A distributed transaction is a

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 ...

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 encompasses all operations against such resources. Distributed transactions, as any other transactions, must have all four ACID (atomicity, consistency, isolation, durability) properties, where atomicity guarantees all-or-nothing outcomes for the unit of work (operations bundle). Open Group, a vendor consortium, proposed the X/Open Distributed Transaction Processing (DTP) Model (X/Open XA), which became a de facto standard for behavior of transaction model components. Databases are common transactional resources and, often, transactions span a couple of such databases. In this case, a distributed transaction can be seen as a

that must be synchronized (or provide ACID properties) among multiple participating

database In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases s ...

s which are

distributed Distribution may refer to: Mathematics *Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations *Probability distribution, the probability of a particular value or value range of a varia ...

among different physical locations. The isolation property (the I of ACID) poses a special challenge for multi database transactions, since the (global)

serializability In concurrency control of databases, Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman (1987)''Concurrency Control and Recovery in Database Systems''(free PDF download), Addison Wesley Publishing Company, Gerhard Weikum, Gottfried Vossen (20 ...

property could be violated, even if each database provides it (see also global serializability). In practice most commercial database systems use strong strict two phase locking (SS2PL) for

concurrency control In information technology and computer science, especially in the fields of computer programming, operating systems, multiprocessors, and databases, concurrency control ensures that correct results for concurrent operations are generated, while ...

, which ensures global serializability, if all the participating databases employ it. (see also

commitment ordering Commitment ordering (CO) is a class of interoperable '' serializability'' techniques in concurrency control of databases, transaction processing, and related applications. It allows optimistic (non-blocking) implementations. With the proliferation ...

for multidatabases.) A common

algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...

for ensuring correct completion of a distributed transaction is the

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). This algorithm is usually applied for updates able to commit in a short period of time, ranging from couple of milliseconds to couple of minutes. There are also long-lived distributed transactions, for example a transaction to book a trip, which consists of booking a flight, a rental car and a hotel. Since booking the flight might take up to a day to get a confirmation, two-phase commit is not applicable here, it will lock the resources for this long. In this case more sophisticated techniques that involve multiple undo levels are used. The way you can undo the hotel booking by calling a desk and cancelling the reservation, a system can be designed to undo certain operations (unless they are irreversibly finished). In practice, long-lived distributed transactions are implemented in systems based on Web Services. Usually these transactions utilize principles of

compensating transaction The execution of a business process consists of one or more transactions. Each transaction may consist of several individual operations yet, as a whole, it moves the system between consistent states. There are two groups of systems where compens ...

s, Optimism and Isolation Without Locking. X/Open standard does not cover long-lived DTP. Several technologies, including

Enterprise Java Beans Jakarta Enterprise Beans (EJB; formerly Enterprise JavaBeans) is one of several Java APIs for modular construction of enterprise software. EJB is a server-side software component that encapsulates business logic of an application. An EJB web c ...

(EJBs) and

Microsoft Transaction Server Microsoft Transaction Server (MTS) was software that provided services to Component Object Model (COM) software components, to make it easier to create large distributed applications. The major services provided by MTS were automated transaction ...

(MTS) fully support distributed transaction standards.

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See also

References

Further reading