Currency Rate Service

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The service is composed of three internal subsystems:

• Polling Subsystem — scheduler triggers rate collection per configured interval; adapter registry dispatches to generic JSON adapters (config-driven) or custom adapters (code); health tracker records per-provider status;
• Data Layer — all business configuration (pairs, providers, intervals) lives in PostgreSQL; sqlc/pgx provide type-safe access; config loader reads pairs and providers at startup and on reload;
• API Layer — gRPC server exposes GetRate, GetRates, and ListSupportedPairs; an HTTP health endpoint is available for infrastructure monitoring.

External providers (fawazahmed0, ExchangeRate-API, Frankfurter) are contacted only by the polling subsystem — gRPC consumers always read from the local database, guaranteeing sub-50ms response times.

Full architecture diagrams (component and sequence) are available in the SRS document.

Currency Rate Service is part of the Expense Tracker ecosystem and serves as its first independent microservice:

• Expense Tracker Backend (v0.3.0) — primary gRPC consumer; retrieves rates to display currency equivalents alongside transactions;
• Flutter Mobile Client (v0.5.0) — indirect beneficiary via enriched transaction data from the backend;
• Invoice Generator (future) — planned direct consumer for EUR↔RSD conversion in international invoices.

The service operates autonomously — it polls and stores rates regardless of consumer availability. If unreachable, consumers gracefully degrade by showing "rate unavailable" rather than stale data. This establishes the inter-service communication pattern documented in the Expense Tracker gRPC Integration TDD (ADR-6).