Distributed transaction
Encyclopedia
A distributed transaction is an operations bundle, 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
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 database transaction
that must be synchronized
(or provide ACID
properties) among multiple participating database
s which are distributed
among different physical locations. The isolation
property (the I of ACID) poses a special challenge for multi database transactions, since the (global) serializability
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
, which ensures global serializability, if all the participating databases employ it. (see also commitment ordering
for multidatabases.)
A common algorithm
for ensuring correct
completion of a distributed transaction is the two-phase commit (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
s, Optimism and Isolation Without Locking. X/Open standard does not cover long-lived DTP.
Several modern technologies, including Enterprise Java Beans (EJBs) and Microsoft Transaction Server
(MTS) fully support distributed transaction standards.
Database transaction
A transaction comprises a unit of work performed within a database management system against a database, and treated in a coherent and reliable way independent of other transactions...
, must have all four ACID
ACID
In computer science, ACID is a set of properties that guarantee database transactions are processed reliably. In the context of databases, a single logical operation on the data is called a transaction...
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
In computing, the XA standard is a specification by The Open Group for distributed transaction processing . It describes the interface between the global transaction manager and the local resource manager...
(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 database transaction
Database transaction
A transaction comprises a unit of work performed within a database management system against a database, and treated in a coherent and reliable way independent of other transactions...
that must be synchronized
Synchronized
Synchronized can refer to the following definitions:*synchronization , or syncronisation , the coordination of events to operate a system in unison.*Synchronized , a 2002 album by sHeavy....
(or provide ACID
ACID
In computer science, ACID is a set of properties that guarantee database transactions are processed reliably. In the context of databases, a single logical operation on the data is called a transaction...
properties) among multiple participating database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
s which are distributed
Distributed computing
Distributed computing is a field of computer science that studies distributed systems. A distributed system consists of multiple autonomous computers that communicate through a computer network. The computers interact with each other in order to achieve a common goal...
among different physical locations. The isolation
Isolation (computer science)
In database systems, isolation is a property that defines how/when the changes made by one operation become visible to other concurrent operations. Isolation is one of the ACID properties.-Isolation levels:...
property (the I of ACID) poses a special challenge for multi database transactions, since the (global) serializability
Serializability
In concurrency control of databases, transaction processing , and various transactional applications , both centralized and distributed, a transaction schedule is serializable, has the serializability property, if its outcome In concurrency control of databases, transaction processing (transaction...
property could be violated, even if each database provides it (see also global serializability
Global serializability
In concurrency control of databases, transaction processing , and other transactional distributed applications, Global serializability is a property of a global schedule of transactions...
). In practice most commercial database systems use strong strict two phase locking (SS2PL)
Two phase locking
In databases and transaction processing two-phase locking, is a concurrency control method that guarantees serializability.It is also the name of the resulting set of database transaction schedules...
for concurrency control
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 getting those results as quickly as possible.Computer...
, which ensures global serializability, if all the participating databases employ it. (see also commitment ordering
Commitment ordering
In concurrency control of databases, transaction processing , and related applications, Commitment ordering is a class of interoperable Serializability techniques, both centralized and distributed. It allows optimistic implementations...
for multidatabases.)
A common algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
for ensuring correct
Correctness
In theoretical computer science, correctness of an algorithm is asserted when it is said that the algorithm is correct with respect to a specification...
completion of a distributed transaction is the two-phase commit (2PC). This algorithm is usually applied for updates able to commit
Commit (data management)
In the context of computer science and data management, commit refers to the idea of making a set of tentative changes permanent. A popular usage is at the end of a transaction. A commit is an act of committing.-Data management:...
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
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....
s, Optimism and Isolation Without Locking. X/Open standard does not cover long-lived DTP.
Several modern technologies, including Enterprise Java Beans (EJBs) and Microsoft Transaction Server
Microsoft Transaction Server
Microsoft Transaction Server was software that provided services to Component Object Model software components, to make it easier to create large distributed applications. The major services provided by MTS were automated transaction management, instance management and role-based security...
(MTS) fully support distributed transaction standards.
Further reading
- Gerhard Weikum, Gottfried Vossen, Transactional information systems: theory, algorithms, and the practice of concurrency control and recovery, Morgan Kaufmann, 2002, ISBN 1558605088