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There are three groups of HTTP web APIs exposed by XQueue, all of which communicate using JSON.

Authentication

XQueue has a separate user model that has no connection to edx-platform users. Service accounts must be created directly with XQueue, and clients must use basic (session) auth to communicate.

API Endpoints

There are three groups of HTTP web APIs exposed by XQueue, all of which communicate using JSON.

LMS-facing

• /xqueue/submit

  • Receive a new task

  • Allowed methods: POST

  • Format: must have three keys defined in a JSON object in the POST body.

    • lms_callback_url: the exact location where XQueue should POST the final result of a task

    • lms_key: an opaque identifier that the caller wants returned in the final callback

    • queue_name

  • Any files included in the POST will be dumped into XQueue's configured file storage system.

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Also called "passive," these graders exposed a web interface that XQueue calls called when a new submission is was available for processing. XServer was a push-grader implementation. We moved away from the use of push graders in favor of more general-purpose XBlock integrations (e.g. an LTI consumer).

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• Consider modifying XQWatcher to have a "slow" poll when the queue is empty, and a "fast" poll when it the queue is probably not empty (as in, if the last poll was successful).

• XQWatcher is more of an instance of a code grader than a code grader framework. A framework/client library may speed adoption of external graders.

Resources

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Source code

• XQueue: https://github.com/edx/xqueue

  • Contains the Python code for the Django-based IDA.

• Pull grader client: https://github.com/edx/xqueue-watcher

  • Contains the base Python code for Flask-based XQueue watchers.

  • The actual graders used on edx.org are maintained by the course authors in separate Github repos.

  • One example of a (private) Github repo where MIT graders are kept for one course: https://github.com/mitodl/graders-mit-600x

• Push grader client: https://github.com/edx/xserver

  • This repo is now archived and is read-only. Push graders are now deprecated and removed.

• LMS: https://github.com/edx/edx-platform

Related documentation

• [How To] Adding a new xqueue queue

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title Pull Flow

participant LMS
participant XQueue
participant XQWatcher
XQWatcher-->XQueue: Poll for submissions
note left of LMS: Initiate a request\n(asynchronously)
LMS->XQueue: Submit request to queue
XQWatcher-->XQueue: Poll for submissions
XQWatcher<->XQueue: Get next submission
note right of XQWatcher: Do some work\n(e.g. grade code)
XQWatcher->XQueue: Push result back
XQueue->LMS: Send result via callback
note left of LMS: Display result
XQWatcher-->XQueue: Poll for submissions

Push Flow

title Push Flow

participant LMS
participant XQueue
participant XServer
note left of LMS: Initiate a request\n(asynchronously)
LMS->XQueue: Submit request to queue
XQueue->XServer: Submit to grader\n(blocking)
note right of XServer: Do some work\n(e.g. grade code)
XServer->XQueue: Respond with result
XQueue->LMS: Send result via callback
note left of LMS: Display result