Database.PostgreSQL.Simple.Transaction

Transaction handling

withTransaction :: Connection -> IO a -> IO a

Execute an action inside a SQL transaction.

This function initiates a transaction with a "begin transaction" statement, then executes the supplied action. If the action succeeds, the transaction will be completed with commit before this function returns.

If the action throws any kind of exception (not just a PostgreSQL-related exception), the transaction will be rolled back using rollback, then the exception will be rethrown.

For nesting transactions, see withSavepoint.

withTransactionLevel :: IsolationLevel -> Connection -> IO a -> IO a

Execute an action inside a SQL transaction with a given isolation level.

withTransactionMode :: TransactionMode -> Connection -> IO a -> IO a

Execute an action inside a SQL transaction with a given transaction mode.

withTransactionModeRetry :: TransactionMode -> (SqlError -> Bool) -> Connection -> IO a -> IO a

Like withTransactionMode, but also takes a custom callback to determine if a transaction should be retried if an SqlError occurs. If the callback returns True, then the transaction will be retried. If the callback returns False, or an exception other than an SqlError occurs then the transaction will be rolled back and the exception rethrown.

This is used to implement withTransactionSerializable.

withTransactionSerializable :: Connection -> IO a -> IO a

Execute an action inside of a Serializable transaction. If a serialization failure occurs, roll back the transaction and try again. Be warned that this may execute the IO action multiple times.

A Serializable transaction creates the illusion that your program has exclusive access to the database. This means that, even in a concurrent setting, you can perform queries in sequence without having to worry about what might happen between one statement and the next.

Think of it as STM, but without retry.

data TransactionMode

Constructors

TransactionMode

Fields

isolationLevel :: !IsolationLevel

readWriteMode :: !ReadWriteMode

Instances

Eq TransactionMode

Show TransactionMode

data IsolationLevel

Of the four isolation levels defined by the SQL standard, these are the three levels distinguished by PostgreSQL as of version 9.0. See http://www.postgresql.org/docs/9.1/static/transaction-iso.html for more information. Note that prior to PostgreSQL 9.0, RepeatableRead was equivalent to Serializable.

Constructors

DefaultIsolationLevel

the isolation level will be taken from PostgreSQL's per-connection default_transaction_isolation variable, which is initialized according to the server's config. The default configuration is ReadCommitted.

ReadCommitted

RepeatableRead

Serializable

Instances

Bounded IsolationLevel

Enum IsolationLevel

Eq IsolationLevel

Ord IsolationLevel

Show IsolationLevel

data ReadWriteMode

Constructors

DefaultReadWriteMode

the read-write mode will be taken from PostgreSQL's per-connection default_transaction_read_only variable, which is initialized according to the server's config. The default configuration is ReadWrite.

ReadWrite

ReadOnly

Instances

Bounded ReadWriteMode

Enum ReadWriteMode

Eq ReadWriteMode

Ord ReadWriteMode

Show ReadWriteMode

defaultTransactionMode :: TransactionMode

defaultIsolationLevel :: IsolationLevel

defaultReadWriteMode :: ReadWriteMode

begin :: Connection -> IO ()

Begin a transaction.

beginLevel :: IsolationLevel -> Connection -> IO ()

Begin a transaction with a given isolation level

beginMode :: TransactionMode -> Connection -> IO ()

Begin a transaction with a given transaction mode

commit :: Connection -> IO ()

Commit a transaction.

rollback :: Connection -> IO ()

Rollback a transaction.

Savepoint

withSavepoint :: Connection -> IO a -> IO a

Create a savepoint, and roll back to it if an error occurs. This may only be used inside of a transaction, and provides a sort of "nested transaction".

See http://www.postgresql.org/docs/current/static/sql-savepoint.html

data Savepoint

Instances

Eq Savepoint

Ord Savepoint

Read Savepoint

Show Savepoint

Typeable Savepoint

newSavepoint :: Connection -> IO Savepoint

Create a new savepoint. This may only be used inside of a transaction.

releaseSavepoint :: Connection -> Savepoint -> IO ()

Destroy a savepoint, but retain its effects.

Warning: this will throw a SqlError matching isFailedTransactionError if the transaction is aborted due to an error. commit would merely warn and roll back.

rollbackToSavepoint :: Connection -> Savepoint -> IO ()

Roll back to a savepoint. This will not release the savepoint.

rollbackToAndReleaseSavepoint :: Connection -> Savepoint -> IO ()

Roll back to a savepoint and release it. This is like calling rollbackToSavepoint followed by releaseSavepoint, but avoids a round trip to the database server.

Error predicates

isSerializationError :: SqlError -> Bool

isNoActiveTransactionError :: SqlError -> Bool

isFailedTransactionError :: SqlError -> Bool