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/*! @m_page{{c,java},transactions,Transactions}
@section transactions_acid ACID properties
Transactions provide a powerful abstraction for multiple threads to
operate on data concurrently because they have the following properties:
- Atomicity: all or none of a transaction is completed.
- Consistency: if each transaction maintains some property when considered
separately, then the combined effect of executing the transactions
concurrently will maintain the same property.
- Isolation: developers can reason about transactions as if they run
single-threaded.
- Durability: once a transaction commits, its updates cannot be lost.
WiredTiger supports transactions with the following caveats to the ACID
properties:
- the maximum level of isolation supported is snapshot isolation.
See @ref transaction_isolation for more details.
- transactional updates are made durable by a combination of checkpoints
and logging. See @ref checkpoint for information on checkpoint durability
and @ref durability for information on commit-level durability.
- each transaction's uncommitted changes must fit in memory: for
efficiency, WiredTiger does not write to the log until a transaction
commits.
@section transactions_api Transactional API
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 (whether opened
before or after the WT_SESSION::begin_transaction call), 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 the
operation retried.
The WT_SESSION::rollback_transaction method implicitly resets all
cursors in the session as if the WT_CURSOR::reset method was called,
discarding any cursor position as well as any key and value.
@snippet ex_all.c transaction commit/rollback
@section transactions_implicit Implicit transactions
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
\c 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 cursors in the
WT_SESSION remain positioned. If an implicit transaction fails, all
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 @ref cursors_transactions for more information.
@section transactions_concurrency Concurrency control
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.
@section transaction_isolation Isolation levels
WiredTiger supports <code>read-uncommitted</code>,
<code>read-committed</code> and <code>snapshot</code> isolation levels;
the default isolation level is <code>read-committed</code>.
- <code>read-uncommitted</code>:
Transactions can see changes made by other transactions before those
transactions are committed. Dirty reads, non-repeatable reads and
phantoms are possible.
- <code>read-committed</code>:
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.
- <code>snapshot</code>:
Transactions read the versions of records committed before the transaction
started. Dirty reads and non-repeatable reads are not possible; phantoms
are possible.<br><br>
Snapshot isolation is a strong guarantee, but not equivalent to a
single-threaded execution of the transactions, known as serializable
isolation. Concurrent transactions T1 and T2 running under snapshot
isolation may both commit and produce a state that neither (T1 followed
by T2) nor (T2 followed by T1) could have produced, if there is overlap
between T1's reads and T2's writes, and between T1's writes and T2's
reads.
The transaction isolation level can be configured on a per-transaction
basis:
@snippet ex_all.c transaction isolation
Additionally, the default transaction isolation can be configured and
re-configured on a per-session basis:
@snippet ex_all.c session isolation configuration
@snippet ex_all.c session isolation re-configuration
*/
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