XBlock Peak Confusion notes

This is a laundry list of things that are maximally confusing as we approach the point where XBlocks run in edx-platform, but most of the code is still written using XModule concepts.

Outline of things to change to move down from Peak Confusion:

• Per-block runtimes (created by modulestore, module_render)
• Per-block field datas (created by modulestore, module_render)
  • FieldDatas should know how to read/write to the DB, rather than just having local memory state that something else persists
• Separate runtimes for XModules and XModuleDescriptors
• Ball-of-mud runtimes (runtimes without segregated responsibilities)
• Use of the word system
• Conglomeration of serialization (xml import) and storage (in-memory) in the XML Module Store
• Modulestore api (divide responsibilities between FieldData, UsageStore, and separate interface for version management)
• Use of to_json and from_json for client facing code.
  • Use singledispatch to allow for many serialization targets?
• Use of handler_prefix to implement JS handler_url code
• ErrorModules and ErrorDescriptors don't act like the XModules and XModuleDescriptors they replace
  • Use ErrorMixin instead that has an is_error attribute, and possibly overrides views with debugging info?

Old-to-new conversions:

• XModules has .location and class Location. These are implementation details of the CMS/LMS world, and pure XBlock code can't assume ids are implemented this way. We need to move away from .location, and toward usage_ids.

Confusing things:

• ParentTracker: this deals with multiple parents for blocks, but with the usage/definition split, we no longer need to have multiple parents.
• Lots of places that need an id for a block use .location.url(), which will be going away. What should replace it?

XModule Guidelines:

• Don't import xmodule.modulestore.django or xmodule.contentstore.django from inside an XModule.

  • Rational: Those libraries are externally facing in order to make it easy for a django project (LMS and Studio) to interface w/ XModules. By importing one into an XModule, you make that XModule dependent on Django, which will make it harder to convert to an XBlock later

• If an XModule (or XBlock) uses the contents of a field, then that field should be defined on that XModule (or XBlock). Don't rely on the definition of the field in a Mixin.

  • Rational: XBlocks should be usable without the Inheritance mechanism (they should be stand-alone), so that they can be tested independently.

• Don't ask about the course in an XModule

  • Rational: XModules (and XBlocks) aren't always going to live inside a course. We might want to embed one directly into a page. As such, they shouldn't be trying to access their containing course to find out information about it.
  • Alternative: A common reason for trying to get to the course object is to allow users to set an attribute on the course to influence all of the blocks of a particular type inside that course. To solve this problem, use inheritance (provided by the LMS and Studio) instead.

Inheritance example:

Say you want to configure the LTI module to use a shared secret. Add that secret as a field on the LTI module:

class LTIModule(XModule):
    lti_secret = String(scope=Scope.settings)
    def get_html(self):
        ... self.lti_secret ...

Then add that same field to the InheritanceMixin (in xmodule/modulestore/inheritance.py):

class InheritanceMixin(XBlockMixin):
    lti_secret = String(scope=Scope.settings)

Now, in Studio, when you edit course settings, the lti_secret will be available for editing, and when you set it on the course, that setting will cascade down to all LTIModules contained in that course. But if you wanted to use the LTIModule outside of a course, you could configure the lti_secret for just that single module.