For

transaction processing Transaction processing is information processing in computer science that is divided into individual, indivisible operations called ''transactions''. Each transaction must succeed or fail as a complete unit; it can never be only partially compl ...

computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

, the X/Open XA

standard Standard may refer to: Symbols * Colours, standards and guidons, kinds of military signs * Standard (emblem), a type of a large symbol or emblem used for identification Norms, conventions or requirements * Standard (metrology), an object th ...

(short for "eXtended Architecture") is a specification released in 1991 by

X/Open X/Open group (also known as the Open Group for Unix Systems and incorporated in 1987 as X/Open Company, Ltd.) was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of info ...

(which later merged with

The Open Group The Open Group is a global consortium that seeks to "enable the achievement of business objectives" by developing "open, vendor-neutral technology standards and certifications." It has over 840 member organizations and provides a number of servi ...

) for

distributed transaction processing A distributed transaction is a database transaction in which two or more network hosts are involved. Usually, hosts provide transactional resources, while the transaction manager is responsible for creating and managing a global transaction that en ...

(DTP).

Goals

The goal of XA is to guarantee atomicity in "global transactions" that are executed across heterogeneous components. A ''transaction'' is a unit of work such as transferring money from one person to another. Distributed transactions update multiple

data store In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted. ...

s (such as databases, application servers, message queues, transactional caches, etc.) To guarantee integrity, XA uses a

two-phase commit In transaction processing, databases, and computer networking, the two-phase commit protocol (2PC) is a type of atomic commitment protocol (ACP). It is a distributed algorithm that coordinates all the processes that participate in a distributed ...

(2PC) to ensure that all of a transaction's changes either take effect (''commit'') or do not (''roll back''), i.e., ''atomically''.

Architecture

Specifically, XA describes the interface between a global transaction manager and a specific application. An application that wants to use XA engages an XA transaction manager using a library or separate service. The transaction manager tracks the participants in the transaction (i.e. the various data stores to which the application writes), and works with them to carry out the two-phase commit. In other words, the XA transaction manager is separate from an application's interactions with servers. XA maintains a log of its decisions to commit or roll back, which it can use to recover in case of a system outage. Many software vendors support XA (meaning the software can participate in XA transactions), including a variety of relational databases and message brokers.

Advantages and disadvantages

Since XA uses two-phase commit, the advantages and disadvantages of that protocol generally apply to XA. The main advantage is that XA (using 2PC) allows an atomic transaction across multiple heterogeneous technologies (e.g. a single transaction could encompass multiple databases from different vendors as well as an email server and a message broker), whereas traditional

database transaction A database transaction symbolizes a unit of work, performed within a database management system (or similar system) against a database, that is treated in a coherent and reliable way independent of other transactions. A transaction generally repr ...

s are limited to a single database. The main disadvantage is that 2PC is a blocking protocol: the other servers need to wait for the transaction manager to issue a decision about whether to commit or abort each transaction. If the transaction manager goes offline while transactions are waiting for its final decision, they will be stuck and hold their database locks until the transaction manager comes online again and issues its decision. This extended holding of locks may be disruptive to other applications that are using the same databases. Moreover, if the transaction manager crashes and its record of decisions cannot be recovered (e.g. due to a bug in how the decisions were logged, or due to data corruption on the server), manual intervention may be necessary. Many XA implementations provide an "escape hatch" for transactions to independently decide whether to commit or abort (without waiting to hear from the transaction manager), but this risks violating the atomicity guarantee and is therefore reserved for emergencies.

Specification

The XA specification describes what a resource manager must do to support transactional access. Resource managers that follow this specification are said to be ''XA-compliant''. The XA specification was based on an interface used in the

Tuxedo Black tie is a semi-formal Western dress code for evening events, originating in British and American conventions for attire in the 19th century. In British English, the dress code is often referred to synecdochically by its principal element ...

system developed in the 1980s, but adopted by several systems since then.

References

External links

Distributed Transaction Processing: The XA Specification
(see als

page) {{DEFAULTSORT:X Open XA Transaction processing Open Group standards