One of the strongest features of the Oracle database is its ability to scale up to handle extremely large volumes of data and users. Oracle scales not only by running on more and more powerful platforms, but also by running in a distributed configuration. Oracle databases on separate platforms can be combined to act as a single logical distributed database.
This section describes some of the basic ways that Oracle handles database interactions in a distributed database system.
Data within an organization is often spread among multiple databases for reasons of both capacity and organizational responsibility. Users may want to query this distributed data or update it as if it existed within a single database.
Oracle first introduced distributed databases in response to the requirements for accessing data on multiple platforms in the early 1980s. Distributed queries can retrieve data from multiple databases. Distributed transactions can insert, update, or delete data on distributed databases. Oracle’s two-phase commit mechanism, which is described in detail in Chapter 12, guarantees that all the database servers that are part of a transaction will either commit or roll back the transaction. Distributed transactions that may be interrupted by a system failure are monitored by a recovery background process. Once the failed system comes back online, the same process will complete the distributed transactions to maintain consistency across the databases.
You can also implement distributed transactions in Oracle by popular transaction monitors (TPs) that interact with Oracle via XA, an industry standard (X/Open) interface. Oracle8i also added native transaction coordination with the Microsoft Transaction Server (MTS), so you can implement a distributed transaction initiated under the control of MTS through an Oracle database.
Heterogeneous Services allow non-Oracle data and services to be accessed from an Oracle database through tools such as Oracle Transparent Gateways. For example, Transparent Gateways allow users to submit Oracle SQL statements to a non-Oracle distributed database source and have them automatically translated into the SQL dialect of the non-Oracle source system, which remains transparent to the user. In addition to providing underlying SQL services, Heterogeneous Services provide transaction services utilizing Oracle’s two-phase commit with non-Oracle databases and procedural services that call third-generation language routines on non-Oracle systems. Users interact with the Oracle database as if all objects are stored in the Oracle database, and Heterogeneous Services handle the transparent interaction with the foreign database on the user’s behalf.
Heterogeneous Services work in conjunction with Transparent Gateways. Generic connectivity via ODBC and OLEDB is included with the database. Optional Transparent Gateways use agents specifically tailored for a variety of target systems.
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