High Level Architecture
[3] Today the HLA is used in a number of domains including defense and security and civilian applications.
The purpose of HLA was to provide one unified standard that would meet the simulation interoperability requirements of all US DoD components.
The full standards texts are available at no cost to SISO members or can be purchased from the IEEE shop.
The development of a new version of HLA started in January 2016 by SISO and is currently ongoing.
In the C++ and Java APIs, services are invoked using calls to an instance of the RTI Ambassador class.
These are federation-wide events where all, or selected federates are required to complete an operation, such as initializing a scenario, before the execution can continue.
Key services are: Save: Restore: The purpose of Declaration Management services, described in chapter 5 of the HLA Interface Specification,[5] is to enable federates to declare what information they wish to publish (send) and subscribe to (receive) based on object and interaction classes in the FOM.
For an object class, publishing and subscribing are performed for a specific set of attributes.
Key services are: Objects: Attributes: Interactions: The purpose of Ownership Management services, described in chapter 7 of the HLA Interface Specification,[5] is to dynamically manage what federate that simulates what aspect of an object instance.
Without pacing, the federation will run as fast as possible (e.g., federations that do not require human interaction at runtime nor interfaces with systems that depend upon a real-time clock can run as fast as computing resources will allow).
Key services for GALT are: More advanced services include: The purpose of DDM, described in chapter 9 of the HLA Interface Specification,[5] is to increase scalability of federations by performing additional filtering of subscribed data beyond class and attribute subscriptions.
A normalization function for the CarBrandDimension could map a set of car brands Kia, Ford, BMW and Peugeot to an integer in the range 0..3.
At runtime, a federate can provide Regions when subscribing to object class attributes and interactions.
MOM objects are defined in a special FOM module called the MIM, that is automatically loaded by the RTI.
A number of predefined classes, datatypes, dimensions and transportation types are provided in the standard.
There are three different XML Schemas for the OMT: The purpose of the identification table is to provide meta-data about the model, to facilitate reuse of the FOM/SOM or federates.
Object class attributes are inherited from superclasses to subclasses based on this hierarchy.
Interaction class parameters are inherited from superclasses to subclasses based on this hierarchy.
A number of predefined basic datatypes are provided in the HLA standard: HLAinteger16BE, HLAinteger32BE, HLAinteger64BE, HLAfloat32BE, HLAfloat64BE, HLAoctetPairBE, HLAinteger16LE, HLAinteger32LE, HLAinteger64LE, HLAfloat32LE, HLAfloat64LE, HLAoctetPairLE and HLAoctet.
A number of predefined simple datatypes are provided in the HLA standard: HLAASCIIchar, HLAunicodeChar, HLAbyte, HLAinteger64time and HLAfloat64time.
The purpose of the enumerated datatypes table is to describe data elements that can take on a finite discrete set of values.
A number of predefined simple datatypes are provided in the HLA standard: HLAASCIIstring, HLAunicodeString, HLAopaqueData and HLAtoken.
Which alternative that applies for a given variant record is indicated by a data element called the Discriminant.
Other major improvements include: In order to ensure the proper interaction between simulations, a way of testing federate conformance is defined.
This involves ensuring that every class and interaction listed in the SOM for a particular federate is used according to the usage described, "PublishSubscribe", "Publish", "Subscribe" or "None".
HLA (in both the current IEEE 1516 version and its ancestor "1.3" version) is the subject of the NATO standardization agreement (STANAG 4603) for modeling and simulation: Modeling And Simulation Architecture Standards For Technical Interoperability: High Level Architecture (HLA).
[13] The Base Object Model (BOM), SISO-STD-003-2006 is a related standard by SISO to provide better reuse and composability for HLA simulations.
As for middleware applications that most closely match HLA features, such as the publish and subscribe feature (P&S) see Data Distribution Service (DDS) which shares many of the same characteristics but having an open on-the-wire protocol for system interoperability.
[15] HLA is a Message-oriented middleware that defines as a set of services, provided by a C++ or Java API.
Participants in a federation must use RTI libraries from the same provider and usually also of the same version, which in some cases is perceived as a drawback.
