docs: add ADR for pathway catalog and content split

[Agrendalath]

This adds an ADR describing a separation of concerns between the marketing and authoring parts of a pathway.

Related PRs:

• docs: add ADR for pathway catalog and content split #761
• docs: add ADR for pathway and pathway item boundaries #762
• docs: add ADR for mapping pathway items to fulfillment #763
• docs: add ADR for pathway credentials #764

Private-ref: BB-10965

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[ormsbee]

Can you also please add something about the dependency relationship, i.e. openedx_content knows about openedx_catalog, but not vice-versa? That has some implications elsewhere (e.g. Items are allowed to directly reference CourseRuns, and the mapping of CatalogPathway to Pathway content has to be in openedx_content).

[ormsbee]

From a code point of view, I think we should always force it to be specified, and we can create a default database entry for "Pathway". If we do it with a hardcoded fallback-in-code, we're more likely to get inconsistent behaviors.

[Agrendalath]

@ormsbee, good point - added in 9218009.

[kdmccormick]

Can you say more about this? It sounds like a credential will tie together a learner, a catalog pathway, and particular content pathway that "implemented" that catalog pathway.

Is that right? Will an enrollment do the same?

[Agrendalath]

@kdmccormick, I added more details about the enrollments in 9218009. I will add the details about the credentials to #764, since it's out of scope for the current ADR (and I wanted to avoid forward refs here).

In short:

1. Enrollment binds User and Catalog Pathway, because a learner enrolls in a pathway, not its content - i.e., any active authoring (content) changes are visible to the learner. Therefore, I believe there is no real reason to track, at the DB level, the exact state in which the learner was enrolled.
2. Credential is different - it binds User and a specific version of the pathway content, because a credential is a claim that a particular definition of requirements (that was active at that point in time) was met.

[bradenmacdonald]

Overall sounds good, though +1 to @ormsbee's comment.

[bradenmacdonald]

Keeping the two apart makes that possible without inventing field-level permissions.

In our new RBAC world, I would expect that there's not much difference between implementing "field-level" permissions and "model-level" permissions, since we're using Casbin to define whatever roles and permissions make sense for each API. Either way, it's simply permissions and needs to be enforced in the REST API via permission_classes as appropriate.

However, I still think this split makes sense for the other reasons you've stated.

[Agrendalath]

@bradenmacdonald, ah, that's good to know - I'm not very familiar with the recent RBAC changes. Should we remove this point from the ADR?