4.2 transaction

Hệ quản trị cơ sở dữ liệu

Transaction
Dư Phương Hạnh
Bộ môn Hệ thống thông tin
Khoa CNTT, trường Đại học Công nghệ
Đại học Quốc gia Hanoi
hanhdp@vnu.edu.vn

Introducing Transactions
 We live in a transactional world and we perceive that things
such as money, files, and data move from one place to
another.
 We understand that data doesn't really move. It gets copied
from storage and the new copy is inserted into a new storage
location, and then the original copy is deleted from its initial
location.
 In a database, a transaction is simply a mechanism to
ensure and verify that data gets to its intended destination.
Just like a purchase or bank transaction, both parties must
be satisfied with the results.
2

Hệ quản trị CSDL @ BM HTTT

Transaction Types
 Explicit Transaction.
– The explicit transaction is defined by the presence of an
explicit BEGIN TRANSACTION statement followed by
one or more dependent data modification statements and
completed with an explicit COMMIT TRANSACTION
statement.
– Error checking is added prior to the COMMIT
TRANSACTION statement so that if an error occurred the
transaction can be reversed with a ROLLBACK
TRANSACTION statement.

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Transaction Types
 Implicit Transaction.
– The implicit transaction follows the behavior of some
other database products in that whenever a data
modification is executed it implicitly begins a
transaction.
– However, it does not complete the transaction and
release the modified data until an explicit COMMIT
TRANSACTION or ROLLBACK TRANSACTION
statement is issued.
– Implicit transactions are enabled on a connection
basis with the SET IMPLICIT_TRANSACTIONS ON
command.
4


Transaction Types
 Auto-Commit Transaction.
– If a data modification statement is executed against
the database without an explicit or implicit transaction,
it is considered an auto-commit transaction.
– The modification contained in an auto-commit
transaction follows the same pattern as other
transactions

5


The ACID Test
 Most of us have been burned enough by data
loss problems to realize that steps must be
taken to ensure that data gets from one
place to another. Although there are a number
of benefits, this is what transactions are all
about.
 A bona fide transaction must meet the following
criteria:

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The ACID Test
Atomic — All steps and operations that are part of a transaction are
treated as an atomic unit. Either all succeed or all fail together.
Consistent — The outcome of any transaction is always
predictable; all of the operations either fail or succeed. All
operations abide by consistency rules and checks to ensure
data integrity within the database.
Isolated — Any operations performed before, during, or after the
transaction will see related data in a consistent state, rather
than in a state of partial completion. Any user or operation that
queries data affected by a transaction will perceive that the
entire transaction was committed instantaneously.
Durable — If a transaction succeeds, data is written to disk and
does not revert to its previous state. Data can survive system
failure.

7



MySQL Transaction
Dư Phương Hạnh
hanhdp@vnu.edu.vn

Using MySQL Transaction
 To start a transaction you use the START TRANSACTION
statement
 To undo MySQL statements you use ROLLBACK statement.
Note that there are several SQL statements you cannot use
ROLLBACK such as:
– CREATE / ALTER / DROP DATABASE
– CREATE /ALTER / DROP / RENAME / TRUNCATE TABLE
– CREATE / DROP INDEX
– CREATE / DROP EVENT
– CREATE / DROP FUNCTION
– CREATE / DROP PROCEDURE
– …
9


Using MySQL Transaction
 To write the changes into the database within a
transaction you use the COMMIT statement.
 It is important to note that MySQL automatically
commit the changes to the database by default. To
force MySQL not to commit changes automatically,
you need to use the following statement:
SET autocommit = 0;

10


Examples
Using MySQL transaction to add new sale order into
our Classicmodels database and add the transaction
processing steps:
– Start a transaction using START TRANSACTION
– Get latest sale order number from orders table, and use
the next sale order number as the new sale order
number.
– Insert a new sale order into orders table for a given
customer
– Insert new sale order items into orderdetails table
– Commit changes using COMMIT statement
– Get data from both table orders and orderdetails tables to
confirm the changes
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MySQL Transaction

12


MySQL Transaction with savepoint

13


MySQL Transaction with
savepoint
SAVEPOINT identifier
ROLLBACK [WORK] TO [SAVEPOINT] identifier
RELEASE SAVEPOINT identifier

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Isolation level
Dư Phương Hạnh
hanhdp@vnu.edu.vn

Introduce
 Isolation is a property that defines how/when the
changes made by one operation become visible to
other concurrent operations.
 Of the four ACID properties in a DBMS, the isolation
property is the one most often relaxed.
 The isolation levels defined by the ANSI/ISO
SQL standard are listed as follows:

16


Read phenomena
 Dirty Read

17


Read phenomena
 Nonrepeatable read

18


Read phenomena
 Phantom Reads

19


Isolation levels

20


4.2 transaction

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4.2 transaction