[FC-0118] docs: ADR for normalizing nested json apis

docs/decisions/0036-normalize-deeply-nested-json-apis.rst

@@ -0,0 +1,182 @@
+
+Reduce Deeply Nested JSON via Minimal/Flattened Views
+=====================================================
+
+:Status: Accepted
+:Date: 2026-04-08
+:Deciders: API Working Group
+
+Context
+=======
+
+Some APIs return deeply nested JSON payloads (course structures, block trees, progress views).
+This makes payloads hard to parse, increases response size, and slows clients and automated agents.
+
+Decision
+========
+
+1. Provide a "minimal" representation option for complex resources:
+
+   * Query param example: ``?view=minimal`` or ``?fields=...``.
+
+2. Normalize/flatten overly nested structures where possible:
+
+   * Prefer references/IDs with follow-up endpoints over embedding entire trees by
+     default. This is the recommended pattern for server-to-server integrations and
+     automated clients (e.g., AI agents), where minimizing coupling to a specific
+     nested shape matters more than minimizing request count.
+
+     **Exception — frontend / MFE clients:** when a client needs to render a complete
+     view in a single page load and the nested data is *always* needed together, an
+     embedded full representation is acceptable to avoid costly sequential requests.
+     In these cases, use an explicit opt-in such as ``?view=full`` or
+     ``?fields=<explicit-list>`` so the heavy payload is never the default and clients
+     can still request only what they need.
+
+3. Document response shapes in OpenAPI, including minimal vs full variants.
+
+.. note::
+   Endpoints that expose a flat list of nodes (e.g. a search result set or an enrollment
+   list) are also subject to :doc:`0032-standardize-pagination-usage`.  For *tree-shaped*
+   responses, see the `Interaction with Pagination (ADR 0032)`_ section below.
+
+Relevance in edx-platform
+=========================
+
+* **Deeply nested payloads**: The course blocks API (``lms/djangoapps/course_api/blocks/views.py``)
+  returns a tree of blocks with ``root`` and ``blocks`` (dict keyed by usage ID), each block
+  containing ``id``, ``type``, ``display_name``, ``children``, ``student_view_data``, etc.
+  Full trees can be large and hard to parse.
+* **Existing flexibility**: Blocks API already supports ``requested_fields`` and
+  ``block_types_filter`` to reduce payload; a ``view=minimal`` or ``fields=...``
+  convention would align with this ADR.
+* **Modulestore/OLX**: ``openedx/core/djangoapps/olx_rest_api/views.py`` returns
+  nested block OLX; providing a minimal (e.g. IDs + types only) view would help
+  clients that only need structure.
+
+Interaction with Pagination (ADR 0032)
+======================================
+
+Tree-shaped endpoints are **out of scope** for ADR 0032's flat-list ``DefaultPagination``
+requirement, because "page N of a tree" is not well-defined when the tree structure itself
+provides the navigation context.  However, unbounded child lists within a tree still pose a
+performance risk.  When a node may have a large number of direct children (e.g. a course
+chapter with hundreds of units, or a library with thousands of components), one of the
+following two patterns MUST be used:
+
+**Pattern A — Depth cap + child-fetch URLs (recommended for tree fidelity)**
+
+Return the full structural representation up to a fixed maximum depth (default ``depth=2``).
+For subtrees beyond that depth, return the child usage ID and a follow-up URL instead of
+embedding the full subtree:
+
+.. code-block:: json
+
+   {
+     "id": "block-v1:edX+Demo+2026+type@chapter+block@week1",
+     "type": "chapter",
+     "display_name": "Week 1",
+     "children": [
+       {
+         "id": "block-v1:...",
+         "type": "sequential",
+         "display_name": "Lesson 1",
+         "children_url": "/api/courses/v1/blocks/block-v1:.../?depth=1"
+       }
+     ]
+   }
+
+This preserves tree semantics while bounding response size.  The depth default and maximum
+MUST be documented and honoured via a ``?depth=N`` query parameter.
+
+**Pattern B — Flat paginated child list (recommended when order/count matters more than
+structure)**
+
+When clients primarily need to enumerate children (e.g. a library component picker or a
+block-type filter), expose a flat paginated sub-resource following ADR 0032:
+
+.. code-block:: text
+
+   GET /api/courses/v1/blocks/<parent_id>/children/?page=1&page_size=50
+
+This separates structural navigation (tree) from bulk enumeration (paginated flat list).
+
+**Guidance for** ``?fields=...`` **with large child sets**
+
+If a client requests a field that would expand a large child collection (e.g.
+``?fields=children.student_view_data``), the server MUST NOT silently return all children
+unbounded.  Apply whichever pattern above is appropriate for the endpoint and document the
+behaviour in OpenAPI.  Returning HTTP 400 with a descriptive message is preferable to
+silently truncating or returning an oversized payload.
+
+The Course Blocks API (``/api/courses/v1/blocks/``) is the canonical reference case: it
+already supports ``?depth=N`` and ``requested_fields``.  Any extension of that API MUST
+continue to honour depth caps and MUST NOT add unbounded field expansions.
+
+Code example
+============
+
+**Query params for minimal representation:**
+
+New endpoints should use ``fields`` (explicit field selection) and/or ``view``
+(named preset) consistently. The existing Blocks API uses ``requested_fields``
+for backwards compatibility; new work should follow the ``fields`` convention.
+
+.. code-block:: text
+
+   GET /api/courses/v1/blocks/<usage_id>/?depth=1&fields=id,type,display_name
+   GET /api/course_structure/v1/?view=minimal
+
+**Response shape (minimal vs full):**
+
+.. code-block:: json
+
+   // minimal (?view=minimal or ?fields=id,type,display_name,children)
+   {
+     "root": "block-v1:...",
+     "blocks": {
+       "block-v1:...": { "id": "...", "type": "chapter", "display_name": "Week 1", "children": ["..."] }
+     }
+   }
+
+   // full: same structure but with student_view_data, completion, block_counts, etc.
+
+**Prefer IDs + follow-up over embedding:**
+
+.. code-block:: text
+
+   GET /api/courses/v1/blocks/  → returns block IDs and types
+   GET /api/courses/v1/blocks/<id>/  → returns full block when needed
+
+Consequences
+============
+
+* Pros
+
+  * Improved performance and developer ergonomics.
+  * Easier integration for external services and AI agents.
+
+* Cons / Costs
+
+  * Must maintain multiple representations: each ``view`` preset or ``fields``
+    variant requires a separate serializer code path, its own test coverage, and a
+    documented schema entry in OpenAPI. All variants must be kept in sync whenever
+    the underlying data model changes (new fields, renamed keys, deprecated sub-objects).
+    Without versioning discipline this becomes a source of subtle divergence bugs
+    and undocumented breaking changes.
+
+Implementation Notes
+====================
+
+* Start with endpoints called out in the standardization notes (course structure, contentstore index,
+  xblock, progress).
+* Measure payload size reduction and client performance improvements.
+
+References
+==========
+
+* :doc:`0032-standardize-pagination-usage` — Pagination standardization ADR. Tree-shaped
+  endpoints are explicitly out of scope for ADR 0032's flat-list pagination requirement;
+  this ADR (0036) governs how deep-structure responses should bound child list sizes at
+  those same endpoints.
+* `Open edX REST API Conventions — “Multiple Formats” <https://openedx.atlassian.net/wiki/spaces/AC/pages/18350757/Open+edX+REST+API+Conventions#10.-Multiple-Formats>`_