Pomerium is an identity-aware access proxy that provides zero-trust security for internal web applications and services. It acts as a reverse proxy that authenticates users via OIDC/OAuth2 providers, authorizes access based on policy rules (Pomerium Policy Language evaluated via OPA/Rego), and proxies HTTP, gRPC, WebSocket, and SSH traffic. The system is built as a multi-service architecture with gRPC-based inter-service communication, using Envoy as the underlying proxy engine. Key components include an authentication service, authorization service with policy evaluator, a distributed databroker for session/config storage (with Raft consensus), and a control plane that generates Envoy configuration dynamically. It supports multiple identity providers (Google, Okta, Azure, Auth0, etc.), mTLS client certificates, WebAuthn, and SSH tunneling with a TUI client. The codebase also includes a Model Context Protocol (MCP) implementation for AI tool access control, and a 'Zero' cloud-managed mode for enterprise deployments. External dependencies include Envoy, PostgreSQL, Pebble (RocksDB), and cloud blob storage (S3, GCS, Azure Blob). Architecturally, Pomerium follows a modular design with clear separation of concerns: configuration management (layered sources, validation, envoy config builders), policy engine (parser, generator, criteria matchers), storage abstraction (in-memory, file, postgres, blob with caching and querier patterns), telemetry (OpenTelemetry, Prometheus, health checks), and utility packages (cryptography, HTTP, gRPC, protobuf, retry, fanout). The system is highly configurable with runtime flags and supports hot-reload via config diffing. The codebase demonstrates strong engineering practices with extensive unit, integration, and end-to-end tests (Playwright-based acceptance suites). Data flows: Incoming requests are intercepted by Envoy, which forwards them to the Pomerium proxy service. The proxy authenticates the user via the authenticate service (OIDC/OAuth2 flows), then authorizes via the authorize service (policy evaluation with OPA/Rego). Session and user data are stored in the databroker (distributed key-value store with Raft replication). Configuration is managed centrally and distributed to all services via gRPC streams. Telemetry data (traces, metrics, logs) is exported via OpenTelemetry and Prometheus.