Crate llkv_transaction 

Transaction management and MVCC (Multi-Version Concurrency Control) for LLKV.

This crate provides transaction isolation using MVCC semantics. Each transaction operates with a consistent snapshot of the database, determined by its transaction ID and snapshot timestamp.

Module Organization

• core: Core MVCC primitives (TxnIdManager, TransactionSnapshot, RowVersion) - currently mvcc module
• table: Table-level MVCC integration (row filtering, builders) - currently helpers module
• context: Transaction context types (SessionTransaction, TransactionSession, TransactionManager)
• types: Data type definitions (TransactionResult, TransactionKind, TransactionCatalogSnapshot)

Key Concepts

• Transaction ID (TxnId): Unique 64-bit identifier for each transaction
• Snapshot Isolation: Transactions see a consistent view of data as of their start time
• Row Versioning: Each row tracks when it was created and deleted via created_by and deleted_by columns
• TransactionSnapshot: Captures transaction ID and snapshot timestamp

Reserved Transaction IDs

• TXN_ID_NONE (u64::MAX): Indicates no transaction (uninitialized state)
• TXN_ID_AUTO_COMMIT (1): Used for auto-commit (single-statement) transactions
• IDs 2+: Multi-statement transactions (allocated by TxnIdManager)

Visibility Rules

A row is visible to a transaction if:

1. It was created before the transaction’s snapshot (created_by <= snapshot_id)
2. It was not deleted, or deleted after the snapshot (deleted_by == TXN_ID_NONE || deleted_by > snapshot_id)

Architecture

• TxnIdManager: Allocates transaction IDs and tracks commit status
• TransactionSnapshot: Immutable view of transaction state for visibility checks
• context::TransactionContext: Main interface for executing operations within a transaction
• context::SessionTransaction: Per-transaction state machine
• context::TransactionSession: Session-level transaction management
• context::TransactionManager: Cross-session transaction coordinator
• RowVersion: Metadata tracking which transaction created/deleted a row
• types::TransactionCatalogSnapshot: Catalog snapshot interface for table lookups

Re-exports

pub use context::SessionTransaction;

pub use context::TransactionContext;

pub use context::TransactionManager;

pub use context::TransactionSession;

pub use context::TransactionSessionId;

pub use helpers::MvccRowIdFilter;

pub use helpers::TransactionMvccBuilder;

pub use helpers::filter_row_ids_for_fk_check;

pub use helpers::filter_row_ids_for_snapshot;

pub use mvcc::RowVersion;

pub use mvcc::TXN_ID_AUTO_COMMIT;

pub use mvcc::TXN_ID_NONE;

pub use mvcc::TransactionSnapshot;

pub use mvcc::TxnId;

pub use mvcc::TxnIdManager;

pub use types::TransactionCatalogSnapshot;

pub use types::TransactionKind;

pub use types::TransactionResult;

Modules

context

Transaction context types for orchestrating transaction lifecycles.

helpers

MVCC row filtering and builder utilities.

mvcc

types

Transaction types and result enums.