No reads or writes occur except within a transaction. Any command that accesses the database (basically, any SQL command, except a few PRAGMA statements) will automatically start a transaction if one is not already in effect. Automatically started transactions are committed when the last SQL statement finishes.
Transactions can be started manually using the BEGIN command. Such transactions usually persist until the next COMMIT or ROLLBACK command. But a transaction will also ROLLBACK if the database is closed or if an error occurs and the ROLLBACK conflict resolution algorithm is specified. See the documentation on the ON CONFLICT clause for additional information about the ROLLBACK conflict resolution algorithm.
END TRANSACTION is an alias for COMMIT.
Transactions created using BEGIN...COMMIT do not nest. For nested transactions, use the SAVEPOINT and RELEASE commands. The "TO SAVEPOINT name" clause of the ROLLBACK command shown in the syntax diagram above is only applicable to SAVEPOINT transactions. An attempt to invoke the BEGIN command within a transaction will fail with an error, regardless of whether the transaction was started by SAVEPOINT or a prior BEGIN. The COMMIT command and the ROLLBACK command without the TO clause work the same on SAVEPOINT transactions as they do with transactions started by BEGIN.
SQLite supports multiple simultaneous read transactions coming from separate database connections, possibly in separate threads or processes, but only one simultaneous write transaction.
A read transaction is used for reading only. A write transaction allows both reading and writing. A read transaction is started by a SELECT statement, and a write transaction is started by statements like CREATE, DELETE, DROP, INSERT, or UPDATE (collectively "write statements"). If a write statement occurs while a read transaction is active, then the read transaction is upgraded to a write transaction if possible. If some other database connection has already modified the database or is already in the process of modifying the database, then upgrading to a write transaction is not possible and the write statement will fail with SQLITE_BUSY.
While a read transaction is active, any changes to the database that are implemented by separate database connections will not be seen by the database connection that started the read transaction. If database connection X is holding a read transaction, it is possible that some other database connection Y might change the content of the database while X's transaction is still open, however X will not be able to see those changes until after the transaction ends. While its read transaction is active, X will continue to see an historic snapshot of the database prior to the changes implemented by Y.
Transactions can be DEFERRED, IMMEDIATE, or EXCLUSIVE. The default transaction behavior is DEFERRED.
DEFERRED means that the transaction does not actually start until the database is first accessed. Internally, the BEGIN DEFERRED statement merely sets a flag on the database connection that turns off the automatic commit that would normally occur when the last statement finishes. This causes the transaction that is automatically started to persist until an explicit COMMIT or ROLLBACK or until a rollback is provoked by an error or an ON CONFLICT ROLLBACK clause. If the first statement after BEGIN DEFERRED is a SELECT, then a read transaction is started. Subsequent write statements will upgrade the transaction to a write transaction if possible, or return SQLITE_BUSY. If the first statement after BEGIN DEFERRED is a write statement, then a write transaction is started.
IMMEDIATE cause the database connection to start a new write immediately, without waiting for a write statement. The BEGIN IMMEDIATE might fail with SQLITE_BUSY if another write transaction is already active on another database connection.
EXCLUSIVE is similar to IMMEDIATE in that a write transaction is started immediately. EXCLUSIVE and IMMEDIATE are the same in WAL mode, but in other journaling modes, EXCLUSIVE prevents other database connections from reading the database while the transaction is underway.
An implicit transaction (a transaction that is started automatically, not a transaction started by BEGIN) is committed automatically when the last active statement finishes. A statement finishes when its last cursor closes, which is guaranteed to happen when the prepared statement is reset or finalized. Some statements might "finish" for the purpose of transaction control prior to being reset or finalized, but there is no guarantee of this. The only way to ensure that a statement has "finished" is to invoke sqlite3_reset() or sqlite3_finalize() on that statement. An open sqlite3_blob used for incremental BLOB I/O also counts as an unfinished statement. The sqlite3_blob finishes when it is closed.
The explicit COMMIT command runs immediately, even if there are pending SELECT statements. However, if there are pending write operations, the COMMIT command will fail with an error code SQLITE_BUSY.
An attempt to execute COMMIT might also result in an SQLITE_BUSY return code if an another thread or process has an open read connection. When COMMIT fails in this way, the transaction remains active and the COMMIT can be retried later after the reader has had a chance to clear.
In very old versions of SQLite (before version 3.7.11 - 2012-03-20) the ROLLBACK will fail with an error code SQLITE_BUSY if there are any pending queries. In more recent versions of SQLite, the ROLLBACK will proceed and pending statements will often be aborted, causing them to return an SQLITE_ABORT or SQLITE_ABORT_ROLLBACK error. In SQLite version 3.8.8 (2015-01-16) and later, a pending read will continue functioning after the ROLLBACK as long as the ROLLBACK does not modify the database schema.
If PRAGMA journal_mode is set to OFF (thus disabling the rollback journal file) then the behavior of the ROLLBACK command is undefined.
If certain kinds of errors occur within a transaction, the transaction may or may not be rolled back automatically. The errors that can cause an automatic rollback include:
For all of these errors, SQLite attempts to undo just the one statement it was working on and leave changes from prior statements within the same transaction intact and continue with the transaction. However, depending on the statement being evaluated and the point at which the error occurs, it might be necessary for SQLite to rollback and cancel the entire transaction. An application can tell which course of action SQLite took by using the sqlite3_get_autocommit() C-language interface.
It is recommended that applications respond to the errors listed above by explicitly issuing a ROLLBACK command. If the transaction has already been rolled back automatically by the error response, then the ROLLBACK command will fail with an error, but no harm is caused by this.
Future versions of SQLite may extend the list of errors which might cause automatic transaction rollback. Future versions of SQLite might change the error response. In particular, we may choose to simplify the interface in future versions of SQLite by causing the errors above to force an unconditional rollback.
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https://sqlite.org/lang_transaction.html