Distributed Databases in Oracle
Oracle provides support for homogeneous, heterogeneous, and client server architectures of distributed databases. In a homogeneous architecture, a minimum of two Oracle databases reside on at least one machine. Although the location and platform of the databases are transparent to client applications, they would need to distinguish between local and remote objects semantically. Using synonyms, this need can be overcome wherein users can access the remote objects with the same syntax as local objects. Different versions of DBMSs can be used, although it must be noted that Oracle offers backward compatibility but not forward compatibility between its versions. For example, it is possible that some of the SQL extensions that were incorporated into Oracle 11i may not be understood by Oracle 9.
In a heterogeneous architecture, at least one of the databases in the network is a non-Oracle system. The Oracle database local to the application hides the underlying heterogeneity and offers the view of a single local, underlying Oracle database. Connectivity is handled by use of an ODBC- or OLE-DB-compliant protocol or by Oracle’s Heterogeneous Services and Transparent Gateway agent components. A discussion of the Heterogeneous Services and Transparent Gateway agents is beyond the scope of this book, and the reader is advised to consult the online Oracle documentation.
In the client-server architecture, the Oracle database system is divided into two parts: a front end as the client portion, and a back end as the server portion. The client portion is the front-end database application that interacts with the user. The client has no data access responsibility and merely handles the requesting, processing, and presentation of data managed by the server. The server portion runs Oracle and handles the functions related to concurrent shared access. It accepts SQL and PL/SQL statements originating from client applications, processes them, and sends the results back to the client. Oracle client-server applications provide location transparency by making the location of data transparent to users; several features like views, synonyms, and procedures contribute to this. Global naming is achieved by using <TABLE_NAME@DATABASE_NAME> to refer to tables uniquely.
Oracle uses a two-phase commit protocol to deal with concurrent distributed trans-actions. The COMMIT statement triggers the two-phase commit mechanism. The RECO (recoverer) background process automatically resolves the outcome of those distributed transactions in which the commit was interrupted. The RECO of each local Oracle server automatically commits or rolls back any in-doubt distributed transactions consistently on all involved nodes. For long-term failures, Oracle allows each local DBA to manually commit or roll back any in-doubt transactions and free up resources. Global consistency can be maintained by restoring the data-base at each site to a predetermined fixed point in the past.
Oracle’s distributed database architecture is shown in Figure 25.12. A node in a distributed database system can act as a client, as a server, or both, depending on the situation. The figure shows two sites where databases called HQ (headquarters) and Sales are kept. For example, in the application shown running at the headquarters, for an SQL statement issued against local data (for example, DELETE FROM DEPT ...), the HQ computer acts as a server, whereas for a statement against remote data (for example, INSERT INTO EMP@SALES), the HQ computer acts as a client.
Communication in such a distributed heterogeneous environment is facilitated through Oracle Net Services, which supports standard network protocols and APIs. Under Oracle’s client-server implementation of distributed databases, Net Services
is responsible for establishing and managing connections between a client application and database server. It is present in each node on the network running an Oracle client application, database server, or both. It packages SQL statements into one of the many communication protocols to facilitate client-to-server communication and then packages the results back similarly to the client. The support offered by Net Services to heterogeneity refers to platform specifications only and not the database software. Support for DBMSs other than Oracle is through Oracle’s Heterogeneous Services and Transparent Gateway. Each database has a unique global name provided by a hierarchical arrangement of network domain names that is prefixed to the database name to make it unique.
Oracle supports database links that define a one-way communication path from one Oracle database to another. For example, CREATE DATABASE LINK sales.us.americas; establishes a connection to the sales database in Figure 25.12 under the network domain us that comes under domain americas. Using links, a user can access a remote object on another database subject to ownership rights without the need for being a user on the remote database.
Data in an Oracle DDBS can be replicated using snapshots or replicated master tables. Replication is provided at the following levels:
Basic replication. Replicas of tables are managed for read-only access. For updates, data must be accessed at a single primary site.
Advanced (symmetric) replication. This extends beyond basic replication by allowing applications to update table replicas throughout a replicated DDBS. Data can be read and updated at any site. This requires additional software called Oracle’s advanced replication option. A snapshot generates a copy of a part of the table by means of a query called the snapshot defining query. A simple snapshot definition looks like this:
CREATE SNAPSHOT SALES_ORDERS AS
SELECT * FROM SALES_ORDERS@hq.us.americas;
Oracle groups snapshots into refresh groups. By specifying a refresh interval, the snapshot is automatically refreshed periodically at that interval by up to ten Snapshot Refresh Processes (SNPs). If the defining query of a snapshot contains a distinct or aggregate function, a GROUP BY or CONNECT BY clause, or join or set operations, the snapshot is termed a complex snapshot and requires additional processing. Oracle (up to version 7.3) also supports ROWID snapshots that are based on physical row identifiers of rows in the master table.
Heterogeneous Databases in Oracle. In a heterogeneous DDBS, at least one database is a non-Oracle system. Oracle Open Gateways provides access to a non-Oracle database from an Oracle server, which uses a database link to access data or to execute remote procedures in the non-Oracle system. The Open Gateways feature includes the following:
Distributed transactions. Under the two-phase commit mechanism, trans-actions may span Oracle and non-Oracle systems.
Transparent SQL access. SQL statements issued by an application are transparently transformed into SQL statements understood by the non-Oracle system.
Pass-through SQL and stored procedures. An application can directly access a non-Oracle system using that system’s version of SQL. Stored procedures in a non-Oracle SQL-based system are treated as if they were PL/SQL remote procedures.
Global query optimization. Cardinality information, indexes, and so on at the non-Oracle system are accounted for by the Oracle server query optimizer to perform global query optimization.
Procedural access. Procedural systems like messaging or queuing systems are accessed by the Oracle server using PL/SQL remote procedure calls.
In addition to the above, data dictionary references are translated to make the non-Oracle data dictionary appear as a part of the Oracle server’s dictionary. Character set translations are done between national language character sets to connect multi-lingual databases.
From a security perspective, Oracle recommends that if a query originates at site A and accesses sites B, C, and D, then the auditing of links should be done in the data-base at site A only. This is because the remote databases cannot distinguish whether a successful connection request and following SQL statements are coming from another server or a locally connected client.
A concept closely related with distributed enterprise systems is online directories. Online directories are essentially a structured organization of metadata needed for management functions. They can represent information about a variety of sources ranging from security credentials, shared network resources, and database catalog.
Lightweight Directory Access Protocol (LDAP) is an industry standard protocol for directory services. LDAP enables the use of a partitioned Directory Information Tree (DIT) across multiple LDAP servers, which in turn can return references to other servers as a result of a directory query. Online directories and LDAP are particularly important in distributed databases, wherein access of meta-data related to transparencies discussed in Section 25.1 must be scalable, secure, and highly available.
Oracle supports LDAP Version 3 and online directories through Oracle Internet Directory, a general-purpose directory service for fast access and centralized man-agement of metadata pertaining to distributed network resources and users. It runs as an application on an Oracle database and communicates with the database through Oracle Net Services. It also provides password-based, anonymous, and certificate-based user authentication using SSL Version 3.
Figure 25.13 illustrates the architecture of the Oracle Internet Directory. The main components are:
Oracle directory server. Handles client requests and updates for information pertaining to people and resources.
Oracle directory replication server. Stores a copy of the LDAP data from Oracle directory servers as a backup.
Directory administrator: Supports both GUI-based and command line-based interfaces for directory administration.
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