Solutions

Reference designs for every task in ../tasks/, plus pattern references. Each design covers requirements, capacity estimates, architecture, deep dives, failure walkthroughs, trade-offs and an evolution path. The ones marked 🔒 have a dedicated Security, Privacy, and Abuse Prevention section. The rest handle security within their main sections (auth, tenancy, guardrails).

Designs

Area	Solution	Task
Distributed systems	Distributed counter 🔒	task
	Instagram feed	task
	Twitter search 🔒	task
	API gateway + rate limiter	task
	Key-value store 🔒	task
	Job scheduler on Postgres	task
	DAG pipeline orchestration 🔒	task
	Workflow orchestration 🔒	task
Fintech	IBKR-style trading platform 🔒	task
Security	FastAPI RBAC	task
ML systems	Feature store	task
	Recommendation system 🔒	task
	ML inference platform 🔒	task
	Parallel ML training 🔒	task
AI / LLM systems	LLM gateway	task
	RAG platform	task
	AI search engine 🔒	task
	Agent orchestration	task
	AI agent platform	task
	Tool platform	task
	AI observability & evaluation	task

Pattern references

Reference	Use it for
API design patterns	REST/gRPC resource design, pagination, versioning, errors
API message patterns	Sync vs. async messaging, events, sagas
Big-tech API standards	How Google, Stripe, Microsoft and others standardize APIs
Database design best practices	Schema design, indexing, scaling from startup to enterprise
Write-ahead log deep dive	Building a WAL: durability, fsync, recovery
Zero-GC Python	Low-latency Python services

Read a solution after attempting its task. Solutions show one defensible design, not the only one. Where you chose differently, check whether your choice still meets the task's numbers.