Struct RoTxn 

pub struct RoTxn<'e, T = AnyTls> { /* private fields */ }

A read-only transaction.

LMDB Limitations

It’s a must to keep read transactions short-lived.

Active Read transactions prevent the reuse of pages freed by newer write transactions, thus the database can grow quickly.

OSX/Darwin Limitation

At least 10 transactions can be active at the same time in the same process, since only 10 POSIX semaphores can be active at the same time for a process. Threads are in the same process space.

If the process crashes in the POSIX semaphore locking section of the transaction, the semaphore will be kept locked.

Note: if your program already use POSIX semaphores, you will have less available for heed/LMDB!

You may increase the limit by editing it at your own risk: /Library/LaunchDaemons/sysctl.plist

This struct is covariant

#[allow(dead_code)]
trait CovariantMarker<'a>: 'static {
    type R: 'a;

    fn is_covariant(&'a self) -> &'a Self::R;
}

impl<'a, T> CovariantMarker<'a> for heed::RoTxn<'static, T> {
    type R = heed::RoTxn<'a, T>;

    fn is_covariant(&'a self) -> &'a heed::RoTxn<'a, T> {
        self
    }
}

Implementations

impl<'e, T> RoTxn<'e, T>

pub fn id(&self) -> usize

Return the transaction’s ID.

This returns the identifier associated with this transaction. For a RoTxn, this corresponds to the snapshot being read; concurrent readers will frequently have the same transaction ID.

pub fn commit(self) -> Result<()>

Commit a read transaction.

Synchronizing some Env metadata with the global handle.

LMDB

It’s mandatory in a multi-process setup to call RoTxn::commit upon read-only database opening. After the transaction opening, the database is dropped. The next transaction might return Io(Os { code: 22, kind: InvalidInput, message: "Invalid argument" }) known as EINVAL.

Trait Implementations

impl<'a> Deref for RoTxn<'a, WithTls>

type Target = RoTxn<'a>

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl<'a> Deref for RoTxn<'a, WithoutTls>

type Target = RoTxn<'a>

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl<T> Drop for RoTxn<'_, T>

fn drop(&mut self)

Executes the destructor for this type.

impl Send for RoTxn<'_, WithoutTls>

Is sendable only if MDB_NOTLS has been used to open this transaction.