OEP-6: Context-scoped XBlock Fields

OEP OEP-6
Title Context-scoped XBlock Fields
Last Modified 2016-12-14
Author Braden MacDonald <braden@opencraft.com>
Arbiter Calen Pennington <cale@edx.org>
Status Provisional
Type Architecture
Created 2016-08-03
Resolution  

Abstract

There is currently no standard way for XBlocks to define data fields such that content, settings, or user input can be shared among multiple XBlocks within a particular “context” such as a course. Such “course-scoped” or “context-scoped” fields are a common requirement; for example, an author may wish to specify the default “number of attempts allowed” once for all problems in a course, or a video player XBlock may want to “remember” the user’s preferred video playback speed for videos within a particular course.

This OEP proposes a new mechanism for defining XBlock fields that are explicitly scoped to a particular context. The generic term “context” refers to a course, content library, blog post, or any other place where XBlocks can be used (see “Rationale”).

Background

The XBlock API uses the abstract notion of “scopes” to associate each field with particular environment(s) and user(s). A field’s Scope is generally a composite property, defined as the combination of a BlockScope, a UserScope, and a name.

For reference, the UserScope can have three values (no changes to these are being proposed):

  • NONE: data not related any particular user, that does not change while users interact with the XBlock
  • ALL: data not related to any particular user, but which may be modified during user interactions
  • ONE: data related to an individual user

The BlockScope can have the following values:

  • USAGE: “The data is related to a particular use of a block in a course.”
  • DEFINITION: “The data is related to the definition of the block” (one

block definition can be used multiple times in a course or even among multiple courses). * TYPE: “The data is related to all instances of this type of XBlock.” * ALL: “The data is common to all blocks.”

XBlock runtimes would generally not be expected to implement every possible combination of BlockScope and UserScope. Instead, certain “named scopes” are defined, which are the only scopes that most runtimes support:

The existing named scopes, and an example use case for each, are as follows:

  • Scope.content (UserScope.NONE, BlockScope.DEFINITION):
    • Example: The text of a multiple choice question
  • Scope.settings (UserScope.NONE, BlockScope.USAGE)
    • Example: The number of attempts allowed for a problem
  • Scope.user_state (UserScope.ONE, BlockScope.USAGE)
    • Example: The student’s answer to a multiple choice question
  • Scope.user_state_summary (UserScope.ALL, BlockScope.USAGE)
    • Example: The number of students who have answered the question (side note: there is no locking or atomicity for XBlock field writes, so in practice this field is currently of little use in a MOOC)
  • Scope.user_info (UserScope.ONE, BlockScope.ALL)
    • Example: user age (note: this scope seems little-used and overlaps with the user service)
  • Scope.preferences (UserScope.ONE, BlockScope.TYPE)
    • Example: (For a video XBlock) Boolean setting to prefer YouTube or non-YouTube video playback.

Motivation

As one would expect, Scope.user_info and Scope.preferences fields in the Open edX platform are implemented globally, and not scoped to the course that uses them. There is currently no standard, extensible way to use XBlock fields to store data that is scoped to a particular “context,” such as a course, although this is a common need.

In the absence of a formal method for achieving context-scoped fields through the XBlock API, many other solutions have been put in place, such as: adding custom Django models to XBlocks, storing fields on the root “course” XBlock, defining inherited fields using InheritanceMixin, using edx-submissions as a generic data store, and more.

It is hoped that the new canonical mechanism described by this OEP can provide consistency, improve interoperability, allow new types of behavior to be implemented by XBlocks, and give both authors and learners additional control over learner interactions with XBlocks.

Specification

New BlockScope value: CONTEXT

To support context-scoped fields, BlockScope will allow a new value: CONTEXT. This new BlockScope will denote data that is related to all instances of this type of XBlock within a particular context.

A “context” is defined by the runtime but is usually a “course” or the nearest equivalent. In the Open edX platform, the context may be a course or a content library. Different runs of the same course would be considered different contexts and would not share data, though UserScope.NONE fields would be copied when creating a new course run. On other platforms, a “context” could be defined as a blog post or a textbook.

New named scopes

As is the situtation today, XBlock runtimes would generally not be expected to implement every possible combination of BlockScope and UserScope. Instead, XBlock runtimes are encouraged to implement these new named scopes, which would be defined in the XBlock python package:

  • Scope.context_settings (UserScope.NONE, BlockScope.CONTEXT)
    • Example: For an LTI XBlock, a list of the LTI provider URLs and secrets used in the course
    • Example: For a discussions XBlock: Enable or disable discussions in this course, as well as settings like “allow anonymous discussion posts”, “allow anonymous discussion posts to peers”, “discussion blackout dates”, etc.
  • Scope.context_preferences (UserScope.ONE, BlockScope.CONTEXT)
    • To avoid performance issues, runtimes are not required to support field values larger than 4 KiB for fields that use this scope. This restriction is intended to avoid the temptation to use this type of XBlock field for logging or message passing. Such uses could cause data loss or performance concerns, because the XBlock API does not support appends nor locking for field data.
    • Example: Enable or disable notifications about new discussion posts in this course
    • Example: Video playback speed for this course
    • Example: An XBlock in which students “accumulate building blocks over the course of the semester” (PLAT-325)

New XBlock view: context_settings_view

An XBlock which uses Scope.context_settings fields will need a way for content authors to edit the data in those fields. (e.g. when creating a course, the course author may wish to change course-wide settings for the discussion XBlock used throughout that course.)

A new XBlock view, context_settings_view shall be used for this purpose. Just like the existing studio_view defined by edX Studio, this new view will allow an XBlock to return an arbitrary HTML Fragment which contains a form or other UI for editing its Scope.context_settings fields.

It is expected (but not required) that runtimes like edX Studio which want to support editing of context-scoped fields will define a list of XBlocks that are enabled within a particular context/course, and will display a list of XBlocks that define the context_settings_view. Clicking on the name of that XBlock would display the HTML fragment and allow the author to edit the context-scoped fields.

Context-scoped fields in OLX (Provisional)

When serializing a course with Scope.context_settings fields to Open Learning XML (OLX), any context-scoped fields could be described within a new <xblock-settings> XML element that is a child of the <course> element. This would typically be found in course/course.xml. Each XBlock class that has Scope.context_settings field data could be represented as a child element of <xblock-settings>, with the name of the element matching the XBlock’s entry point name.

An example follows:

<course display_name="Example Course" language="en" advanced_modules='["drag-and-drop-v2", "lti_consumer", "xblock-dalite"]'>
  <chapter url_name="6915ee3dd6ab403d8c05c0ea3180a0ee"/>
  <wiki slug="A.B.C"/>
  <xblock-settings>
    <discussion allow_anonymous_discussion_posts="true" discussion_blackout_dates='[["2015-09-15", "2015-09-21"], ["2015-10-01", "2015-10-08"]]' />
    <lti_consumer lti_passports='["myapp:key1:secret1", "otherapp:key2:secret2"]' />
    <xblock-dalite dalite_lti_passports='["dalite-ng-demo:https://dalite-ng-demo.example:key:secret"]' />
  </xblock-settings>
</course>

The reasoning behind the above provisional spec is as follows: * Conceptually, in the OLX format, all course content and all XBlock fields are

serialized as XML nodes, within a root <course> node (docs). The entire course’s XML may be in a single course.xml file or spread out into multiple files and subdirectories, where the subdirectory name is the XBlock entry point name (XML node name) and the file name is the block ID (url_name).
  • Since context-scoped fields are XBlock fields just like Scope.content fields, they should be serialized to OLX the same way (XML, not JSON).
  • Many existing course-scoped settings that are analogous to context-scoped fields (discussion settings, LTI passports, etc.) are currently stored as attributes on the <course> XML element.
  • The option of serializing context-scoped fields as namespaced attributes on the <course> element is not feasible as it would be messy and conflicts with the current approach where almost every attribute is an XBlock field belonging to the course XBlock.
  • <xblock-settings> is suggested as a wrapper to keep these new XML nodes organized and indicate their purpose clearly.

Rationale

Many of the ideas in this proposal are driven by the desire for new types of functionality to be implemented via XBlocks, and thus to be pluggable and easily installed, enabled, or disabled for each Open edX installation or each course.

The idea of naming the new scope “context” comes from other technologies such as LTI, where the term “context” is used analogously. For example, when embedding an LTI tool into the Open edX LMS, the LMS will pass the current course ID as the LTI “context” parameter.

Previous discussions of note include:

Other related proposals of note include:

Backward Compatibility

This OEP introduces new field scopes but does not remove or modify any existing functionality. XBlock authors that add new fields to their XBlock may opt-in to using this feature.

XBlock runtimes may choose to implement context-scoped fields or not. Attempting to access a context-scoped field in a runtime that does not support such fields should simply return the default value of the field. Attempting to modify such a context-scoped field in a runtime that doesn’t support such fields should raise an InvalidScopeError.

Some existing XBlocks currently use workarounds for the lack of context-scoped fields, such as storing data in the fields of the root ‘course’ XBlock. Converting such existing techniques to use context-scoped fields instead would be best achieved with an XBlock data migration API, which currently does not exist and is outside the scope of this OEP (however, see here for an example implementation).

Details on the current implementation of field scopes in edx-platform can be found in this gist. The field test XBlock can be used to test how a runtime behaves with different field scopes.

Open Questions

  1. Should runtimes like the LMS be expected to implement a “Preferences” tab for each course that allows students to edit all Scope.context_preferences fields for XBlocks used in that course?
  2. If yes (see previous bullet), can we use a field attribute to allow XBlocks to opt-out of this auto-generated settings UIs on a per-field basis? This may best be addressed at the same time as a standard API for definining editor UI options such as allow_reset, multiline_editor, list_style, and values_provider as implemented in xblock-utils.
  3. Do we need to consider supporting contexts smaller than a course, such as a section or exam? And if so, could that be done in a future change, such as adding a non-scope field property to select a context which defaults to “course”?
    • The author’s thought on this is that we likely cannot come up with a reasonable proposal to support every possible use case, and it’s better to offer simple, flexible, easily understandable support for the main “course” use case now.
  4. Should there be a way to define a field such that the runtime will search through a hierarchy of BlockScope scopes and use the first value found, such as “check for a Scope.settings value, then a Scope.context_settings value”?

Reference Implementation

TBD. Sample code to implement this feature in the XBlock package and the XBlock SDK/Workbench will be produced once the draft has had some initial scrutiny and feedback.

Rejected Alternatives

Additional Scopes

This proposal originally included two new BlockScope values: CONTEXT, and CONTEXT_ALL_TYPES; the latter was a proposed new scope that would be used for field data that is accessible to XBlocks of any type, within a particular context.

It also defined two named scopes that used CONTEXT_ALL_TYPES:

  • Scope.context_shared_preferences (UserScope.ONE, BlockScope.CONTEXT_ALL_TYPES)
    • This scope has been dropped from the proposal as it seems to lack a clear and compelling use case.
  • Scope.context_shared_settings (UserScope.NONE, BlockScope.CONTEXT_ALL_TYPES)
    • This scope has been dropped from the proposal as it seems to lack a clear and compelling use case; further, it is unclear how an authoring experience for such fields would work, since no one XBlock could provide an authoring UI for fields that could be shared by many unrelated XBlocks.

Inheritance

The current proposal deliberately does not support inheritance (i.e. a “context” in the LMS would always be a course, not a “section” or any smaller unit, and field values don’t propagate from parent blocks to children), so it is not a replacement for InheritanceMixin.

Initial discussions of this OEP agreed that context-scoped fields should not participate in inheritance. Improving inheritance of fields or making inheritable fields pluggable are worthy goals, but for a separate proposal.

Context as a New Scope Dimension

Currently, a BlockScope, a UserScope, and a name are combined to make a Scope for a field. Under this alternative option, a new RelevanceScope dimension would be added, which could have one of two values: ALL (default) or CONTEXT. The ALL value would represent the current XBlock API’s definitions, and the CONTEXT value would mean that the field’s relevance is limited to one particular context (e.g. that field’s value relates to one particular course.)

The downside of this is that it creates a large matrix/tensor of scopes: 4 BlockScopes × 3 UserScopes × 2 RelevanceScopes = 24 distinct scope types. Not only is this more difficult to understand, but there is no value in defining a “context-specific” version of any BlockScope.USAGE or BlockScope.DEFINITION fields. As a result of that and other conflicting or not useful Scope combinations, it is likely that fewer than half of the 24 possible Scope values would be used.

Shared data service

Another alternate option is to create a “shared data” or “course-scoped data” XBlock service that XBlocks can use to read and write data at a course level, rather than making this a new type of field. Such an approach seems less consistent, and would not allow the automatic creation of a UI for editing context-scoped XBlock settings. (With the field approach, we can replace the “Advanced Settings” page in Studio with an auto-generated UI that allows authors to configure the course-scoped settings of all XBlocks enabled for that course. We can also generate a “preferences” tab in the LMS that allows students to change XBlock preferences for that course.)