Building an e-commerce platform means you need shopping carts that work seamlessly whether users are browsing as guests or logged-in customers. A robust shopping cart service sounds simple on the surface, but it becomes complex fast when you factor in device synchronization, session management, and the critical moment when a guest decides to create an account. This is Day 6 of our 365-day system design challenge, and today we're exploring a distributed architecture that keeps carts reliable and user experiences smooth.
Architecture Overview
A distributed shopping cart service sits at the intersection of several key concerns: persistence, availability, and real-time synchronization. At its core, the system needs multiple layers to handle guest carts, authenticated carts, and the merging logic that bridges them. The typical design includes a Cart API that receives requests, a cache layer (usually Redis) for lightning-fast reads, a persistent database for durability, and a session management system that tracks user context across devices and browsers.
The flow works like this: when a user adds an item to their cart, the request hits the Cart API, which immediately writes to the cache for speed and queues a database operation for durability. This dual-write pattern ensures that even if the database is slow or temporarily unavailable, the user experiences instant feedback. For guest carts, the system assigns a unique identifier (often stored in browser cookies or local storage) that maps to a temporary cart record. When a guest logs in, the service triggers a merge operation that intelligently combines their guest cart with any existing authenticated cart.
Device synchronization relies on a combination of techniques. A cart identifier is tied to either the user's account or session ID, allowing the same cart to be retrieved from any device. When a user switches from mobile to desktop, they're fetching the same cart state rather than a duplicate. This requires careful handling of concurrent updates, typically solved through versioning or optimistic locking patterns. The architecture also needs to account for offline scenarios, where clients cache cart state locally and sync when connectivity returns.
Design Insight
Session expiration presents a particularly interesting challenge in cart architecture. When a user's session expires, their authentication token becomes invalid, but their cart shouldn't disappear. The solution lies in treating the cart and session as decoupled concerns. When a session expires, the cart remains persisted in the database, tied to the user's account ID rather than the temporary session token. If the user logs back in within a reasonable window, they retrieve their cart automatically. For guest carts, expiration is handled differently, typically with a time-to-live (TTL) policy that cleans up abandoned carts after days or weeks. The key insight is that session state and cart state follow different lifecycle rules, and conflating them creates data loss and poor user experience.
Watch the Full Design Process
Want to see how this architecture comes together? Watch as AI generates a complete shopping cart service diagram in real-time, visualizing all these components and their interactions:
Try It Yourself
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