CORBA Object Transaction Service Telcordia Contact: Paolo Missier - - PDF document

1 CORBA Object Transaction Service

Telcordia Technologies Proprietary – Internal Use Only This document contains proprietary information that shall be distributed, routed or made available only within Telcordia Technologies, except with written permission of Telcordia Technologies.

Telcordia Contact: Paolo Missier paolo@research.telcordia.com +1 (973) 829 4644 March 29th, 1999

An SAIC Company

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Transactional Support for Distributed Objects

Goals of Distributed Transaction Processing in CORBA:

– Ensure data integrity across multiple heterogeneous sources – Compatibility with existing CORBA services – No extensions to the CORBA core (IDL, ORB) – Interoperability with existing TP Monitors (CICS, Tuxedo, etc.)

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Basic ACID Transactional Model: quick review

Atomicity

– Either all or none of the transaction’s operations are performed – If a transaction is interrupted by a failure, its partial results must be undone

Consistency

– Consistent database state: integrity constraints are not violated – Upon conclusion, transactions leave the database in a consistent state – Consistency degrees are defined to specify the acceptable behavior of concurrent transactions

Isolation

– Serializability: the effect of concurrent transaction execution must be the same as that of some serial order – An incomplete transaction cannot reveal its partial results

Durability

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The need for (Distributed) Transactions:

Canonical example: transactional credit-debit application Other uses of DTP: keeping replicas synchronized

– alternative: use replica managers – may use Corba to handle cached data with write-through propagation to the storage

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Resource Managers

RMs provide ACID operations on a set of data “any subsystem that implements transactional data can be a RM” [Gray] Examples: DBMS, transactional file systems, transactional queue managers

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Transactional Applications

Transaction execution spans several app servers and Resource Managers

Transaction Manager Application Application Servers Application Servers Resource Managers Resource Managers

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The Transaction Manager

Monitors the transaction progress Connects clients to servers Coordinates commit and rollback operations Manages failures (system recovery)

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The X/Open DTP Model

Portable Interfaces: XA and TX

– TX defines the 2-Phase Commit Protocol Application Resource Managers Transaction Manager

Requests Begin Commit Abort Join Prepare Commit Abort

TX XA

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The 2-Phase Commit Protocol

Prepare: Invoke each RMs involved in the transaction, asking for a vote Decide:

– if all RMs vote Yes:

write the commit record in the transaction log Commit: send the commit decision to each RM Complete: When all the RMs acknowledge the commit message, write the end-

• f-transaction record in the log, and deallocate the transaction.

– if at least one RM votes No:

Abort: send the abort decision to each RM Complete: When all the RMs acknowledge the abort message, write the end-of- transaction record in the log, and deallocate the transaction.

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X/Open’s DTP Reference Model for DTP - summary

XA and TX: Protocols for Distributed Transactions Main entities involved in TP:

– The application (AP) – The Resource Manager (RM) – The Transaction Manager (TM)

Procedural interfaces:

– XA between TM and RM – TX between AP and TM

Limitations:

– Interfaces are defined at the programming language level (C, COBOL) – The model does not indicate how invocations are propagated across process/address spaces

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Transaction Processing Monitors

Built as a Transactional layer on top of the Basic OS (TPOS)

– TRPC

Enforce transactional properties of client/server interactions Provide load balancing, execution monitoring and control Intrusive (performance overhead)

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Known Limitations of TP Monitors

Its presence is not transparent to applications and servers Not amenable to distributed object-oriented computing General lack of Java support

– Tuxedo provides a Java-based gateway (JOLT)

Expensive, requires substantial management

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CORBA OTS = DTP + Distributed object computing

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Relation with the X/Open Ref. Model

Two main improvements:

– The procedural XA and TX interfaces are replaced with a set of CORBA IDL interfaces

Resource [10.3.7], Current [10.3.1]

– All inter-component communication (i.e., TM, RMs, AP) occur via CORBA calls on instances of implementations for these interfaces

OTS is fully compatible with X/open-compliant software

– tTransactions can be imported from and exported to [10.4.11] XA- compliant RMs and TX-compliant TMs

OTS defines [optional] support for Nested Transactions

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OTS Functional Goals

Co-existence with legacy transactional environments Support for nested transactions (optional) Model Interoperability:

– interoperability with the X/Open DTP model – Object access to existing Resource Managers

Network interoperability:

– multiple TSs and/or multiple ORBs

Support for TP Monitors

– clients, servers, and TSs may run in separate processes – TS must be usable in a TP Monitor Environment

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OTS Design Goals

Low implementation cost:

– Minimal impact on the existing ORB – Reuse of existing Transaction Managers – Low maintenance, simpler than typical TPM

Portability

– Applications (across OTS implementations) – OTS implementations (across ORBs)

No impact on IDL for existing interfaces

– Implementation of the same IDL can be transactional or non-transactional

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OTS Performance Goals

Achieve performance benchmarks similar to X/Open DTP for:

– number of network messages – number of disk accesses – amount of data logged

Note that X/Open is procedural, not OO. Actual benchmarking of Inprise’s ITS in progress in our group

– ITS performance for Oracle-based transactions against native Oracle distributed transaction facility

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Approaches to OTS implementation: Inprise’s ITS

Fully integrated with the ORB No need for a separate TS on each node Scalable together with the underlying ORB Entirely Java-based RMs can be Corba servers

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Approaches to OTS implementation: IONA’s OTM

The ORBs interfaces with a back-end TP Monitor TS provides interface to specific TPM => users locked into choice of TPM TS cannot be discovered dynamically => TPM required at each node Calls to TPM are not IIOP High maintenance

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Approaches to OTS implementation: BEA’s M3

Existing TP Monitor (Tuxedo) + ORB The TP Framework provides:

– Object state management (activation, deactivation) and lifecycle – Transaction management: interface with M3 transaction manager – System events notification to clients

Clear distinction among:

– Development: framework-based – Deployment

OTS, Security, LifeCycle Fault Management, Routing, Load Balancing

– Operations: management tools

Advantage: tight integration with TPM. Useful framework Disadvantage: tight integration with TPM. Rigid framework

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OTS elements: Transaction Context

Goal: to provide trasnaction synchronization across the elements of a distributed c/s application Originator: the client app that requests transaction initiation to the OTS Transaction Context (Scope):

– Created and managed by the OTS when originator requests transaction initiation; – Shared by participants; – Associated to the originator’s thread; – Propagated to objects that participate in the transaction (transactional

• bjects)

– Propagation can be implicit (done by the OTS), or explicit (as a parameter in CORBA calls)

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OTS Elements: Transactional Objects

An object whose behavior is affected by being invoked within the scope of a transaction Contains (a reference to) persistent data that is manipulated by the transaction Scope of the transaction context:

– a tr.obj. can decide which methods are within the context. Some methods may remain non-transactional

The same interface can have both transactional and nontransactional implementations Transactional objects are used to implement two kind of application servers:

– Transactional Server – Recoverable Server

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Recoverable and Resource Objects

Recoverable objects are Transactional objects that participate in the recovery protocol (after a failure) Resources are server objects that implement the Resource interface (X/Open TX interface):

– prepare() – rollback() – commit() – commit_one_phase()

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Basic scenario for OTS use

To1 Res1 Originator OTS To2 Res2

Begin() Commit() Register() Op1() Op2() SQL1 SQL2 Register() Prepare() Commit() Prepare() Commit()

x ya za {a | }a yb zb {b }b

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Interaction Diagram for Basic Scenario

• riginator

OTS To1 Res1 To2 Res2 1: begin() 2: op1() 3: register_resource() 4: SQL1 5: return 6: return 7: op2() 8: register_resource() 9: SQL2 10: return 11: return CORBA OTS – 26 Telcordia Technologies Proprietary - Internal use only. See proprietary restrictions on title page.

Interaction Diagram for Basic Scenario - 2PC

• riginator

OTS To1 Res1 To2 Res2 12: commit() 13: prepare() 15: prepare() 14: yes 16: yes 17: commit() 18: commit() 19: ack 20: ack

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Transaction Creation: Current interface

Used to begin, end and query the status of a transaction Current defines operations for the management of associations between threads and transactions

– one transaction can be associated to more than one thread

Simple to use. Typical idiom:

• rg.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init();
• rg.omg.CORBA.Object initRef =
• rb.resolve_initial_references("

TransactionCurrent"); current = CurrentHelper.narrow(initRef); current.begin();

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Transaction Creation: TransactionFactory

Idiom: More restrictive than Current. The Control object may be restricted for use in other environments. Can be used to import transactions that originates outside of the TS

CosTransaction::Control c;

• rg.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init();

TransactionFactory Tfactory = TransactionFactoryHelper .bind(orb); c = TFactory->create(timeout);

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Context Management: the Control interface

One Control object is associated with each transaction Control is obtained by:

– Current::get_control() – Current::suspend() – TransactionFactory::create()

It contains a Coordinator and a Terminator Coordinator can be used by a transactional object to:

– register resources: Coordinator::register_resource(in Resource r); – force a transaction to rollback: Coordinator::rollback_only();

Terminator is used by the originator to:

– commit: Terminator::commit(); – rollback: Terminator::rollback();

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Transaction Propagation

A call on a transactional object may or may not be transactional. In the example scenario:

Op1(), Op2() are transactional calls

How is the transaction context passed to To1, To2? Explicit propagation:

– theControl object is passed as a parameter in the call, e.g.

void To1.Op1(in CosTransaction::Control c, …)

Implicit propagation: transactional objects inherit from a marker interface in the IDL:

To : CosTransaction::TransactionalObject

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Resource Registration

Manual registration:

– Res1 implements the CosTransactions::Resource interface – To1 registers Res1 using the Coordinator – Typically, resource registration is the first action in a transactional call – Note: in some implementations, the Coordinator object is not directly available (i.e. in M3). In those cases, this method cannot be used.

Automatic registration:

– Resource has native support from OTS, or it supports the XA/TX interface – Registration is static and occurs before any request to the resource. – OTS can reach the resource’s XA interface

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Notes on XA and native support

Inprise provides integrated XA Resource Directors for some XA- compliant resources. However, it is restricted to specific DBMS (e.g. Oracle) and specific versions Orbix only provides filters for registering XA resources Native support depends on specific arrangements among ORB- DBMS/TP Monitor vendors

– Orbix natively supports the Encina TP monitor by Transarc – Inprise supports generic connection to a TP Monitor – M3 is designed around the Tuxedo TP Monitor

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Transaction Termination

Only the originator should terminate the transaction. Indirect termination:

– Use

Current::commit(in boolean report_heuristics) Current::rollback()

– heuristics = True: call blocks until all resources are done – heuristics = False: call returns after prepare phase (faster for client)

Direct termination:

– Use

Terminator::commit(in boolean report_heuristics) Terminator ::rollback()

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OTS interfaces summary

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Scenario II: Resources as Corba services

To1 Originator OTS To2

Begin() Commit() Register() Op1() Op2() Register() Prepare() Commit() Prepare() Commit()

x ya za { yb zb Res1 Rm1

SQL1

{a Res2 Rm2

SQL2

|a |b

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Multithreaded transaction execution

In Scenario I and II, there is a potential for concurrent execution of parts (a) and (b) However, transaction Control is associated with one thread in the

• riginator

Solution:

– in the originator, split the client into two threads – in each thread, the same Control object ctrl is visible – in each thread, the resume() method is called on ctrl

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Multi-tier Application Deployment Scenario

Client App DBMS TPM

O T S

TX TX/XA SQL IIOP IIOP IIOP IIOP

OrderManager Server Data Wrapper Data Wrapper

presentation B.O. middle tier Wrappers Data Layer

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Multi-tier Scenario: ORB-centric view

Client App

Data Wrapper Data Wrapper

DBMS TPM

OrderManager Server OTS ORB

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Multi-tier deployement detail

Client App Inventory Manager Account Manager Order Manager (originator) Inventory DB Account DB Inventory Storage Manager Oracle client Account Storage Manager Oracle client

O T S

presentation B.O. middle tier Wrappers Data Layer TX TX SQL SQL IIOP IIOP IIOP IIOP