Transactions provide a powerful abstraction for multiple threads to operate on data concurrently because they have the following properties:
WiredTiger supports transactions with the following caveats to the ACID properties:
In WiredTiger, transaction operations are methods off the WT_SESSION class.
Applications call WT_SESSION::begin_transaction to start a new transaction. Operations subsequently performed using that WT_SESSION handle, including operations on any cursors open in that WT_SESSION handle, are part of the transaction and their effects committed by calling WT_SESSION::commit_transaction, or discarded by calling WT_SESSION::rollback_transaction.
If WT_SESSION::commit_transaction returns an error for any reason, the transaction was rolled-back, not committed.
When transactions are used, data operations can encounter a conflict and fail with the WT_DEADLOCK error. If this error occurs, transactions should be rolled back with WT_SESSION::rollback_transaction and retried.
The WT_SESSION::begin_transaction, WT_SESSION::commit_transaction and WT_SESSION::rollback_transaction methods all implicitly reset all open cursors in the WT_SESSION, as if WT_CURSOR::reset were called, discarding any position or key/value information they may have.
If a cursor is used when no explicit transaction is active in a session, reads are performed at the isolation level of the session, set with the isolation
key to WT_CONNECTION::open_session, and successful updates are automatically committed before the update operation returns.
Any operation consisting of multiple related updates should be enclosed in an explicit transaction to ensure the updates are applied atomically.
If an implicit transaction successfully commits, the open cursors in the WT_SESSION remain open and positioned. If an implicit transaction fails, all open cursors in the WT_SESSION are reset, as if WT_CURSOR::reset were called, discarding any position or key/value information they may have.
See Cursors and Transactions for more information.
WiredTiger uses optimistic concurrency control algorithms. This avoids the bottleneck of a centralized lock manager and ensures transactional operations do not block: reads do not block writes, and vice versa.
Further, writes do not block writes, although concurrent transactions updating the same value will fail with WT_DEADLOCK. Some applications may benefit from application-level synchronization to avoid repeated attempts to rollback and update the same value.
Operations in transactions may also fail with the WT_DEADLOCK error if some resource cannot be allocated after repeated attempts. For example, if the cache is not large enough to hold the updates required to satisfy transactional readers, an operation may fail and return WT_DEADLOCK.
WiredTiger supports read-uncommitted
, read-committed
and snapshot
isolation levels; the default isolation level is read-committed
.
read-uncommitted
: Transactions can see changes made by other transactions before those transactions are committed. Dirty reads, non-repeatable reads and phantoms are possible.read-committed
: Transactions cannot see changes made by other transactions before those transactions are committed. Dirty reads are not possible; non-repeatable reads and phantoms are possible. Committed changes from concurrent transactions become visible when no cursor is positioned in the read-committed transaction.snapshot
: Transactions read the versions of records committed before the transaction started. Dirty reads and non-repeatable reads are not possible; phantoms are possible.The transaction isolation level can be configured on a per-transaction basis:
Additionally, the default transaction isolation can be configured and re-configured on a per-session basis:
Recovery is run automatically when a data source is opened. Any changes since the last checkpoint are discarded, and the application restarts from a consistent point in the transaction history.
This demonstrates the importance of regular checkpoints: they limit the volume of commits that may be lost in a crash. See WT_SESSION::checkpoint and Checkpoints for further information.