GC & TG 概况

Oracle offers two conectivity solutions for the heterogeneous environment
1.Oracle Generic Connectivity
2.Oracle Transparent Gateways
Applications developed using a consistent Oracle interface for both Oracle and non-Oracle systems to eliminate the need for application developers to customize their application to access data from different non-Oracle systems.

SQL translations, data dictionary translations and data type translations are required even if the non-Oracle systems are based on SQL standards for smooth interoperability between disparate systems. GC & TGs have the ability to translate one systems'dialect to another. ( In a word , it is a translator that accomplishes the communication between different dialects of disparate systems)

1.HS(Heterogeneous Services component) 2. Agent composing the gateway tech.

Introduction to HS.
HS can offer the generic tech for connecting to non-Oracle systems . GC & TG are based on HS.
HS具有强大的解析SQL能力。扩展于Oracle SQL 引擎。利用mappings 获得必需的数据字典信息。提供了两种转换方式(1)Oracle SQL => dialect of non-Oracle system (2)数据字典的转换（Q）data dictionary translations which display the metadata of non-Oracle systems.不过 HS并不保证所有的都可以translation，通过提供了pass-through的功能-当没有translations可以使用的时候，使用pass-through直接将native SQL发送到non-Oracle system.
HS也提供了在Oralce和non-Oracle系统之间的事务间协调机制。例如提供two-phase commit 协议来保证分布式环境下事务的完整性(Q)。


Agent 和HS 交互 从而实现了oracle &non-oracle之间的透明连接。
两种类型的agent：1.HS Agents 2.Transparent Gateway agents

HS Agents 也有两种：a. HS ODBC b.HS OLEDB
HS Agents are part of the Oracle database 默认安装。GC正是使用这些agents连接到non-Oracle systems.

Transparent Gateway agents 独立于oralce server.它可以安装在任何机器上。并且可以oracle database 并存。什么时候安装TG agents必须考虑网络阻塞状况、os平台性能、硬件资源和存储情况。要获得最大的性能，TG是最佳选择。TGs use the native interfaces to communicate with non-oracle systems.

Oracle Generic Connectivity

Generic Connectivity is a feature of the Oracle database. It is a generic solution that uses an ODBC or OLE DB driver to access any ODBC or OLEDB compliant non-Oracle system. It addresses the needs of data access to many data stores for which Oracle does not have a gateway solution. This feature enables transparent connectivity using industry standards such as ODBC and OLEDB. Oracle Generic connectivity makes it possible to access low-end data stores such as Foxpro, Access, dBase and non-relational targets like Excel.

Oracle Transparent Gateways

In contrast to Generic Connectivity, which is a generic solution, Transparent Gateways are tailored solutions, specifically coded for the non-Oracle system. They provide an optimized solution, with more functionality and better performance than Generic Connectivity. Generic Connectivity relies on industry standards, whereas Transparent Gateways accesses the non-Oracle systems using their native interface. Transparent Gateways are also end-to-end certified. Oracle has transparent gateways to many sources, DB2, Sybase, Informix, Microsoft SQL Server, Ingres, Teradata to name a few.

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KEY FEATURES

* Improved SQL Generation - Fine grained capability handling
* Generated SQL queries cache - Non-Oracle language queries are generated and cached in Oracle's Shared Pool, making it more efficient for reuse when such queries are frequently used by multiple client sessions
* Better performance - Improved remote connectivity techniques of HS, provides better performance
* Local Cache - Information about capabilities, SQL translations, and data dictionary translations are stored locally, reducing network traffic
* Distributed SQL Optimizations - Improves performance of multi-site JOIN and SET operations
* Pass-Through SQL- Allows native SQL (including DDL) of target non-Oracle system to be directly transmitted
* Read/Write Access - Both query and update capabilities are supported for most non-Oracle systems.
* Data Type Translations - Performs automatic data type translations between the Oracle Sever and the non-Oracle system
* Data Dictionary Translations - Metadata of the non-Oracle system is translated and displayed in Oracle format
* Gateway Mobility - The Transparent Gateway, the non-Oracle system and the Oracle server can all reside on different machines
* Remote Stored Procedures - Supports execution of stored procedures defined in non-Oracle system
* Two Phase Commit - Oracle supports two-phase commit with non-Oracle stores. When a public two-phase commit interface is not available, it can be supported when distributed transactions involve Oracle and a single non-Oracle store
* Logon Security - Enforces access authorizations for remote login
* Data Encryption - Available with Oracle's Advanced Security Option
* Location Transparency - Users do not need to be aware of the physical location of data, just the names of the tables to be accessed
* Commit Transparency - Distributed and non-distributed transactions are committed using the ANSI-standard SQL COMMIT. Oracle automatically detects when a transaction is distributed and uses a two-phase commit protocol to ensure transaction integrity and consistency
* Network and Operating System Transparency - No special coding is required, even if the data resides on different systems
* NLS and NCHAR Support - Enables connectivity to non-Oracle systems using multi-byte character sets