⏱ 4 min read Sitemap XML Format urlset with url entries: loc, lastmod, changefreq, priority. Get the required loc field right first — it […] Read article

⏱ 3 min read URL Frontier Priority queue of URLs to crawl, prioritized by PageRank estimate + freshness. Interviewers probe how you stop one Read article

⏱ 3 min read Query Syntax A full-featured query parser supports: AND/OR/NOT boolean operators, phrase queries (exact ordered match), field-specific search (title:python), wildcards (* Read article

⏱ 4 min read Invoice Service: Overview An invoice service manages the full lifecycle of invoices: creation, line items, tax, PDF generation, payment collection, Read article

⏱ 4 min read Overview A payout service disburses funds to recipients via multiple rails: ACH (batch, 1-2 day settlement), wire (same-day, high fee), Read article

⏱ 3 min read Double-Entry Accounting Primer Every financial event produces at least two journal lines: one debit and one credit. The sum of Read article

⏱ 4 min read Experimentation Platform: Low Level Design An experimentation platform enables A/B and multivariate tests with rigorous statistical guarantees. This design covers Read article

⏱ 4 min read Feature Flag Targeting Service: Low Level Design A feature flag targeting service lets teams release features safely by controlling visibility Read article

⏱ 4 min read Cursor State { user_id, user_name, color, position: { line, col } // for text, or { x, y } for Read article

⏱ 3 min read OT for Text Operations are insert(pos, text) and delete(pos, len). These two primitives cover every edit — a replacement is Read article

⏱ 3 min read Quiz Engine Data Model A quiz engine supports timed assessments, randomization, auto-grading, leaderboards, and anti-cheat mechanisms. Core Tables Quiz (id, Read article

⏱ 5 min read Content Archival Service: Storage Tiers The archival service manages long-term content retention across four S3 storage tiers with different cost Read article

⏱ 4 min read Reading List Service: Core Schema The reading list service saves articles for later reading with content extraction and offline availability. Read article

⏱ 4 min read Bookmark Service: Core Schema The bookmark service persists saved URLs with rich metadata and organizational structure. Bookmark Table id BIGSERIAL Read article

⏱ 3 min read Data Model Dashboard Table Dashboard ( id, name, owner_id, layout JSONB, — grid positions created_at ) Widget Table Widget ( Read article