feat: bookmarks rm user foreign key db constraint

[johanseto]

Description

Title: Refactor: Remove Database Foreign Key Constraint on User Table in Bookmarks to Optimize Write Performance

Inspired by the courseware student module's good performance.

openedx-platform/lms/djangoapps/courseware/models.py

Line 95 in b1b59dd

student = models.ForeignKey(User, db_index=True, db_constraint=False, on_delete=models.CASCADE)

1. Context & Architecture

The Bookmarks service allows students to save specific course components (e.g., videos, text blocks, or assignments) for quick reference later. Historically, the Bookmark model mapped directly to the central User table using a strict, database-enforced Foreign Key (FK) constraint. This ensured rigid referential integrity at the database layer.

2. The Problem: Write Bottlenecks and Lock Contention

In a high-traffic environment where thousands of students are interacting with the platform simultaneously, the act of creating or deleting a bookmark becomes a micro-transaction that can disproportionately impact the wider database ecosystem.

Because of the hard Foreign Key constraint, every time a new bookmark is inserted or updated, the database engine must acquire a shared lock on the parent User table row to verify the user exists and maintain referential integrity.

• The User table is a "hot" table, concurrently accessed and modified by almost every other model in the system (e.g., CourseEnrollment, Completion, tracking logs).
• Stacking these shared locks for frequent, low-priority write operations like bookmarking causes unnecessary transaction queueing.
• This lock contention drags down the performance of the User table globally, resulting in latency spikes and reducing overall write throughput across the platform.

3. The Solution

This PR drops the explicit database-level Foreign Key constraint linking Bookmarks to the User table (utilizing db_constraint=False in the model definition).

The application will continue to store the user_id as a conceptual relationship to track ownership of the bookmark, but the database will no longer execute strict referential integrity checks or place shared locks on the User table during INSERT or UPDATE operations on bookmarks.

4. Impact & Benefits

• Optimized Write Performance: INSERT operations for bookmarks are now completely decoupled from the User table's lock state, executing immediately without queueing.
• Reduced Database Contention: Eliminating shared locks on the core User table during bookmarking events heavily reduces transaction overhead, improving the latency and responsiveness of the platform during high-traffic spikes.
• Better Scalability for Low-Priority Actions: Bookmarking is a high-frequency, low-criticality action. Isolating its database impact protects core services like authentication, enrollment, and grade calculation from being slowed down by users interacting with course materials.

5. Trade-offs & Considerations

• Application-Level Responsibility: We are trading strict database referential integrity for speed and scale. The database will no longer automatically block user deletions if dependent bookmarks exist.
• Orphaned Record Management: Any cleanup of orphaned bookmarks (e.g., when a user is permanently deleted) must now be handled at the application layer or offloaded to an asynchronous background cleanup job (e.g., via Celery tasks).

Test

run migrations

Inspired also by PR nelc#78

similar approach in openedx/completion#443

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