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XQueue defines an interface for the LMS to communicate with external grader services.
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Part of edX code.


XQueue defines an interface for the LMS to communicate with external grader services. For example, when a student submits a problem in the LMS, it gets sent to the XQueue. The XQueue then has the problem graded by an external service and sends the response back to the LMS.

How the LMS Interacts with the XQueue

1. The LMS pushes student submissions to the XQueue with an HTTP POST request to the URL /xqueue/submit. The submission contains a callback URL indicating where the graded response should be sent.

2. When the submission has been graded, the XQueue pushes a response back to the LMS with an HTTP POST request to the callback URL.

How External Graders Interact with the XQueue

There are two ways kinds of grading services: passive and active. These interact with the XQueue in different ways.

Passive Graders (aka Push Graders)

Passive graders wait for the XQueue to send them submissions. They then respond synchronously with a graded response.

  1. The LMS sends messages to a particular queue.

2. XQueue checks its settings and finds that the queue has a URL associated with it. XQueue forwards the message to that URL.

3. The passive grader receives a POST request from the XQueue and responds synchronously with the graded response.

4. XQueue forwards the graded response to the callback URL the LMS provided in its original message.

Active Graders (aka Pull Graders)

Active graders pull messages off the XQueue and push responses back to the XQueue.

  1. The test client sends messages to a particular queue.

2. The active grader polls the XQueue using a REST-like interface. When it receives a submission, it pushes a response back to the XQueue, also using a REST-like interface.

If a submission is retrieved but not graded, it will be available in the queue later following the SUBMISSION_PROCESSING_DELAY when it is requeued.

  1. XQueue pushes the response back to the LMS.

Local Testing

XQueue is now available as a docker image used as part of Docker Devstack. You can start and provision it by using xqueue specific commands configured in your devstack directory

make dev.provision.xqueue make dev.up.xqueue

XQueue will use the shared MySQL image.


You can run the unit/integration test suite using:

make test

after connecting to your container on Docker Devstack

make xqueue-shell make test


The code in this repository is licensed under version 3 of the AGPL unless otherwise noted.

Please see LICENSE.txt for details.

How to Contribute

Contributions are very welcome. The easiest way is to fork this repo, and then make a pull request from your fork. The first time you make a pull request, you may be asked to sign a Contributor Agreement.

Reporting Security Issues

Please do not report security issues in public. Please email

Mailing List and other support options

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