Full Raft consensus implementation with optimizations including leadership transfer, joint consensus for membership changes, pre-vote, check-quorum, flow control, and lease-based reads. Used as the replication foundation for the KV layer. Well-tested with extensive testdata scenarios and TLA+ specifications.

Cascades-style cost-based optimizer with memoization, expression normalization, physical property enforcement, and extensible rule system. Supports complex transformations including join reordering, predicate pushdown, index selection, and distributed execution planning. The optgen DSL generates operator definitions and rewrite rules from declarative specifications.

Vectorized columnar execution engine using batch-oriented processing with SIMD-friendly operations. Includes hash joins, hash aggregation, sorting, window functions, and external spill-to-disk operators. The execgen code generator produces type-specialized operator implementations from templates, avoiding reflection overhead.

Declarative online schema changer that breaks DDL into discrete, verifiable steps with automatic rollback. Uses a dependency graph to order operations, supports multi-version upgrades, and validates against a formal specification. The scplan package generates execution plans from SQL statements from high-level schema operations.

Distributed backup/restore system with incremental chains, locality-aware destination URIs, and three restore modes (legacy, fast, online). Uses span covering to map backup SSTs to restore ranges, supports encryption, multi-region, and cross-version restores. The revlog format provides efficient metadata streaming for continuous backup.

Change data capture with support for multiple sink formats (Avro, JSON, Protobuf, CSV, Parquet) and destinations (Kafka, cloud storage, webhooks, SQL). Includes schema registry integration, exactly-once semantics, backfill with protected timestamps, and parallel processing with configurable batching.

CPU and I/O admission control system using token buckets with linear modeling for token estimation. Implements priority-based scheduling, elastic work queues, replicated write admission with tenant fairness, and disk bandwidth limiting. Integrates with the SQL layer for per-statement resource governance.

Custom time-series storage engine for cluster metrics with automatic rollups (10s, 30m, 1h resolutions), efficient compression, and SQL-compatible query interface. Used for the DB Console graphs and cluster observability. Supports multi-tenant isolation and downsampling policies.

Multi-region SQL capabilities including REGIONAL BY ROW/TABLE, super regions, survival goals, and locality-optimized query planning. Implements automatic row rehoming, regional lease preferences, and cross-region query routing with follower reads for bounded staleness.

Pebble LSM storage engine integration with MVCC encoding, intent management, range tombstones, and incremental iterators. Provides batch writing, snapshot isolation, and efficient range operations. The metamorphic testing framework validates correctness against a reference implementation.