This is the Flashbots MEV-Boost Relay, a production-grade Go service that mediates between Ethereum block proposers (validators) and block builders. It implements the proposer-builder separation (PBS) specification, providing REST APIs for builder block submission (with simulation), validator registration, and proposer block retrieval (getHeader/getPayload). The relay is the critical infrastructure that enables validators to outsource block construction to specialized builders, maximizing MEV extraction while maintaining trustless verification. The architecture is a multi-service system with three independently scalable components: an API server (builder, proposer, data, and internal endpoints), a housekeeper background service (validator updates, proposer duties, metrics), and a website service (public status page). It uses PostgreSQL for persistent storage, Redis for low-latency bid caching, optional Memcached for redundant payload storage, and connects to one or more beacon nodes (consensus layer clients) for chain data. Block simulation is performed via an external validation node with a concurrency rate limiter. Key engineering highlights include a multi-beacon-client with failover and least-used ordering, a datastore fallback chain (Redis → Memcached → PostgreSQL), streaming JSON decoding for large validator sets, SSZ/JSON content negotiation with version detection across Ethereum forks (Capella through Fulu), and extensive Prometheus/OpenTelemetry metrics. The codebase is well-tested with integration tests, mock databases, and test data for multiple fork versions. It is actively maintained with recent commits, CI/CD pipelines, and a clear migration path for database schema changes.