| name | cdc |
| description | VelociDB Change Data Capture - the in-memory change log in src/cdc.rs and its executor hooks. Use when modifying cdc.rs, adding CDC hooks to new write paths, changing ChangeEvent fields, or debugging missing/duplicate/out-of-order change events. |
Change Data Capture (CDC)
Turso-inspired real-time change tracking. Opt-in, in-memory, bounded.
Design (src/cdc.rs)
CdcManager holds a VecDeque<ChangeEvent> behind a parking_lot::RwLock,
a monotonic AtomicU64 sequence counter (starts at 1), and an enabled flag.- Capacity-bounded (
DEFAULT_CDC_CAPACITY = 65_536); oldest events are
dropped when full. Consumers detect gaps by comparing sequence numbers.
- Disabled by default.
disable() also clears the log.
- NOT persisted — the log is lost on restart. This is documented behavior.
ChangeEvent: seq, table, op (Insert/Update/Delete), rowid
(primary key), before: Option<Row>, after: Option<Row>.
Insert has no before; Delete has no after.
Public API
Database::{enable_cdc, disable_cdc, cdc_enabled, changes_since(seq), cdc_latest_seq}AsyncConnection::changes_since(seq).await- REPL:
.cdc on|off, .changes [seq]
Hook rules (executor)
One Arc<CdcManager> lives on Database and is passed into Executor::new.
Hooks exist in execute_insert, execute_update, execute_delete in
src/executor.rs. When adding a hook to a new write path:
- Record only after the statement succeeded. UPDATE/DELETE collect
cdc_events into a local Vec inside the B-tree block and call
cdc.record(...) only after the result is Ok — a failed statement's
pages are rolled back by abort_group, so recording early would emit
phantom events.
- Guard row-image cloning with
self.cdc.is_enabled() to keep the disabled
path zero-cost.
- For primary-key-changing UPDATEs,
rowid is the OLD key; the new key is
visible in after.
- ALTER TABLE / DDL is intentionally not captured (matches "data" capture).
Tests
Unit tests in src/cdc.rs (disabled-by-default, ordering, capacity bound);
end-to-end in tests/advanced_features_tests.rs::test_cdc_capture_and_poll
(asserts op sequence, before/after images, incremental polling).
