docs: Add ADR for ensuring GET requests are idempotent

Add edx-platform/docs/decisions/0030-ensure-get-requests-are-idempotent.rst as an accepted ADR.
Define policy that GET endpoints must be strictly read-only, with side effects moved to explicit write endpoints or async event pipelines.
Include edx-platform relevance, anti-pattern vs preferred code examples, and rollout guidance for testing and migration.

Author: Taimoor Ahmed

docs/decisions/0030-ensure-get-requests-are-idempotent.rst

@@ -0,0 +1,92 @@
+===============================
+Ensure GET is Idempotent
+===============================
+
+:Status: Proposed
+:Date: 2026-03-31
+:Deciders: API Working Group
+
+Context
+=======
+
+Some Open edX endpoints use ``GET`` requests that have side-effects (e.g., writing tracking logs,
+recording first access events). This violates REST safety/idempotency expectations and can break
+caching/proxy behavior and automated clients/agents.
+
+Decision
+========
+
+1. Treat ``GET`` as strictly read-only for all REST APIs.
+2. Move side-effect behavior out of ``GET`` handlers:
+
+   * Create explicit write endpoints (``POST``, ``PUT``, ``PATCH``) for state changes.
+   * If telemetry must exist, decouple it using async event pipelines (emit events without
+     mutating domain state) and ensure API responses are not dependent on state mutation.
+
+3. Add regression tests to ensure ``GET`` handlers do not modify domain state.
+4. Document exceptions (if any) and provide migration notes for clients.
+
+Relevance in edx-platform
+=========================
+
+* **GET used with side-effects**: Various views use ``@require_GET`` while
+  triggering writes (e.g. tracking, first-access, or logging). Discussion views
+  (``lms/djangoapps/discussion/views.py``) use ``@require_GET`` for thread/topic
+  listing; any implicit tracking on read should be moved to separate endpoints or
+  async events.
+* **Event emission on read**: ``common/djangoapps/student`` and courseware code
+  sometimes emit events (e.g. ``tracker.emit``, streak updates) in code paths
+  triggered by GET; these should be decoupled so GET handlers do not mutate
+  domain state.
+
+Code example
+============
+
+**Anti-pattern (GET that writes):**
+
+.. code-block:: python
+
+   @require_GET
+   def get_progress(request, course_id):
+       # BAD: recording "first access" or analytics on every GET
+       record_first_access(request.user, course_id)
+       return JsonResponse(compute_progress(...))
+
+**Preferred: read-only GET + optional separate track endpoint**
+
+.. code-block:: python
+
+   @require_GET
+   def get_progress(request, course_id):
+       return Response(ProgressSerializer(compute_progress(...)).data)
+
+   @require_POST
+   def track_progress_view(request, course_id):
+       # Or emit via async pipeline; response does not depend on write
+       emit_progress_viewed_event(request.user, course_id)
+       return Response(status=204)
+
+Consequences
+============
+
+* Pros
+
+  * REST-compliant behavior; safer automated consumption (AI agents, integrations).
+  * Predictable caching/proxy semantics.
+
+* Cons / Costs
+
+  * Requires refactoring legacy courseware/analytics endpoints that currently log on read.
+  * Potential behavior changes for internal analytics that relied on implicit GET-triggered writes.
+
+Implementation Notes
+====================
+
+* Inventory endpoints with GET side-effects.
+* For each, define a read-only GET representation and a separate write/track endpoint (or async
+  event emission) if needed.
+
+References
+==========
+
+* “Non-Idempotent GET Requests” recommendation in the Open edX REST API standardization notes.