X/Open XA分布式事务处理规范详解

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《DTP分布式事务处理规范XA》是X/Open（现为The Open Group）制定的一项关键标准，用于协调和管理分布式系统中的事务处理。该规范在1991年由X/Open Company Ltd.发布，旨在解决分布式环境中的数据一致性问题，确保在多个独立数据库或资源之间进行交易操作时的一致性和可靠性。在X/Open的XA规范中，分布式事务处理（DTP）是一个核心概念，它定义了一种标准的方式来发起、管理和完成跨越多个参与者的事务，这些参与者可能位于不同的数据库管理系统（DBMS）中。X/Open DT模型和定义是该规范的核心组成部分，它为开发者提供了一个清晰的框架，来设计和实现跨系统的事务处理逻辑。第1章介绍了XA规范的目的和背景，阐述了在分布式环境下处理复杂业务流程时，确保事务的原子性、一致性、隔离性和持久性（ACID特性）的重要性。章节详细讨论了为何传统的单体事务处理模式不能满足分布式需求，并解释了XA如何通过提供一个可扩展的解决方案来克服这些问题。第2章“Model and Definitions”深入探讨了XA的模型和术语。其中的关键概念包括： 1. **参与者**（Participants）：指参与分布式事务的系统，它们可能是数据库或其他资源。 2. **全局事务管理器**（Global Transaction Manager, GTM）：负责协调整个事务的生命周期，确保所有参与者按照相同的方式执行事务。 3. **资源管理器**（Resource Manager, RM）：每个参与系统的数据库管理组件，负责处理特定资源上的局部事务。 4. **事务上下文**（Transaction Context）：存储有关事务状态和配置信息的数据结构，由GTMs和RMs共享。 5. **两阶段提交协议（Two-Phase Commit, 2PC）**：XA规范中的关键机制，分为预提交（Prepare）和提交（Commit）两个阶段，确保所有参与者一致同意执行事务或者回滚。 XA规范强调了跨资源的事务透明性，使得应用程序无需关心底层的分布式细节，只需遵循统一的接口即可。这大大简化了开发和维护分布式应用的复杂性，同时也提高了系统的可扩展性和可用性。《DTP分布式事务处理规范XA》是现代IT基础设施中的基石，它确保了分布式系统在进行大规模、多节点交互时的数据一致性，对于企业级应用程序、云计算和微服务架构等场景尤为重要。理解并遵循这一标准，是构建可靠分布式应用的基础。

Chapter 2

Model and Deﬁnitions

This chapter discusses the XA interface in general terms and provides necessary

background material for the rest of the speciﬁcation. The chapter shows the

relationship of the interface to the X/Open DTP model. The chapter also states the

design assumptions that the interface uses and shows how the interface addresses

common DTP concepts.

2.1 X/Open DTP Model

The ﬁgure below illustrates a local instance of a DTP system where an AP calls a TM to

structure transactions. The boxes indicate software components in the X/Open DTP

model (see the deﬁnitions in Section 2.2 on page 4). The arrows indicate the directions

in which control ﬂows.

There may be several DTP systems coexisting on the same processor. The boxes in the

ﬁgure below are not necessarily separate processes, nor necessarily a single thread of

control (see Section 2.2.8 on page 6). Furthermore, the components of this model do not

have invariable roles. For example, an RM might use the TX interface to do work in

support of a transaction.

(3) TM and RMs exchange transaction information

(2) AP deﬁnes

transaction

boundaries

through the

TX interface

Resource

Managers

(RMs)

Transaction

Manager

(TM)

Application Program (AP)

(1) AP uses

resources from

a set of RMs

2.1.1 Interfaces between Local TP Components

The subject of this X/Open speciﬁcation is interface (3) in the diagram above, the XA

interface by which TMs and RMs interact.

For more details on this model and diagram, including detailed deﬁnitions of each

component, see the referenced DTP guide.

Distributed Transaction Processing: The XA Speciﬁcation 3

Deﬁnitions Model and Deﬁnitions

2.2 Deﬁnitions

For additional deﬁnitions see the referenced DTP guide.

2.2.1 Transaction

A transaction is a complete unit of work. It may comprise many computational tasks,

which may include user interface, data retrieval, and communications. A typical

transaction modiﬁes shared resources. (The referenced OSI DTP speciﬁcation (model)

deﬁnes transactions more precisely.)

Transactions must be able to be rolled back. A human user may roll back the transaction

in response to a real-world event, such as a customer decision. A program can elect to

roll back a transaction. For example, account number veriﬁcation may fail or the

account may fail a test of its balance. Transactions also roll back if a component of the

system fails, keeping it from retrieving, communicating, or storing data. Every DTP

software component subject to transaction control must be able to undo its work in a

transaction that is rolled back at any time.

When the system determines that a transaction can complete without failure of any

kind, it commits the transaction. This means that changes to shared resources take

permanent effect. Either commitment or rollback results in a consistent state.

Completion means either commitment or rollback.

2.2.2 Distributed Transaction Processing

Within the scope of this document, DTP systems are those where work in support of a

single transaction may occur across RMs. This has several implications:

• The system must have a way to refer to a transaction that encompasses all work

done anywhere in the system.

• The decision to commit or roll back a transaction must consider the status of work

done anywhere on behalf of the transaction. The decision must have uniform effect

throughout the DTP system.

Even though an RM may have an X/Open-compliant interface, such as Structured

Query Language (SQL), it must also address these two items to be useful in the DTP

environment.

2.2.3 Application Program

The AP deﬁnes transactions and accesses resources within transaction boundaries.

Each AP speciﬁes a sequence of operations that involves resources such as terminals

and databases. This speciﬁcation generally uses the term AP to refer to a single

instance of an application program.

X/Open CAE Speciﬁcation (1991)

Model and Deﬁnitions Deﬁnitions

2.2.4 Resource Manager

An RM manages a certain part of the computer’s shared resources. Many other

software entities can request access to the resource from time to time, using services

that the RM provides. Here are some examples of RMs:

• A database management system (DBMS) is an RM. Typical DBMSs are capable of

deﬁning transactions and committing work atomically.

• A ﬁle access method such as the Indexed Sequential Access Method (ISAM) can be

the basis for an RM. Typically, an ISAM RM must be enhanced to support

transactions as deﬁned herein.

• A print server might be implemented as an RM.

A single RM may service multiple independent resource domains. An RM instance

services one of these domains. (See also Section 3.2 on page 13.) Unless speciﬁed

otherwise, operations this speciﬁcation allows on an RM are allowed on each RM

instance.

2.2.5 Global Transactions

Every RM in the DTP environment must support transactions as described in Section

2.2.1 on page 4. Many RMs already structure their work into recoverable units.

In the DTP environment, many RMs may operate in support of the same unit of work.

This unit of work is a global transaction. For example, an AP might request updates to

several different databases. Work occurring anywhere in the system must be

committed atomically. Each RM must let the TM coordinate the RM’s recoverable

units of work that are part of a global transaction.

Commitment of an RM’s internal work depends not only on whether its own

operations can succeed, but also on operations occurring at other RMs, perhaps

remotely. If any operation fails anywhere, every participating RM must roll back all

operations it did on behalf of the global transaction. A given RM is typically unaware

of the work that other RMs are doing. A TM informs each RM of the existence, and

directs the completion, of global transactions. An RM is responsible for mapping its

recoverable units of work to the global transaction.

2.2.6 Transaction Branches

A global transaction has one or more transaction branches (or branches). A branch is a

part of the work in support of a global transaction for which the TM and the RM

engage in a separate but coordinated transaction commitment protocol (see Section 2.3

on page 8). Each of the RM’s internal units of work in support of a global transaction is

part of exactly one branch.

A global transaction might have more than one branch when, for example, the AP uses

multiple processes or is involved in the same global transaction by multiple remote

APs.

After the TM begins the transaction commitment protocol, the RM receives no

additional work to do on that transaction branch. The RM may receive additional work

Distributed Transaction Processing: The XA Speciﬁcation 5

Deﬁnitions Model and Deﬁnitions

on behalf of the same transaction, from different branches. The different branches are

related in that they must be completed atomically.

Each transaction branch identiﬁer (or XID — see Section 4.2 on page 19) that the TM

gives the RM identiﬁes both a global transaction and a speciﬁc branch. The RM may

use this information to optimise its use of shared resources and locks.

2.2.7 Transaction Manager

TMs manage global transactions, coordinate the decision to commit them or roll them

back, and coordinate failure recovery. The AP deﬁnes the start and end of a global

transaction by calling a TM. The TM assigns an identiﬁer to the global transaction (see

Section 4.2 on page 19). The TM manages global transactions and informs each RM of

the XID on behalf of which the RM is doing work. Although RMs can manage their

own recoverable work units as they see ﬁt, each RM must accept XIDs and associate

them with those work units. In this way, an RM knows what recoverable work units to

complete when the TM completes a global transaction.

2.2.8 Thread of Control

A thread of control (or a thread) is the entity, with all its context, that is currently in

control of a processor. A thread of control is an operating-system process: an address

space and single thread of control that executes within that address space, and its

required system resources. The context may include the process’ locks on shared

resources, and the ﬁles the process has open. For portability reasons, the notion of

thread of control must be common among the AP, TM and RM.

The thread concept is central to the TM’s coordination of RMs. APs call RMs to request

work, while TMs call RMs to delineate transaction branches. The way the RM knows

that a given work request pertains to a given branch is that the AP and the TM both call

it from the same thread of control. For example, an AP thread calls the TM to declare the

start of a global transaction. The TM records this fact and informs RMs. After the AP

regains control, it uses the native interface of one or more RMs to do work. The RM

receives the calls from the AP and TM in the same thread of control.

Certain XA routines, therefore, must be called from a particular thread. The reference

manual pages in Chapter 5 indicate which routines require this.

2.2.9 Tightly- and Loosely-coupled Threads

Many application threads of control can participate in a single global transaction. All

the work done in these threads is atomically completed. Within a single global

transaction, the relationship between any pair of participating threads is either tightly-

coupled or loosely-coupled:

• A tightly-coupled relationship is one where a pair of threads are designed to share

resources. In addition, with respect to an RM’s isolation policies, the pair are

treated as a single entity. Thus, for a pair of tightly-coupled threads, the RM must

guarantee that resource deadlock does not occur within the transaction branch.

X/Open CAE Speciﬁcation (1991)

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