mvcc_cell code review

mvcc_cell code review

(Jin Qing's Column, Aug., 2024)

mvcc_cell: Software-transactional memory for Rust

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Transactions are fully isolated and serialized:

• Each transaction sees a fixed snapshot
• Any concurrent commits will be prevented

Example:

let mvcc = Mvcc::new();

// Create a transactional slot
let slot = MvccCell::new(&mvcc, Box::new(0));

// Start concurrent transactions
let mut t1 = mvcc.begin();
let mut t2 = mvcc.begin();

// Uncommitted values are not visible outside the transaction
t1[&slot] = 42;
assert_eq!(t2[&slot], 0);

// First committer wins, regardless of actual modification order
t2[&slot] = 7;
assert!(t2.try_commit().is_ok());
assert!(t1.try_commit().is_err());

// Transactions always read the values that were current when
// begin() was called.
assert_eq!(mvcc.begin()[&slot], 7);

Features

• The minimal implementation of MVCC
• Uses 2 transaction IDs: begin ID and commit ID
  • 2 transactions can have the same begin ID
  • Commit will increase the global transaction ID
    • TxnId only increased on commit
  • Any overlap of 2 transactions is regarded as confliction
• MvccCell stores a value with its history and pending writings
• Snapshot isolation: reading and writing are within the snapshot
• Non-blocking: no lock on reading and writing
• Transaction
  • The ID of a transaction is private
  • Transaction is not Send nor Sync
• Not two-phase commit: try_commit() will commit
• Writing implies reading
  • Low Performance: any concurrent read or write to the same slot will fail
• Consistency: Serializable
• Vacuum can cleanup the history