As applications became more sophisticated and were gradually separated into

As applications became more sophisticated and were

This preview shows page 23 - 26 out of 28 pages.

As applications became more sophisticated and were gradually separated into independent components (notably distinguishing database components from processing components), it became clear that integration should also take place by letting applications communicate directly with each other. This has now lead to a huge industry that concentrates on Enterprise Application Integration (EAI) . Distributed Transaction Processing In a mail system, there might be primitives to send, receive, and forward mail. In an accounting system, they might be quite different. READ and WRITE are typical examples, however. Ordinary statements, procedure calls, and so on, are also allowed inside a transaction. In particular, remote procedure calls (RPCs) , that is, procedure
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calls to remote servers, are often also encapsulated in a transaction , leading to what is known as a transactional RPC . This all-or-nothing property of transactions is one of the four characteristic properties that transactions have. More specifically, transactions adhere to the so-called ACID properties : Atomic : To the outside world, the transaction happens indivisibly Consistent : The transaction does not violate system invariants Isolated : Concurrent transactions do not interfere with each other Durable : Once a transaction commits, the changes are permanent In distributed systems, transactions are often constructed as a number of subtransactions, jointly forming a nested transaction . The top-level transaction may fork off children that run in parallel with one another, on different machines, to gain performance or simplify programming. Each of these children may also execute one or more subtransactions, or fork off its own children. Nested transactions are important in distributed systems, for they provide a natural way of distributing a transaction across multiple machines. They follow a logical division of the work of the original transaction. For example, a transaction for planning a trip by which three different flights need to be reserved can be logically split up into three subtransactions. Each of these subtransactions can be managed separately and independently of the other two. In the early days of enterprise middleware systems, the component that handled distributed (or nested) transactions formed the core for integrating applications at the server or database level. This component was called a transaction processing monitor or TP monitor for short. Its main task was to allow an application to access multiple server/databases by offering it a transactional programming model. Essentially, the TP monitor coordinated the commitment of subtransactions following a standard protocol known as distributed commit .
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Enterprise application integration The more applications became decoupled from the databases they were built upon, the more evident it became that facilities were needed to integrate applications independently from their databases. In particular, application components should be
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