Design a meeting scheduler and shopping cart

You are asked to do two system-design prompts.

Prompt A: Meeting Scheduler

Design a meeting scheduling system (similar to a calendar + scheduling assistant) that allows users to:

• Create meetings with a set of attendees.
• Find available time slots for all attendees within a time range.
• Send invitations and track RSVP status (accepted/declined/tentative).
• Prevent double-booking (or clearly define how conflicts are handled).

Discuss:

• Core APIs.
• Data model and storage choices.
• How you compute availability efficiently.
• Consistency requirements and concurrency control.
• Indexing strategy and performance at scale.

Assume:

• Millions of users, each with many events.
• Time zones matter.

Prompt B: Online Shopping Cart (Multi-merchant)

Design an online shopping cart service where:

• A user can add/update/remove items in their cart.
• The platform is a marketplace: each merchant has a separate cart per user (i.e., user has one cart per merchant).
• You must handle frequent cart updates and decide how to index and store carts.

Discuss:

• Core APIs.
• DB schema (or document model) and indexing.
• How to handle concurrency (multiple devices/tabs).
• How pricing/promotions/inventory are handled (cart vs checkout).
• Caching strategy and data retention (TTL) for abandoned carts.
• Failure modes and consistency trade-offs.