantages include:

* The process of running a pipeline is not done in a controlled environment. This effectively reduces the utility of reproducibility and provenance features.

* Pipelines are currently encoded as JSON which is awkward to use as a DSL.

h3. Tradeoffs

_TODO_

h3. High Level Design

Before executing a job_task that qualifies for re-use, crunch-job uses the API to discover existing job_tasks that are functionally identical, are marked as "pure", and have already finished.

h2. Specifics

h3. Detailed Design

The JobTask schema has a new boolean flag @is_pure@ (not null, default @false@).

Just before starting a task having @is_pure==true@, crunch-job does an API query look up other tasks with @is_pure=true@ and identical inputs, parameters, script_version, etc.

* Some attributes like script and script_version are currently stored in the job record, not the job_task record. This will make the lookup interesting, in the absence of a generic "join" API.

Job tasks have one especially noteworthy side effect: queueing additional tasks. In order to reuse tasks safely without races, we need additional restraints:

* Tasks with @is_pure==true@ cannot queue additional tasks, *and* @is_pure@ cannot change from @false@ to @true@.

* Tasks do not qualify for reuse until they have completed.[1] When reusing a task, copy (and reset to "todo" state) each task whose @created_by_job_task_uuid@ attribute references the task being reused.

fn1. At least in the short term, this constraint is a good way to limit the complexity of implementation without sacrificing too much of the user benefit.

h3. Code Location

@sdk/cli/bin/crunch-job@ will have new task reuse logic.

@services/api/db/migrate@ will have a new migration, which will be reflected in @services/api/db/structure.sql@.

@services/api/app/models/job_task.rb@ will add :is_pure to the API response and prohibit @is_pure@ from changing from @false@ to @true@.

@doc/api/schema/JobTask.html.textile.liquid@ will document the :is_pure flag.

h3. Testing Plan

_TODO_

h3. Logging

@crunch-job@ will log the fact that it has copied its output attribute (and, if applicable, queued additional tasks) from an existing completed task.

h3. Debugging

_TODO_

h3. Caveats

To be determined.

h3. Security Concerns

_TODO_

h3. Open Questions and Risks

_TODO_

h3. Work Estimates

_TODO_

h3. Future Work

_TODO_

h3. Revision History

|_.Date            |_.Revisions Made |_.Author            |_.Reviewed By     |

| October 6, 2014 | Initial Draft         | Tom Clegg |=. ----              |