Transactions and MVCC

EntDB uses MVCC row versioning with transaction snapshots.

MVCC Visibility Flow

+-------------------------------------------+
| row version                               |
| (created_txn, deleted_txn?)               |
+-------------------------------------------+
                     |
                     v
+-------------------------------------------+
| lookup txn status for created/deleted txns|
+-------------------------------------------+
                     |
                     v
+-------------------------------------------+
| compare commit ts against reader snapshot |
+-------------------------------------------+
                     |
                     v
         +---------------------------+
         | created visible to reader?|
         +---------------------------+
            | no               | yes
            v                  v
   +----------------+   +---------------------------+
   | skip row       |   | deleted txn visible?      |
   +----------------+   +---------------------------+
                             | yes             | no
                             v                 v
                    +----------------+   +----------------+
                    | hide row       |   | return row     |
                    +----------------+   +----------------+

How EntDB applies MVCC

Highlights:

• transaction API and SQL control (BEGIN / COMMIT / ROLLBACK),
• snapshot-based visibility rules,
• write-write conflict detection,
• persisted transaction metadata (*.txn.wal, *.txn.json),
• vacuum controls for version cleanup policy.

Why this is useful

• readers get stable visibility while writers progress concurrently,
• write-write conflicts fail fast instead of silently corrupting visibility,
• restart behavior preserves committed state and hides incomplete work.

Validation coverage

• concurrent transaction matrix tests,
• restart visibility tests,
• crash/recovery scenario tests.

Reference files

• crates/entdb/src/tx.rs
• crates/entdb/src/query/executor/mod.rs
• crates/entdb/src/query/executor/update.rs
• crates/entdb/src/query/executor/delete.rs
• crates/entdb/src/query/tests/mvcc_tests.rs