# transactions.rdoc

Path: | doc/transactions.rdoc
Last Update: | Sat May 19 14:08:31 +0000 2012

# Database Transactions

Sequel uses autocommit mode by default for all of its database adapters, so in general in Sequel if you want to use database transactions, you need to be explicit about it. There are a few cases where transactions are used implicitly by default:

- Dataset#import to insert many records at once
- Model#save
- Model#destroy
- Migrations
- A few model plugins

Everywhere else, it is up to use to use a database transaction if you want to.

## Basic Transaction Usage

In Sequel, the `Database#transaction` method should be called if you want to use a database transaction. This method must be called with a block. If the block does not raise an exception, the transaction is committed:

      DB.transaction do # BEGIN
        DB[:foo].insert(1) # INSERT
      end # COMMIT

If the block raises a Sequel::Rollback exception, the transaction is rolled back, but no exception is raised outside the block:

      DB.transaction do # BEGIN
        raise Sequel::Rollback
      end # ROLLBACK
      # no exception raised

If any other exception is raised, the transaction is rolled back, and the exception is raised outside the block:

      # ArgumentError raised
      DB.transaction do # BEGIN
        raise ArgumentError
      end # ROLLBACK
      # ArgumentError raised

If you want Sequel::Rollback exceptions to be reraised, use the `:rollback => :reraise` option:

      DB.transaction(:rollback => :reraise) do # BEGIN
        raise Sequel::Rollback
      end # ROLLBACK
      # Sequel::Rollback raised

If you always want to rollback (useful for testing), use the `:rollback => :always` option:

      DB.transaction(:rollback => :always) do # BEGIN
        DB[:foo].insert(1) # INSERT
      end # ROLLBACK

If you want to check whether you are currently in a transaction, use the Database#in_transaction? method:

      DB.in_transaction? # false
      DB.transaction do
        DB.in_transaction? # true
      end

## Transaction Hooks

You can add hooks to an in progress transaction that are called after the transaction commits or rolls back:

      x = nil
      DB.transaction do
        DB.after_commit{x = 1}
        DB.after_rollback{x = 2}
        x # nil
      end
      x # 1

      x = nil
      DB.transaction do
        DB.after_commit{x = 1}
        DB.after_rollback{x = 2}
        raise Sequel::Rollback
      end
      x # 2

## Nested Transaction Calls / Savepoints

You can nest calls to transaction, which by default just reuses the existing transaction:

      DB.transaction do # BEGIN
        DB.transaction do
          DB[:foo].insert(1) # INSERT
        end
      end # COMMIT

You can use the `:savepoint => true` option in the inner transaction to explicitly use a savepoint (if the database supports it):

      DB.transaction do # BEGIN
        DB.transaction(:savepoint => true) do # SAVEPOINT
          DB[:foo].insert(1) # INSERT
        end # RELEASE SAVEPOINT
      end # COMMIT

If a Sequel::Rollback exception is raised inside the savepoint block, it will only rollback to the savepoint:

      DB.transaction do # BEGIN
        DB.transaction(:savepoint => true) do # SAVEPOINT
          raise Sequel::Rollback
        end # ROLLBACK TO SAVEPOINT
        # no exception raised
      end # COMMIT

Other exceptions, unless rescued inside the outer transaction block, will rollback the savepoint and the outer transactions, since they are reraised by the transaction code:

      DB.transaction do # BEGIN
        DB.transaction(:savepoint => true) do # SAVEPOINT
          raise ArgumentError
        end # ROLLBACK TO SAVEPOINT
      end # ROLLBACK
      # ArgumentError raised

## Prepared Transactions / Two-Phase Commit

Sequel supports database prepared transactions on PostgreSQL, MySQL, and H2. With prepared transactions, at the end of the transaction, the transaction is not immediately committed (it acts like a rollback). Later, you can call `commit_prepared_transaction` to commit the transaction or `rollback_prepared_transaction` to roll the transaction back. Prepared transactions are usually used with distributed databases to make sure all databases commit the same transaction or none of them do.

To use prepared transactions in Sequel, you provide a string as the value of the :prepare option:

      DB.transaction(:prepare => 'foo') do # BEGIN
        DB[:foo].insert(1) # INSERT
      end # PREPARE TRANSACTION 'foo'

Later, you can commit the prepared transaction:

      DB.commit_prepared_transaction('foo')

or roll the prepared transaction back:

      DB.rollback_prepared_transaction('foo')

## Transaction Isolation Levels

The SQL standard supports 4 isolation levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE. Not all databases implement the levels as specified in the standard (or implement the levels at all), but on PostgreSQL, MySQL, and Microsoft SQL Server, you can specify which transaction isolation level you want to use via the :isolation option to `Database#transaction`. The isolation level is specified as one of the following symbols: :uncommitted, :committed, :repeatable, and :serializable. Using this option make Sequel use the correct transaction isolation syntax for your database:

      DB.transaction(:isolation => :serializable) do # BEGIN
        # SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
        DB[:foo].insert(1) # INSERT
      end # COMMIT

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