Struct lmdb_zero::ReadTransaction
[−]
[src]
pub struct ReadTransaction<'env>(_);
A read-only LMDB transaction.
In addition to all operations valid on ConstTransaction
, a
ReadTransaction
can additionally operate on cursors with a lifetime
scoped to the environment instead of the transaction.
Ownership
ReadTransaction
s can be created with all three ownership modes (but owned
mode is not useful).
Example — Shared mode
use std::sync::Arc; let env = Arc::new(create_env()); let db = Arc::new(lmdb::Database::open( env.clone(), None, &lmdb::DatabaseOptions::defaults()).unwrap()); // Type and lifetime annotated explicitly for clarity let txn: lmdb::ReadTransaction<'static> = lmdb::ReadTransaction::new( env.clone()).unwrap(); // Do stuff with `txn`...
Lifetime
All notes for ConstTransaction
apply.
Methods
impl<'env> ReadTransaction<'env>
[src]
pub fn new<E>(env: E) -> Result<Self> where
E: Into<NonSyncSupercow<'env, Environment>>,
[src]
E: Into<NonSyncSupercow<'env, Environment>>,
Opens a new, read-only transaction within the given environment.
Note
A transaction and its cursors must only be used by a single thread
(enforced by the rust compiler), and a thread may only have a single
transaction at a time. If NOTLS
is in use, this does not apply to
read-only transactions. Attempting to open a read-only transaction
while the current thread holds a read-write transaction will deadlock.
pub fn dissoc_cursor<'txn, 'db>(
&self,
cursor: Cursor<'txn, 'db>
) -> Result<StaleCursor<'db>> where
'env: 'db,
[src]
&self,
cursor: Cursor<'txn, 'db>
) -> Result<StaleCursor<'db>> where
'env: 'db,
Dissociates the given cursor from this transaction and its database,
returning a StaleCursor
which can be reused later.
This only fails if cursor
does not belong to this transaction.
Example
let mut saved_cursor; { let txn = lmdb::ReadTransaction::new(&env).unwrap(); let cursor = txn.cursor(&db).unwrap(); // Do some stuff with `txn` and `cursor` // We don't want to realloc `cursor` next time, so save it away saved_cursor = txn.dissoc_cursor(cursor).unwrap(); } // Read transaction goes away, but our saved cursor remains { let txn = lmdb::ReadTransaction::new(&env).unwrap(); // Rebind the old cursor. It continues operating on `db`. let cursor = txn.assoc_cursor(saved_cursor).unwrap(); // Do stuff with txn, cursor // We can save the cursor away again saved_cursor = txn.dissoc_cursor(cursor).unwrap(); }
Example — Shared ownership mode
Cursors can also be dissociated and reassociated with transactions with
shared ownership mode. This can also include changing the ownership
mode. To be able to use shared ownership mode, make sure that the
AssocCursor
trait is imported or else you will simply borrow the
inner transaction instead of taking a copy of the Rc
, etc.
use std::sync::Arc; use lmdb::traits::{AssocCursor, CreateCursor}; // N.B. Unnecessary type and lifetime annotations included for clarity let env: Arc<lmdb::Environment> = Arc::new(create_env()); let db: Arc<lmdb::Database<'static>> = Arc::new(lmdb::Database::open( env.clone(), None, &lmdb::DatabaseOptions::defaults()).unwrap()); let mut saved_cursor: lmdb::StaleCursor<'static>; { // `Arc` is unnecessary in this trivial example, but let's pretend // there was good use for this. let txn: Arc<lmdb::ReadTransaction> = Arc::new( lmdb::ReadTransaction::new(env.clone()).unwrap()); let cursor: lmdb::Cursor<'static, 'static> = txn.cursor(db.clone()).unwrap(); // Do some stuff with `txn` and `cursor` // We don't want to realloc `cursor` next time, so save it away saved_cursor = txn.dissoc_cursor(cursor).unwrap(); } { let txn: Arc<lmdb::ReadTransaction<'static>> = Arc::new(lmdb::ReadTransaction::new(env.clone()).unwrap()); // Rebind the old cursor. It continues operating on `db`. let cursor: lmdb::Cursor<'static, 'static> = txn.assoc_cursor(saved_cursor).unwrap(); // Do stuff with txn, cursor // We can save the cursor away again saved_cursor = txn.dissoc_cursor(cursor).unwrap(); }
pub fn assoc_cursor<'txn, 'db>(
&'txn self,
cursor: StaleCursor<'db>
) -> Result<Cursor<'txn, 'db>>
[src]
&'txn self,
cursor: StaleCursor<'db>
) -> Result<Cursor<'txn, 'db>>
Associates a saved read-only with this transaction.
The cursor will be rebound to this transaction, but will continue using the same database that it was previously.
This method is functionally equivalent to the method on AssocCursor
and exists for convenience and backwards-compatibility.
If you have an, e.g., Rc<ReadTransaction>
and want to get a
Cursor<'static,'db>
, make sure you have the AssocCursor
trait
imported so that the needed alternate implementations of this method
are available.
pub fn reset(self) -> ResetTransaction<'env>
[src]
Resets this transaction, releasing most of its resources but allowing it to be quickly renewed if desired.
Example
let mut saved_txn; { let txn = lmdb::ReadTransaction::new(&env).unwrap(); { let access = txn.access(); // Do stuff with `txn`, `access` } // Save our transaction so we don't have to reallocate it next time, // but we also don't keep locks around and will later move to the // latest version of the environment. saved_txn = txn.reset(); } { // Instead of creating a brand new transaction, renew the one we // saved. let txn = saved_txn.renew().unwrap(); { let access = txn.access(); // Do stuff with `txn`, `access` } // We can save the transaction away again saved_txn = txn.reset(); }
Methods from Deref<Target = ConstTransaction<'env>>
pub fn access(&self) -> ConstAccessor
[src]
Returns an accessor used to manipulate data in this transaction.
Ownership
Unlike most other lmdb-zero APIs, accessors do not support shared
ownership modes (e.g., where the accessor would hold on to a
Rc<ConstTransaction>
). If you need dynamically-managed lifetime,
instead simply drop the accessor and get a new one the next time one is
needed.
Panics
Panics if this function has already been called on this transaction and the returned value has not yet been dropped.
Example
let txn = lmdb::ReadTransaction::new(&env).unwrap(); // Get access the first time let access = txn.access(); // You can't get the accessor again in the same scope, since this // would create two references to the same logical memory and allow // creating aliased mutable references and so forth. let access2 = txn.access(); // PANIC!
pub fn cursor<'txn, 'db, DB>(&'txn self, db: DB) -> Result<Cursor<'txn, 'db>> where
DB: Into<Supercow<'db, Database<'db>>>,
[src]
DB: Into<Supercow<'db, Database<'db>>>,
Creates a new cursor scoped to this transaction, bound to the given database.
This method is functionally equivalent to the method on CreateCursor
and exists for convenience and backwards-compatibility.
If you have an, e.g., Rc<ReadTransaction>
and want to get a
Cursor<'static,'db>
, make sure you have the CreateCursor
trait
imported so that the needed alternate implementations of this method
are available.
pub fn id(&self) -> usize
[src]
Returns the internal id of this transaction.
pub fn db_stat(&self, db: &Database) -> Result<Stat>
[src]
Retrieves statistics for a database.
pub fn db_flags(&self, db: &Database) -> Result<Flags>
[src]
Retrieve the DB flags for a database handle.
Trait Implementations
impl<'txn, 'env: 'txn> CreateCursor<'txn> for &'txn ReadTransaction<'env>
[src]
fn cursor<'db, DB>(&self, db: DB) -> Result<Cursor<'txn, 'db>> where
DB: Into<Supercow<'db, Database<'db>>>,
[src]
DB: Into<Supercow<'db, Database<'db>>>,
Create a cursor using self
as the reference to the containing transaction and db
as the database the cursor will read from and write into. Read more
impl<'txn, 'env: 'txn> AssocCursor<'txn> for &'txn ReadTransaction<'env>
[src]
fn assoc_cursor<'db>(
&self,
cursor: StaleCursor<'db>
) -> Result<Cursor<'txn, 'db>>
[src]
&self,
cursor: StaleCursor<'db>
) -> Result<Cursor<'txn, 'db>>
Associates a saved read-only with this transaction. Read more
impl<'env> Debug for ReadTransaction<'env>
[src]
fn fmt(&self, __arg_0: &mut Formatter) -> Result
[src]
Formats the value using the given formatter. Read more
impl<'env> Deref for ReadTransaction<'env>
[src]
type Target = ConstTransaction<'env>
The resulting type after dereferencing.
fn deref(&self) -> &ConstTransaction<'env>
[src]
Dereferences the value.
impl<'env> DerefMut for ReadTransaction<'env>
[src]
fn deref_mut(&mut self) -> &mut ConstTransaction<'env>
[src]
Mutably dereferences the value.