Double-Entry Ledger A digital wallet service is built on a double-entry ledger: every financial event creates exactly two entries — […]

Ride Matching Service Low-Level Design A ride matching service connects riders with nearby drivers in real time, computing ETAs, scoring

Interview Scheduling System Low-Level Design An interview scheduling system matches candidate time preferences against interviewer availability, handles calendar integration, prevents

Knowledge Graph Overview A knowledge graph stores entities (people, organizations, places, events) and the typed relationships between them, enabling structured

Search Indexer Overview A search indexer transforms raw documents into a data structure that supports fast full-text queries. The pipeline

Abuse Detection System Overview An abuse detection system protects users and the platform from account takeover, credential stuffing, scraping, fake

Spam Detection System Overview Spam detection combines fast rule-based velocity checks, text classification, and graph-based account analysis. No single signal

Personalization Engine Overview A personalization engine ranks content by predicted user interest rather than global popularity. The core idea: represent

Search Ranking System Overview A search ranking system determines the order in which retrieved documents are presented to users. The

What an Access Log Service Provides Every HTTP request to a web service generates a log record. At scale, this

Two-Token Architecture Modern authentication uses two token types with different properties: Access token: Short-lived (15 minutes), self-contained JWT. Resource servers

What a Session Store Does A session store maintains server-side state for authenticated users across stateless HTTP requests. The client

What an Orchestrator Does A data pipeline orchestrator schedules and executes directed acyclic graphs (DAGs) of computational tasks. Each DAG

What a Consensus Log Provides A consensus log is a fault-tolerant, ordered sequence of records that all nodes in a

Why PII Scrubbing Is Hard PII (Personally Identifiable Information) appears in many forms: structured fields like email addresses and SSNs,