Create a Cloud SQL for MySQL instance. Implement a data model to store to-do list information. Create indexes for the most heavily and frequently used queries.

Create a Bigtable instance. Design a database schema to avoid hotspots when writing data. Use a Bigtable change stream to capture data changes.

Use Firestore as the database. Configure Firestore offline persistence to cache a copy of the Firestore data. Listen to document changes to update applications whenever there are document changes.

Implement a SQLite database on each user's device. Use a scheduled job to synchronize each device database with a copy stored in Cloud Storage.