Struct SnapshotId 

pub struct SnapshotId(pub u64);

Snapshot identifier - points to committed transaction state

Only data committed at or before this snapshot_id is visible. If a value cannot be tied to a committed snapshot_id → it does not exist.

Invariant

snapshot_id.0 MUST correspond to a committed transaction. Uncommitted transactions do not create valid snapshots.

Representation

Internally, SnapshotId wraps a TransactionId (u64). This is valid because:

• TransactionId is allocated at begin_transaction()
• SnapshotId is created at commit_transaction()
• The 1:1 mapping ensures snapshot_id uniquely identifies committed state

Tuple Fields

0: u64

Implementations

impl SnapshotId

pub fn current() -> Self

The “current” snapshot - sees only committed data

This returns the most recent committed transaction ID. All reads using this snapshot are guaranteed to see only data that has been durably committed.

Implementation Note

• For native-v2 backend: Returns the maximum committed LSN from the WAL manager
• For SQLite backend: Returns 0 to indicate “all committed data”

Example

let snapshot = SnapshotId::current();
// snapshot now points to the most recent committed transaction

pub fn from_tx(tx_id: u64) -> Self

Create from explicit transaction ID

Arguments

• tx_id - A committed transaction ID

Important

The caller MUST ensure that tx_id corresponds to a committed transaction. Using an uncommitted transaction ID violates snapshot isolation guarantees.

Example

// After commit returns SnapshotId
let snapshot = coordinator.commit_transaction(tx_id)?;
// Later, reuse same snapshot for repeatable reads
let node = backend.get_node(snapshot, node_id)?;

pub fn from_lsn(lsn: u64) -> Self

Create snapshot from explicit LSN (Log Sequence Number)

Arguments

• lsn - A commit LSN representing a committed transaction

Important

The caller MUST ensure that lsn corresponds to a committed transaction. Using an uncommitted LSN violates snapshot isolation guarantees.

Example

// Create snapshot at specific LSN
let snapshot = SnapshotId::from_lsn(12345);

pub fn invalid() -> Self

Invalid snapshot - used for error cases

This sentinel value indicates that no valid snapshot exists. Read operations receiving this snapshot should return an error.

Example

fn validate_snapshot(snapshot: SnapshotId) -> Result<()> {
    if snapshot == SnapshotId::invalid() {
        return Err("Invalid snapshot");
    }
    Ok(())
}

pub fn is_valid(&self) -> bool

Check if this snapshot is valid

Returns false for the invalid sentinel value.

pub fn as_u64(&self) -> u64

Get the underlying transaction ID

pub fn as_lsn(&self) -> u64

Get snapshot as LSN (Log Sequence Number)

Since SnapshotId wraps a commit LSN, this returns the LSN directly. Used for WAL record visibility checks.

Trait Implementations

impl Clone for SnapshotId

fn clone(&self) -> SnapshotId

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for SnapshotId

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Hash for SnapshotId

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for SnapshotId

fn eq(&self, other: &SnapshotId) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for SnapshotId

impl Eq for SnapshotId

impl StructuralPartialEq for SnapshotId