!!! Overview
[{$pagename}] is a computing term that refers to an application outside the [operating system] that provides a layer of security to various system artifacts.


The term is typically seen when referring to security products on [IBM] [z/OS|zOS] platforms. An example is the [RACF] security package. [1]


[{$pagename}] is best known on [IBM] [Mainframe]


On [z/OS|zOS] there are three primary External Security Managers (ESM) 
* [RACF] - Short for [Resource Access Control Facility], is an [IBM] software product. It is a security system that provides access control and auditing functionality for the z/OS and z/VM operating systems. [RACF] was introduced in [1976|Year 1976]
* [Top Secret] -  [Computer Associates]
* [ACF2] ([Access Control Facility]) 

In 1987, acquisitions made [Computer Associates] the largest independent vendor of mainframe infrastructure software and dominant vendor of OS/MVS security software with CA-Top Secret (#2 market share) and CA-ACF2 (#1 market share). IBM's RACF product held the #3 market share position. 
I really have no idea where market share of these products are today.

[{$pagename}] is an [Authentication Agent] for [z/OS|zOS] platforms and can be utilized to provide [Single Sign-On] for applicaitons on the platform.

There is a [DirXML] [ZOS BIDIRECTIONAL DRIVER] that will work with most of the  [{$pagename}] on [z/OS|zOS] platforms.

!! More Information
There might be more information for this subject on one of the following:
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[#1] - [http://en.wikipedia.org/wiki/External_Security_Manager] retrieved 2012-10-15