TiDB is a production-grade, open-source distributed SQL database with MySQL compatibility, horizontal scalability, strong consistency (ACID), and HTAP capabilities via TiKV (row store) and TiFlash (columnar store). The architecture separates compute (TiDB Server) from storage (TiKV/TiFlash) with PD for cluster management. Key entry points: cmd/tidb-server (SQL layer), lightning/cmd/tidb-lightning (TB-scale import), br/cmd/br (backup/restore), dumpling/cmd/dumpling (export). The codebase demonstrates exceptional craftsmanship with a modular design: the parser (pkg/parser) is a standalone MySQL-compatible SQL parser used by external projects; pkg/objstore provides a unified storage abstraction (S3/GCS/Azure/local/HDFS) with compression/encryption; pkg/dxf/framework implements a distributed execution framework with scheduler/task-executor/planner; pkg/timer provides distributed cron/interval scheduling with persistence; pkg/extension enables plugin-based authentication/session-hooks/custom-functions; pkg/util/chunk/arena/zeropool/mvmap/intset/bitmap provide high-performance columnar processing and memory management; pkg/expression uses code generation for vectorized SIMD evaluation; pkg/planner implements a Cascades-based cost-based optimizer with plan caching; pkg/ddl executes online schema changes via job-based distributed backfill with schema versioning; pkg/statistics maintains histograms/CMS/FM-sketch with auto-analyze priority queues. The repo includes comprehensive testing (unit, integration, realtikv, chaos) and modern Go practices (generics, interfaces, DI).
Kronotop is a distributed database system built on FoundationDB that provides Redis-compatible (RESP2/RESP3) interfaces for multiple data models including key-value (stash), document (bucket with BSON storage and querying), and sorted sets (zmap). It features ACID transactions, secondary indexing (including vector search via HNSW), clustering, sharding, replication, and namespace isolation for multi-tenancy. The architecture is modular with clear separation between network handling (Netty), protocol processing, service layers (transaction, session, data services), and storage (volume-based segment store). Key entry points include the server entry point (KronotopInstanceStarter), CLI (kronotop-cli), and control tool (kronotop-ctl). The system relies on external dependencies such as FoundationDB, Netty, Jackson, Lettuce, Guava, JVector, and Typesafe Config. The codebase demonstrates strong engineering practices with reusable components like versionstamp utilities, retry mechanisms, and protocol handling.