Change events

# use corvid::{Db};
# let db = Db::open_in_memory()?;
let id = db.subscribe(|ev| {
    println!("{:?} {} {:?}", ev.kind, ev.collection, ev.key);
});
// ... writes fire the callback ...
db.unsubscribe(id);
# Ok::<(), corvid::Error>(())

In-process subscriptions piggyback on the write path — there is no separate event log. Every event carries:

pub struct ChangeEvent {
    pub kind: ChangeKind,     // Insert | Delete
    pub collection: String,
    pub key: Vec<u8>,
}
pub struct SubscriptionId(/* opaque */);

Dispatch semantics

• Synchronous, post-commit, in mutation order. Callbacks run on the writing thread after the transaction commits — when your callback returns, the event is delivered.
• Multiple subscribers all receive identical event vectors, in order.
• Cross-collection tagging: each event names its own collection.
• Callbacks should be fast and non-blocking; a slow callback stalls the writer. Keep a lock-free hand-off if you process asynchronously.
• unsubscribe reports whether the subscription existed; ids are distinct.

Exact event vectors per path

Path	Events
insert (new key)	one Insert
insert (overwrite)	one Insert (the new value)
insert_batch	one Insert per applied key, in batch order
insert_auto	one Insert keyed by the generated key
update returning Some	one Insert
update returning None (delete)	one Delete
update on missing key creating	one Insert
patch creating	one Insert
patch merging	one Insert
compare_and_set applied (write)	one Insert
compare_and_set applied (delete)	one Delete
compare_and_set compare failed	none
delete / delete_batch / delete_where	one Delete per existing key, in order
delete of absent key (incl. edge cascade)	none
link (new or duplicate)	one Insert keyed by the from endpoint — including links to missing endpoints
unlink	none
TTL purge	none (silent cascade)
stranded TTL purge	none

The “duplicate link re-emits insert” and “TTL purge is silent” rows are the surprising ones — both are pinned by the conformance suite.

What events are for

Cache invalidation, derived-state maintenance, audit trails within the process, live UIs. For cross-process notification, run the MCP sidecar or front the engine with your own service — the engine has no networking, by design, and the C ABI excludes subscriptions in v1 (reentrancy across languages; see ABI exclusions).

Next: administration.