Struct Snapshot 

pub struct Snapshot<S: VersionStore = MemoryStore> { /* private fields */ }

A read-only, point-in-time view of the database.

A snapshot is created by Db::snapshot and reads as of the moment it was taken. It has no write buffer and nothing to commit, so it is cheaper than a transaction when all you need is to read several keys at one consistent instant. While it is alive it pins that instant: garbage collection will not reclaim a version the snapshot can still observe.

Examples

use txn_db::Db;

let db = Db::new();
let mut tx = db.begin();
tx.put(b"k".to_vec(), b"v1".to_vec());
tx.commit()?;

// Capture a snapshot, then change the database.
let snap = db.snapshot();
let mut tx = db.begin();
tx.put(b"k".to_vec(), b"v2".to_vec());
tx.commit()?;

// The snapshot still sees the value as of when it was taken.
assert_eq!(snap.get(b"k")?.as_deref(), Some(&b"v1"[..]));
assert_eq!(db.snapshot().get(b"k")?.as_deref(), Some(&b"v2"[..]));

Implementations

impl<S: VersionStore> Snapshot<S>

pub fn read_timestamp(&self) -> Timestamp

The timestamp this snapshot reads at.

Examples

use txn_db::Db;

let db = Db::new();
assert_eq!(db.snapshot().read_timestamp(), txn_db::Timestamp::ZERO);

pub fn get(&self, key: &[u8]) -> Result<Option<Arc<[u8]>>>

Read the value of key as of this snapshot.

Returns the newest version committed at or before the snapshot timestamp, or None if the key does not exist as of that instant.

Errors

Returns TxnError::Store if the backing store fails the read. The default in-memory store never fails.

Examples

use txn_db::Db;

let db = Db::new();
assert_eq!(db.snapshot().get(b"missing")?, None);