Client/server applications can communicate with an Oracle database in several ways. The most common of these is a middleware layer called the Oracle Call Interface (OCI). As you install Oracle's server software on the database server machine, you must install some sort of client software on the machines that will run the client application, so they can make OCI calls.
The regular Oracle Client package, however, is big—a full install with all components selected is over 700MB. Plus, it must be installed via the Oracle Universal Installer onto each machine that will run the client application. Although a silent install option is available, it's extra work during a deployment.
The original Oracle Instant Client, introduced with 10gR1, simplified the deployment of applications. Available as a free download from Oracle Technology Network, Instant Client's shared libraries require only 216MB. It's also easier to install; you copy its files into a folder on the client and then set an environment variable to point to the files. The Universal Installer is not required.
New in Oracle 10g Release 2 (10gR2), Instant Client Light is even smaller. The shared libraries are only 32MB, further reducing the footprint that your application must load in order to make calls to Oracle databases. It achieves this by removing the multi-language error message texts and character sets from the libraries, leaving only English language error messages. So you'll have to decide if you can live with this English-only limitation to use Instant Client Light.
An Instant Client Software Development Kit (SDK) was also released in 10gR2, containing all the headers needed to compile applications that use the OCI libraries directly. In the past, even though Instant Client could be used for deployment, the full Client package was required to link code with Oracle's libraries. A separate SDK eliminates the need for this.
Instant Client can connect with all versions of Oracle that the regular client supports.
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