Accessibility of configuration variables and data exchange between plugins (processing blocks) are an important issue in the openMHA.

In general, variable types in the openMHA are distinguished by their different access methods. The variable types in the openMHA are:

Configuration variables : Read and write accesses are possible through the openMHA configuration language interface. Configuration variables are implemented as C++ classes with a public data member of the underlying C type. Configuration variables can be read and modified from `‘outside’' using the configuration language. The plugin which provides the configuration variable can use the exposed data member directly. All accesses through the openMHA configuration language are checked for data type, valid range, and access restrictions.
Monitor variables : Read access is possible through the openMHA configuration language. Write access is only possible from the C++ code. Internally, monitor variables have a similar C++ class interface as configuration variables.
AC variables (algorithm communication variables): Any C or C++ data structure can be shared within an openMHA chain. Access management and name space is realised in openMHA chain plugin ('mhachain'). AC variables are not available to the openMHA configuration language interface, although a read-only converter plugin acmon is available.
Runtime configuration : Algorithms usually derive more parameters (runtime configuration) from the openMHA configuration language variables. When a configuration variable changes through configuration language write access, then the runtime configuration has to be recomputed. Plugin developers are encouraged to encapsulate the runtime configuration in a C++ class, which recomputes the runtime configuration from configuration variables in the constructor. The openMHA supports lock-free and thread-safe replacement of the runtime configuration instance (see example5.cpp and references therein).

Variables that describe physical facts to the MHA user should be given in SI units, e.g. meters for distances (not centimeters or inches), seconds for times (not milliseconds or minutes) etc for reasons of uniformity and simplicity of handling derived units.

The C++ data types are shown in the figure below. These variables can be accessed via the openMHA host application using the openMHA configuration language. For more details see the openMHA application manual.