Wayfair Technical Assessment Screening Fulltime PM Experience

Real-Time Group Chat Architecture

Problem Statement The objective is to design a Minimum Viable Product (MVP) for a real-time group chat application consisting of a backend server and a fronte

Real-Time Group Chat Architecture

Problem Statement The objective is to design a Minimum Viable Product (MVP) for a real-time group chat application consisting of a backend server and a frontend web client. The system must support multiple users joining specific groups, broadcasting messages to all group members, and receiving messages in real-time.

Solution To achieve low-latency, bidirectional communication, the application should utilize

WebSockets. Unlike HTTP, which requires a new request for every interaction, WebSockets establish a persistent, full-duplex connection between the client and server. This allows the server to push new messages to all connected group members instantly without clients needing to poll for updates. If WebSockets are unavailable,

HTTP Long-Polling or

Server-Sent Events (SSE) can act as fallback protocols to maintain real-time functionality. ***

High-Scale Social Network Friend Count

Problem Statement A social network platform anticipating high scalability needs to display the total count of an author's friends next to their posts. The database schema consists of a USER table and a USER_RELATIONSHIP table. Calculating this count dynamically via SQL aggregations (e.g., SELECT COUNT(*)) for every post view is computationally expensive and will not scale effectively as traffic increases.

Solution To handle high traffic, the system should implement denormalization. A friend_count integer column should be added directly to the USER table. This value is updated incrementally using transactions whenever a friendship is established or removed in the USER_RELATIONSHIP table. This approach shifts the cost to the write operation (which is less frequent) and allows for O(1) read performance when loading posts, eliminating the need for expensive join operations or real-time counting. ***

Load Balancing for Collaborative Document Editing

Problem Statement A document collaboration tool manages exclusive document access across 100 server instances using a static modulo load-balancing strategy (document_id % 100).

Performance Analysis As the system grows, this specific routing strategy presents two major scalability issues: 1.

Hotspots: Modulo hashing does not account for load distribution based on usage intensity. If several popular documents hash to the same instance index, that single machine will be overwhelmed while others sit idle. 2.

Inflexibility: Scaling the cluster (adding or removing instances) changes the divisor in the modulo equation. This results in a near-total remapping of document-to-server assignments, causing massive cache invalidation and connection resets. A

Consistent Hashing strategy would resolve this by minimizing the remapping required when the cluster size changes. ***

Consistency Models by Use Case

Problem Statement Different system architectures require different trade-offs between data consistency and availability/latency.

Solution Application *

Media Metadata API (<20ms response requirement):

Eventual Consistency. To meet a strict sub-20ms latency requirement, the system cannot afford the overhead of synchronous replication or locking required for strong consistency. Serving slightly stale metadata is acceptable to maintain speed. *

Web Analytics Platform:

Eventual Consistency. The priority is high write throughput to capture every click without blocking. Analytical data does not need to be instantly consistent across all nodes; a delay in aggregation is acceptable. *

Banking System:

Strong Consistency. Financial transactions (deposits and payments) require strict atomicity and isolation. The system must guarantee that a balance is accurate immediately after a transaction to prevent double-spending or incorrect overdrafts, making latency a necessary trade-off for data integrity.