09 transactions new1

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Transactions
Managing concurrent data access

© JBoss, Inc. 2003, 2004. 07/17/04 1

Transactions

 All operations inside a transaction either complete or fail:

Transaction Succeeded

commit

begin
Transaction
Initial State
rollback

Transaction Failed

© JBoss, Inc. 2003, 2004. 2

Acid Properties

 Atomic—Transactions are made up of one or more activities bundled
together as a single unit of work. Atomicity ensures that all the operations in
the transaction happen or that none of them happen. If all the activities
succeed, the transaction is a success. If any of the activities fail, the entire
transaction fails and is rolled back.
 Consistent—Once a transaction ends (whether successful or not), the
system is left in a state that is consistent with the business that it models. The
data should not be corrupted with respect to reality.
 Isolated—Transactions should allow multiple users to work with the same
data, without each user’s work getting tangled up with the others. Therefore,
transactions should be isolated from each other, preventing concurrent reads
and writes to the same data from occurring. (Note that isolation typically
involves locking rows and/or tables in a database.)
 Durable—Once the transaction has completed, the results of the transaction
should be made permanent so that they will survive any sort of system crash.
This typically involves storing the results in a database or some other form of
persistent storage.

© JBoss, Inc. 2003, 2004. 3

Programmatic transaction demarcation

 In a nonmanaged environment, the JDBC API is used to mark
transaction boundaries. You begin a transaction by calling
setAutoCommit(false) on a JDBC Connection and end it by calling
commit(). You may, at any time, force an immediate rollback by
calling rollback().

 In a system that manipulates data in several databases, a particular
unit of work involves access to more than one resource. In this case,
you can’t achieve atomicity with JDBC alone. You need a transaction
manager that can handle several resources in one system
transaction. Such transaction-processing systems expose the Java
Transaction API (JTA) for interaction with the developer. The main
API in JTA is the UserTransaction interface with methods to begin()
and commit() a system transaction.

© JBoss, Inc. 2003, 2004. 4

Hibernate Transaction Interface

 Programmatic transaction management in a Hibernate application
 is exposed to the application developer via the Hibernate
Transaction interface.
 You aren’t forced to use this API—Hibernate also lets you begin and
end JDBC transactions directly, but this usage is discouraged
because it binds your code to direct JDBC.

 In a Java EE environment, a JTA-compatible transaction manager is
available, so you should call the JTA UserTransaction interface to
begin and end a transaction programmatically. However, the
Hibernate Transaction interface also works on top of JTA.

© JBoss, Inc. 2003, 2004. 5

Hibernate Transaction Interface

 org.hibernate.Transaction—Unified transaction demarcation in
Hibernate applications. It works in a nonmanaged plain JDBC
environment and also in an application server with JTA as the
underlying system transaction service. The main benefit, however, is
tight integration with persistence context management—for example,
a Session is flushed automatically when you commit. A
persistence context can also have the scope of this transaction. Use
this API in Java SE if you can’t have a JTA-compatible transaction
service.

 javax.transaction.UserTransaction—Standardized interface for
programmatic transaction control in Java; part of JTA. This should be
your primary choice whenever you have a JTA-compatible transaction
service and want to control transactions programmatically.

© JBoss, Inc. 2003, 2004. 6

Contextual sessions

 Starting with version 3.0.1, Hibernate added the
SessionFactory.getCurrentSession() method. Initially, this
assumed usage of JTA transactions, where the JTA transaction
defined both the scope and context of a current session.

 As of version 3.1, the processing behind
SessionFactory.getCurrentSession() is now pluggable. To that end, a
new extension interface
(org.hibernate.context.CurrentSessionContext) and a new
configuration parameter
(hibernate.current_session_context_class) have been added to
allow pluggability of the scope and context of defining current
sessions.

© JBoss, Inc. 2003, 2004. 7

CurrentSessionContext

 Hibernate comes with three implementations of
org.hibernate.context.CurrentSessionContext :
 1) org.hibernate.context.JTASessionContext - current sessions
are tracked and scoped by a JTA transaction. The processing here is
exactly the same as in the older JTA-only approach.

 2) org.hibernate.context.ThreadLocalSessionContext - current
sessions are tracked by thread of execution.

 3) org.hibernate.context.ManagedSessionContext - current
sessions are tracked by thread of execution. However, you are
responsible to bind and unbind a Session instance with static
methods on this class, it does never open, flush, or close a Session.

© JBoss, Inc. 2003, 2004. 8

hibernate.current_session_context_class

 The hibernate.current_session_context_class configuration
parameter defines which
org.hibernate.context.CurrentSessionContext implementation should
be used.

 For backwards compatibility, if this config param is not set but a
org.hibernate.transaction.TransactionManagerLookup is
configured, Hibernate will use the
org.hibernate.context.JTASessionContext.

 Typically, the value of this parameter would just name the
implementation class to use; for the three out-of-the-box
implementations, there are two corresponding short names, "jta",
"thread", and "managed".

© JBoss, Inc. 2003, 2004. 9

Programmatic transactions in Java SE

 Following is the configuration required in Java SE :
– The hibernate.transaction.factory_class option defaults to
org.hibernate.transaction.JDBCTransactionFactory, which is
the correct factory for the Transaction API in Java SE and for
direct JDBC.
 After you commit the transaction (or roll it back), the database
connection is released and unbound from the Session. Beginning a
new transaction with the same Session obtains another connection
from the pool.

© JBoss, Inc. 2003, 2004. 10

Hibernate in an environment with managed resources

© JBoss, Inc. 2003, 2004. 11

Programmatic transactions with JTA

 Following is the configuration to be done to switch to JTA :
– The hibernate.transaction.factory_class option must be set to
org.hibernate.transaction.JTATransactionFactory.
– Hibernate needs to know the JTA implementation on which you’re
deploying, for two reasons:
• First, different implementations may expose the JTA UserTransaction, which
Hibernate has to call internally now, under different names.
• Second, Hibernate has to hook into the synchronization process of the JTA
transaction manager to handle its caches.
You have to set the
hibernate.transaction.manager_lookup_class option to
configure both: for example, to
org.hibernate.transaction.JBossTransactionManagerLookup.

© JBoss, Inc. 2003, 2004. 12

Configuring for JTA

 In a managed environment, Hibernate no longer creates and
maintains a JDBC connection pool—Hibernate obtains database
connections by looking up a DataSource object in the JNDI registry.
Hence, your Hibernate configuration needs a reference to the JNDI
name where managed connections can be obtained.
 hibernate.connection.datasource=java:/MyDatasource
 hibernate.transaction.factory_class =
org.hibernate.transaction.JTATransactionFactory
 hibernate.transaction.manager_lookup_class =
 org.hibernate.transaction.JBossTransactionManagerLookup

© JBoss, Inc. 2003, 2004. 13

Sample code using UserTransaction

© JBoss, Inc. 2003, 2004. 14


 With default settings, it’s also your responsibility to flush() each
Session manually to synchronize it with the database. You also have
to close all Sessions manually.

 you can enable the
hibernate.transaction.flush_before_completion and/or the
hibernate.transaction.auto_close_session configuration options
and let Hibernate take care of session flushing and closing
automatically. Hibernate will flush and close the session when the
JTATransaction ends.

© JBoss, Inc. 2003, 2004. 15

What does JTATransactionFactory do?

 It enables correct Session scoping and propagation for JTA if you decide to
use the SessionFactory.getCurrentSession() method instead of opening and
closing every Session manually.
– The current Session is bound automatically to the current JTA system
transaction. When the transaction completes, either through commit or
rollback, the persistence context is flushed and the internally bound
current Session is closed.
– It tells Hibernate that you’re planning to call the JTA UserTransaction
interface in your application to start, commit, or roll back system
transactions.
– It also switches the Hibernate Transaction API to JTA, in case you don’t
want to work with the standardized UserTransaction. If you now begin a
transaction with the Hibernate API, it checks whether an ongoing JTA
transaction is in progress and, if possible, joins this transaction. If no JTA
transaction is in progress, a new transaction is started. If you commit or
roll back with the Hibernate API, it sets the system transaction to commit
or roll back.

© JBoss, Inc. 2003, 2004. 16

Why to use JTA’s UserTransaction insteadof Hibernate Transaction?

 U can’t use the Hibernate Transaction interface together with the
getCurrentSession() feature and JTA. You need a Session to call
beginTransaction(), but a Session must be bound to the current JTA
transaction—a chicken and egg problem.

 This again emphasizes that you should always use JTA ‘s
UserTransaction when possible and Hibernate Transaction only if
you can’t use JTA.

© JBoss, Inc. 2003, 2004. 17

JNDI-bound SessionFactory

 If you deploy your application in an environment that supports JNDI,
Hibernate can bind a SessionFactory to JNDI, and you can look it up
there when needed.
 The Hibernate SessionFactory automatically binds itself to JNDI if the
hibernate.session_factory_name property is set to the name of the
JNDI node. If your runtime environment doesn’t provide a default
JNDI context you need to specify a JNDI initial context using the
hibernate.jndi.url and hibernate.jndi.class properties.
 hibernate.session_factory_name = java:/hibernate/MySessionFactory
 hibernate.jndi.class = com.sun.jndi.fscontext.RefFSContextFactory
 hibernate.jndi.url = file:/auction/jndi
 The SessionFactory is bound to JNDI when you build it, which means
when Configuration.buildSessionFactory() is called. To keep your
application code portable, you may want to implement this build and
the lookup in HibernateUtil, and continue using that helper class in
your data access code

© JBoss, Inc. 2003, 2004. 18

Hibernate transaction support

 Hibernate supports system transactions with
– JDBC transaction management if we use a connection pool directly
– JTA transactions in Application Servers
– any custom TransactionFactory and Transaction implementation

The Hibernate Transaction API hides the underlying
strategy and keeps our persistence layer code portable.

© JBoss, Inc. 2003, 2004. 19

Controlling concurrent access

 Databases attempt to ensure transaction isolation, meaning that,
from the point of view of each concurrent transaction, it appears that
no other transactions are in progress.

 This isolation is implemented by Locking.

© JBoss, Inc. 2003, 2004. 20

Transaction isolation issues

 A lost update occurs if two transactions both update a row and then
the second transaction aborts, causing both changes to be lost. This
occurs in systems that don’t implement locking. The concurrent
transactions aren’t isolated.
 A dirty read occurs if a one transaction reads changes made by
another transaction that has not yet been committed. This is
dangerous, because the changes made by the other transaction may
later be rolled back, and invalid data may be written by
 the first transaction.

Lost Update Dirty Read
© JBoss, Inc. 2003, 2004. 21


 An unrepeatable read occurs if a transaction reads a row twice and
reads different state each time. For example, another transaction may
have written to the row and committed between the two reads

 A special case of unrepeatable read is the second lost updates
problem. Imagine that two concurrent transactions both read a row:
One writes to it and commits, and then the second writes to it and
commits. The changes made by the first writer are lost.

Unrepeatable read

© JBoss, Inc. 2003, 2004. 22


 A phantom read is said to occur when a transaction executes a query
twice, and the second result set includes rows that weren’t visible in
the first result set or rows that have been deleted.This situation is
caused by another transaction inserting or deleting rows between the
 execution of the two queries

© JBoss, Inc. 2003, 2004. 23

Read Uncommitted isolation

 A system that permits dirty reads but not lost updates is said to
operate in read uncommitted isolation.
 One transaction may not write to a row if another uncommitted
transaction has already written to it. Any transaction may read
 any row, however.
 This isolation level may be implemented in the database
 management system with exclusive write locks.

© JBoss, Inc. 2003, 2004. 24

Read Committed isolation

 A system that permits unrepeatable reads but not dirty reads is said
to implement read committed transaction isolation.

 This may be achieved by using shared read locks and exclusive write
locks.
 Reading transactions don’t block other transactions from accessing a
row. However, an uncommitted writing transaction blocks all other
transactions from accessing the row.

© JBoss, Inc. 2003, 2004. 25

Repeatable Read Isolation

 A system operating in repeatable read isolation mode permits neither
unrepeatable reads nor dirty reads.

 Phantom reads may occur.

 Reading transactions block writing transactions (but not other
reading transactions), and writing transactions block all other
transactions.

© JBoss, Inc. 2003, 2004. 26

Serializable Isolation

 Serializable provides the strictest transaction isolation. This isolation
level emulates serial transaction execution, as if transactions were
executed one after another, serially, rather than concurrently.

 Serializability may not be implemented using only row-level locks.
There must instead be some other mechanism that prevents a newly
inserted row from becoming visible to a transaction that has already
executed a query that would return the row.

© JBoss, Inc. 2003, 2004. 27

Choosing an isolation Level

 First, eliminate the read uncommitted isolation level. It’s extremely
dangerous to use one transaction’s uncommitted changes in a
different transaction. The rollback or failure of one transaction will
affect other concurrent transactions.

 Secondly, most applications don’t need serializable isolation and this
isolation level tends to scale poorly.

 Repeatable read isolation level eliminates the possibility that one
 transaction can overwrite changes made by another concurrent
transaction (the second lost updates problem) if all data access is
performed in a single atomic database transaction. A read lock held
by a transaction prevents any write lock a concurrent transaction may
wish to obtain. This is an important issue, but enabling repeatable
read isn’t the only way to resolve it.

© JBoss, Inc. 2003, 2004. 28

Choosing an isolation Level

 The combination of the persistence context cache and versioning
already gives you most of the nice features of repeatable read
isolation. In particular, versioning prevents the second lost updates
problem, and the persistence context cache also ensures that the
state of the persistent instances loaded by one transaction is isolated
from changes made by other transactions. So, read-committed
isolation for all database transactions is acceptable if you use
versioned data.

© JBoss, Inc. 2003, 2004. 29

Setting an isolation level

 Every JDBC connection to a database is in the default isolation level
of the DBMS—usually read committed or repeatable read. You can
change this default in the DBMS configuration. You may also set the
transaction isolation for JDBC connections on the application side,
with a Hibernate configuration option:

 hibernate.connection.isolation = 2

 The sensible values for this option are as follows :
 1—Read uncommitted isolation
 2—Read committed isolation
 4—Repeatable read isolation
 8—Serializable isolation

© JBoss, Inc. 2003, 2004. 30

Optimistic concurrency control

 An optimistic approach always assumes that everything will be OK
and that conflicting data modifications are rare. Optimistic
concurrency control raises an error only at the end of a unit of work,
when data is written. Multiuser applications usually default to
optimistic concurrency control and database connections with a read-
committed isolation level.

© JBoss, Inc. 2003, 2004. 31

A Scenario

 Let’s assume that two users select the same piece of data at the
same time. The user in conversation A submits changes first, and the
conversation ends with a successful commit of the second
transaction. Some time later ,the user in conversation B submits
changes. This second transaction also commits successfully. The
changes made in conversation A have been lost, and modifications
of data committed in conversation B may have been based on stale
information.

© JBoss, Inc. 2003, 2004. 32


 U have three choices for how to deal with lost updates in these
second transactions in the conversations:
 Last commit wins—Both transactions commit successfully, and the
second commit overwrites the changes of the first. No error message
is shown.
 First commit wins—The transaction of conversation A is committed,
and the user committing the transaction in conversation B gets an
error message. The user must restart the conversation by retrieving
fresh data and go through all steps of the conversation again with
nonstale data.
 Merge conflicting updates—The first modification is committed, and
the transaction in conversation B aborts with an error message when
it’s committed. The user of the failed conversation B may however
apply changes selectively, instead of going through all the work in the
conversation again.

© JBoss, Inc. 2003, 2004. 33


 If you don’t enable optimistic concurrency control, and by default it
isn’t enabled, your application runs with a last commit wins strategy
 Obviously, first commit wins is much more attractive. If the
application user of conversation B commits, he gets an error message

 Hibernate uses managed versioning for optimistic locking to
implement the second and third strategy

© JBoss, Inc. 2003, 2004. 34

Application Transaction: An example

 Two system administrator edit the same user account and submit:

Last commit wins: Both succeed, the second update overwrites the changes,
no error message is shown.

First commit wins: The first succeeds and the second administrator gets an
error message, he restarts the process with fresh data.

Merge conflicting changes: The first succeeds and the second administrator
merges his changes manually.

 Hibernate can help:
– The first strategy is the default if we don't change anything
– Hibernate uses managed versioning for optimistic locking to implement
the second and third strategy.

© JBoss, Inc. 2003, 2004. 35

Managed versioning

 A timestamp/version is incremented every time the data is updated
– we have to add a new property to our persistent class
– we have to map it with <version> or <timestamp>
– If we are using <timestamp>, instance variable should
be Date
public class Item {
...
private int version;
...
private void setVersion(int version) {
this.version = version;
}
private int getVersion() {
return version;
}
}

<class name="Item" table="ITEM">
<id .....
<version name="version" column="VERSION"/>
...
</class>

© JBoss, Inc. 2003, 2004. 36

Managed versioning in action

 Hibernate will
– automatically set the value of the version/timestamp property
– increment it automatically whenever an item is modified
– throw an exception if the version has been increment meanwhile
– use only a single UPDATE by default (checking the return value)

update ITEM set
DESCRIPTION='New Description',
SELLER=45,
VERSION=3
where
ID=123
and
VERSION=2

 If this update returns zero updated records, someone has increment
this version of the record already and we have stale data!

© JBoss, Inc. 2003, 2004. 37

Managed version without a version/timestamp property

 Optimistic concurrency can be used without extra columns:
Hibernate will use the "old" object state to check all columns for modification
when updating and throw an exception if anything changed.

<class name="Item" table="ITEM" optimistic-lock="all">

<id .....
...
</class>

update ITEM set
DESCRIPTION='New Description', SELLER=45
where ID=123
and DESCRIPTION='Old Description'
and SELLER=45

 Only works for version checks in a
 single Session, no detached objects!

© JBoss, Inc. 2003, 2004. 38

Explicit pessimistic locking

 Consider the following example :

This unit of work executes two reads. The first retrieves an entity instance by
identifier. The second read is a scalar query, loading the description of the
already loaded Item instance again. There is a small window in this unit of work
in which a concurrently running transaction may commit an updated item
description between the two reads. The second read then returns this committed
data, and the variable description has a different value than the property
i.getDescription().
This example is simplified, but it’s enough to illustrate how a unit of work that
mixes entity and scalar reads is vulnerable to reads, if the database transaction
isolation level is read committed. nonrepeatable
© JBoss, Inc. 2003, 2004. 39


 Instead of switching all database transactions into a higher and
nonscalable isolation level, you obtain stronger isolation guarantees
when necessary with the lock() method on the Hibernate Session:

Using LockMode.UPGRADE results in a pessimistic lock held on the
database for the row(s) that represent the Item instance. Now no
concurrent transaction can obtain a lock on the same data .

© JBoss, Inc. 2003, 2004. 40

The Hibernate lock modes

 LockMode.NONE—Don’t go to the database unless the object isn’t in
any cache.
 LockMode.READ—Bypass all caches, and perform a version check
to verify that the object in memory is the same version that currently
exists in the database.
 LockMode.UPDGRADE—Bypass all caches, do a version check (if
applicable), and obtain a database-level pessimistic upgrade lock, if
that is supported. This mode transparently falls back to
LockMode.READ if the database SQL dialect doesn’t support
 a SELECT ... FOR UPDATE option.
 LockMode.UPGRADE_NOWAIT—The same as UPGRADE, but use
a SELECT …FOR UPDATE NOWAIT, if supported. This disables
waiting for concurrent lock releases, thus throwing a locking
exception immediately if the lock can’t be obtained.

© JBoss, Inc. 2003, 2004. 41

The Hibernate lock modes

 LockMode.FORCE—Force an increment of the objects version in the
database, to indicate that it has been modified by the current
transaction.
 LockMode.WRITE—Obtained automatically when Hibernate has
written to a row in the current transaction.

© JBoss, Inc. 2003, 2004. 42

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