Applications designed according to documented behavior of documented, external OpenVMS interfaces will continue to function without change or recompilation, across all future
versions of OpenVMS on one hardware architecture. On occasion existing interfaces may be obsoleted, meaning they are removed from the OpenVMS documentation. However, the
interfaces continue to be present and continue to function. In rare circumstances interfaces may be withdrawn, meaning that they no longer function. This only occurs after
careful study to ensure that the set of affected applications is minimal.
The internal operation of OpenVMS services may change from version to version, and applications that exploit internal interfaces or undocumented behavior may be affected by
changes in internal operation. Since it is possible to inadvertantly build dependencies on unspecified behavior of documented interfaces, it is prudent to test applications
on new versions of OpenVMS before putting them in production.
There are four classes of OpenVMS releases, with different expectations on the magnitude of change:
1. Major releases for OpenVMS contain substantial new functionality. The version number increases to the next integer (for example, from 6.2-1H1 to 7.0).
Application impact: Significant change in internal operation can be expected in some areas of the operating system. Applications that depend on internal interfaces
or access internal data structures will at minimum need to be recompiled and relinked, and in some cases may require code changes. Unspecified behavior of documented interfaces
may change in some areas.
2. New feature releases for OpenVMS contain new features, enhancements, and maintenance updates. The version number increases to the next decimal number (for example,
from 7.2 to 7.3).
Application impact: Internal interfaces are mostly stable. However, there may be changes in internal operation in selected areas, and internal data structures that are
not generally published may change in some instances. Applications that depend on internal operation and interfaces, such as device drivers, will in general continue to function
without recompilation or modification, but should be tested. Unspecified behavior of documented interfaces may change in some circumstances.
3. Enhancement releases for OpenVMS contain enhancements to existing features and maintenance updates. The version number increases to show a revision by using a dash
(for example, OpenVMS Version 7.3-2).
Application impact: The release may contain new hardware support, software enhancements, and maintenance, but the changes are minor. There is no impact on applications
that use published APIs, and impact on internal interfaces is minimal. There is no need for ISVs to test on the new release or to produce a new application kit.
4. Hardware releases provide current version-level support until 12 months after a subsequent release containing that particular hardware support. Hardware releases are
shipped with new hardware sales only and are not distributed to existing service customers.