The Opaque question type and behaviour
https://moodle.org/plugins/qtype_opaque

Opaque (http://docs.moodle.org/en/Development:Opaque) is the Open protocol for
accessing question engines.

The Opaque protocol was originally created by sam marshall of the Open
University (http://www.open.ac.uk/) as part of the OpenMark project
(http://java.net/projects/openmark/). The Moodle implementation of Opaque was
done by Tim Hunt.

As well as OpenMark, this question type can also be used to connect to
ounit (http://code.google.com/p/ounit/) and possibly other question systems
we don't know about.

Opaque has been available since Moodle 1.8, but this version is compatible with
Moodle 3.4.

This question behaviour also requires the Opaque question type to be installed.
https://moodle.org/plugins/qbehaviour_opaque

You can install from the Moodle plugins database using the links above.
Or to install using git, type this command in the root of your Moodle install

    git clone https://github.com/moodleou/moodle-qtype_opaque.git question/type/opaque
    echo '/question/type/opaque/' >> .git/info/exclude
    git clone https://github.com/moodleou/moodle-qbehaviour_opaque.git question/behaviour/opaque
    echo '/question/behaviour/opaque/' >> .git/info/exclude

Once installed you need to go to the question type settings page
(Site administration -> Plugins -> Question types -> Opaque) to
set up the URLs of the question engines you wish to use.

https://github.com/moodleou/moodle-local_testopaqueqe can be used to test that
Opaque is working.

To be able to run all the unit tests, you need a working OpenMark install, then you need to add
    define('QTYPE_OPAQUE_TEST_ENGINE_QE',      'http://example.com/om-qe/services/Om');
    define('QTYPE_OPAQUE_TEST_ENGINE_TN',      'http://example.com/openmark/!question');
    define('QTYPE_OPAQUE_TEST_ENGINE_PASSKEY', 'abc123');
    define('QTYPE_OPAQUE_TEST_ENGINE_TIMEOUT', '5');
to your config.php file. Of these, only the first is required. The remaining
ones are optional. Specify them if your set-up needs them.