Can you give a high-level description of what a distributed transaction is?
Also, can you give an example of why an application or database should perform a transaction that updates data on two or more networked computers? I'm understand the classic bank example; I care more about distributed transactions in Web-scale databases like Dynamo, Bigtable, HBase, or Cassandra.
Distributed transactions span multiple physical systems, whereas standard transactions do not. Synchronization amongst the systems becomes a need which traditionally would not exist in a standard transaction.
From your Wikipedia reference...
...a distributed transaction can be seen as a database transaction that must be synchronized (or provide ACID properties) among multiple participating databases which are distributed among different physical locations...
Usually, transactions occur on one database server:
SELECT something FROM myTable
UPDATE something IN myTable
A distributed transaction involves multiple servers:
UPDATE amount = amount - 100 IN bankAccounts WHERE accountNr = 1
UPDATE amount = amount + 100 IN someRemoteDatabaseAtSomeOtherBank.bankAccounts WHERE accountNr = 2
The difficulty comes from the fact that the servers must communicate to ensure that transactional properties such as atomicity are satisfied on both servers: If the transaction succeeds, the values must be updated on both servers. If the transaction fails, the transaction must be rollbacked on both servers. It must never happen that the values are updated on one server but not updated on the other.
A distributed transaction is a transaction on a distributed database (i.e., one where the data is stored on a number of physically separate systems). It's noteworthy because there's a fair amount of complexity involved (especially in the communications) to assure that all the machines remain in agreement, so either the whole transaction succeeds, or else it appears that nothing happened at all.
A distributed transaction is a transaction that works across several computers. Say you start a transaction in some method in a program on computer A. You then make some changes to data in the method on computer A, and afterwords the method calls a web service on computer B. The web service method on computer B fails and rolls the transaction back. Since the transaction is distributed, this means that any changes made on computer A also need to be rolled back. The combination of the distributed transaction coordinator on windows and the .net framework facilitate this functionality.