Transaction Services in Component Frameworks

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Transaction Services in Component Frameworks

• Bruce Kessler
• Comp250CBS
• March 2, 2004

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Overview

• Transactions as Concept
• A Transaction Standard
• Transactions in CORBA
• Transactions in EJB
• Transactions in COM

3
Transactions as Concept

• Definition
• An atomic unit of work. The work in a
  transaction must be completed as a whole if any
  part of the transaction fails, the entire
  transaction fails. - MSDN ODBC Glossary
• The standard that applies to Transactions is the
  ACID definition.

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Transactions as Concept

• ACID is an acronym describing attributes of a
  transaction
• ATOMIC
• CONSISTENT
• ISOLATED
• DURABLE

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Transactions As Concept

• The concept of transactions grew out of RDBMS
  systems. Given the encapsulation and use of
  databases the concept fits cleanly.
• In component frameworks guaranteeing the same
  reliability across components for a distributed
  transaction is more difficult.

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Transactions as Concept

• For Example
• Three systems across a distributed network that
  need to work in unison and guarantee that they
  have all completed their work before anyone can
  commit. If the network goes down, the rollback
  procedure has to be defined by the framework.

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Transactions as Concept

• For Example
• In a bank transaction there is a component that
  updates customer accounts and then another
  component that formats and prints checks. The
  output of this component is a printed check. If
  a rollback is called for by the account
  component, the check has to be rolled back.

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Transaction as Concept

• Given the diversity of the component environment,
  how do you handle transaction processing and
  guarantee the data?

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Transaction as Concept

• Given the diversity of the component environment,
  how do you handle transaction processing and
  guarantee the data?
• Transaction Standards

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A Transaction Standard

• A transaction standard shared by CORBA, EJB, and
  the COM framework is called XA.
• XA is a Distributed Transaction Processing (DTP)
  protocol specified by the X/Open Organization.
• The standard is now maintained by the Open Group.
  

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A Transaction Standard

• X/Open is known for being the standards body for
  the X Windowing System.
• XA started as a system to allow for concurrency
  in operating systems. It was designed with an
  operating system component framework in mind.

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A Transaction Standard

• The parts of the XA standard are
• Application Program (AP)
• Transaction Manager (TM)
• Resource Manager (RM)
• (eg RDBMS, File System)
• The XA standard defines the interaction between
  the TM and the RM.

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A Transaction Standard
From Distributed Transaction Processing The XA
Specification from the Open Group
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A Transaction Standard

• The fundamental process that underlies DTP is the
  two-phase commit.
• Phase 1 Poll all resources to determine whether
  they are able to commit. If one is not able to
  commit, all resources rollback.
• Phase 2 If all resources can commit, then send
  the commit signal to all resources. The resource
  then returns whether it committed successfully.

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Overview

• Transactions as Concept
• A Transaction Standard
• Transactions in CORBA
• Transactions in EJB
• Transactions in COM

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Transactions in CORBA

• CORBA implements the XA DTP standard.
• Objects in CORBA can opt out of providing support
  for transactions, even in the context of a
  transaction taking place in other objects in the
  same context.
• The following diagram is from the OMGs
  Transaction Service Specification v1.4 Sept. 2003

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Transactions in CORBA
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Transactions in CORBA

• The Transaction Service in CORBA is required to
  support flat transactions, but there is a
  description of nested transactions in the
  specification.
• Nested Transactions are transactions that have a
  hierarchical relationship with each other, that
  allow parts to rollback independently of other
  parts.

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Transactions in CORBA

• An example of a nested transaction would allow a
  parent transaction that encapsulates a child
  transaction to continue if part of the child
  transaction fails.
• Nested Transactions allow systems to carry out
  transactions that may have partially failed, but
  are still acceptable for a commit.

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Overview

• Transactions as Concept
• A Transaction Standard
• Transactions in CORBA
• Transactions in EJB
• Transactions in COM

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Transactions in EJB

• Enterprise Java Beans (EJB) use their containers
  to implement transactions but it is interoperable
  with the XA Standard.
• EJB Containers allow transaction management to be
  done with the Java Transaction API (JTA) which is
  a XA compliant implementation or through the
  container itself.

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Transactions in EJB

• Containers allow two types of transactions
• Container Managed Transactions which is usually
  configured in the ejb-jar.xml file and leaves
  transactions to the Container. Options are
• NotSupported
• Supports
• Requires
• RequiresNew
• Mandatory
• Never

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Transactions in EJB

• Additionally, transactions can be managed by
  beans themselves using EJB methods that allow DTP
  within the application server.

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Overview

• Transactions as Concept
• A Transaction Standard
• Transactions in CORBA
• Transactions in EJB
• Transactions in COM

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Transactions in COM

• COM supports transactions through the Microsoft
  Transaction Server (MTS), which is also based on
  the XA standard.
• MTS allows COM components to act as resources in
  the XA resource model, with itself as the
  transaction manager.
• MTS is now incorporated into COM.

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Transactions in COM

• In terms of the XA framework, COM components are
  resources to be managed and the transaction
  manager is the Distributed Transaction
  Coordinator (DTC).
• The DTC is responsible for handling distributed
  transactions and error recovery. Think of it as
  being like the container in EJB.

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Transactions in COM

• In COM there is a concept of Bring Your Own
  Transaction.
• This means that transaction contexts can be
  assigned to components dynamically and they can
  inherit them from other components.
• For example, a transaction that is happening
  within a CORBA OTS, could be associated with a
  COM component and the framework could accept
  this as the transaction context.

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References

• Transaction Service Specification from OMG, v1.4
  Sept 2003
• Distributed Transaction Processing The XA
  Specification from the Open Group
• Java Transaction API Specification from Sun
  Microsystems
• COM Documentation from MSDN (msdn.microsoft.com)