Core Entities Wallet: (wallet_id, user_id, currency, balance_cents, status=ACTIVE|FROZEN|CLOSED, created_at). One user may have multiple wallets (different currencies). LedgerEntry: (entry_id, wallet_id, […]
The Consensus Problem In a distributed system, multiple nodes must agree on a single value (which node is the leader,
Core Entities LeaveType: (leave_type_id, name, accrual_rate_days_per_month, max_balance_days, carry_over_max_days, is_paid, requires_documentation). Examples: Annual Leave (1.67 days/month, max 30 days), Sick Leave
The Three Pillars of Observability Observability is the ability to understand the internal state of a system from its external
Core Entities Ticket: ticket_id, customer_id, subject, description, priority (LOW, MEDIUM, HIGH, URGENT), status (OPEN, IN_PROGRESS, WAITING_ON_CUSTOMER, RESOLVED, CLOSED), category, assigned_agent_id,
When to Use 2D DP 2D DP uses a table dp[i][j] where i and j represent positions in two sequences,
Core Entities PaymentIntent: intent_id (idempotency key), amount_cents, currency, customer_id, payment_method_id, status (CREATED, PROCESSING, SUCCEEDED, FAILED, CANCELLED), created_at, metadata. PaymentMethod: method_id,
The Sliding Window Technique for Strings A sliding window maintains a contiguous substring without re-scanning from scratch. Two pointers (left
Core Entities Post: post_id, user_id, content, media_urls[], platforms[] (TWITTER, INSTAGRAM, LINKEDIN, FACEBOOK), scheduled_at (UTC), status (DRAFT, SCHEDULED, PUBLISHING, PUBLISHED, FAILED),
Why Distributed Locks? A mutex works within a single process. A database row lock works within a single database. When
Core Entities Order: order_id, customer_id, restaurant_id, driver_id, items[], total_cents, status, placed_at, estimated_delivery_at. OrderStatus (state machine): PLACED -> CONFIRMED -> PREPARING
The Problem with Distributed Transactions In a monolith with a single database, ACID transactions guarantee atomicity: either all operations succeed
The Core Challenge A flash sale sells a limited quantity (e.g., 1000 units) at a discounted price for a short
What Is a Config Management Service? A configuration management service stores application settings that need to change without a code
Core Entities Provider: provider_id, name, timezone, working_hours (JSONB: {“MON”: [“09:00″,”17:00”], …}), slot_duration_minutes (e.g., 30). Appointment: appointment_id, provider_id, client_id, start_time (UTC),
