High Level Architecture
Article by: Chris Turrell  (hla@msis.dmso.mil) Welcome to the inaugural edition of the HLA column. In this first article we want to remind those familiar with the High Level Architecture (HLA) of the infrastructure in place to support their transition to this program. For those not familiar with this important DoD initiative, we've included some background and history to help set the stage. In future pieces we'll use this column to keep the M&S community at large informed about the latest developments surrounding the HLA. The High Level Architecture (HLA) is a reusable software architecture for the development and execution of distributed simulation applications. It consists of: Rules: which govern how federates and federations are constructed Interface Specification: which governs how federations and federates interact with the Runtime Infrastructure Object Model Template: which provides a method for documenting key information about simulations and federations. The development of the HLA is based on the premises that no single, monolithic simulation could satisfy the needs of all users, that all uses of simulations and useful ways of combining them in the future could not be anticipated in advance and that future technological capabilities and a variety of operating considerations would have to be accommodated. That being the case, then, it was determined that the DoD would be best served by adopting a composable approach to constructing simulation federations. The resulting design principles called for federations of simulations constructed from modular components with well defined interfaces, and a separation of the specific simulation functionality from the general purpose supporting runtime infrastructure. Development of the HLA began in 1994 with a DARPA led Program Evaluation Team which reviewed previous architecture efforts and involved industry solicitations. In March of 1995 the Program Evaluation Team presented the initial definition of the HLA. At this point in the development of the architecture, the Architecture Management Group (AMG) assumed responsibility for the development of the specification. The AMG membership consisted of the technical leads from a variety of DoD M&S programs that were selected by the Executive Council for Modeling and Simulation. AMG members offered to bring their programs into the HLA environment and, in so doing, provide feedback on the specification. At the same time they were charged to ensure that recommendations made by other members of the AMG were compatible with the needs of their simulation environment as well. In August of 1996 the AMG had completed a baseline definition of the HLA specification, and in September of that year, the Under Secretary of Defense for Acquisition and Technology issued the mandate that the HLA would be applicable to all DoD simulations. The next year and a half, then, was characterized by a series of 6 month specification update check points which matured the specification and resulted in the HLA entering into the IEEE standardization process. That process is now well underway. For additional details on the HLA standardization process, please visit the Simulation Interoperability Standards Organization website at {http://siso.itcenter.org/}. Last fall we also saw a shift in emphasis at DMSO from the development of the HLA specification to the support of HLA implementations. In addition to the Object Modeling tools, which were developed early in the process of baselining the HLA specification, a suite of "bread and butter" runtime tools were also released. At the same time DMSO sponsored an HLA tools workshop for government and industry developers interested in the HLA tools market. A second workshop will be held June 6th following the DMSO Industry Days. To learn more about government and commercial tools available to support HLA implementations, visit the HLA Tools Bulletin Board accessible through the HLA website at {http://hla.dmso.mil} or download the DMSO sponsored tools from the Software Distribution Center accessible through the HLA website at { http://www.dmso.mil}. The first software developed in support of the HLA was the Runtime Infrastructure (RTI). Development of the RTI has been evolutionary and has mirrored the development of the specification itself. The most current release, RTI 1.3, supports all of the Interface Specification functionality and is available through the DMSO sponsored Software Distribution Center accessible from the HLA website {http://hla.dmso.mil}. In addition to RTI 1.3 there is a DMSO sponsored commercial RTI development called RTI 1.3 NG (Next Generation). This RTI will be available through the Software Distribution Center this summer. As with RTI 1.3, it too supports all interface specification functionality and will be available to the community at large free of charge. Other commercially developed RTIs are also being developed and marketed. An RTI Verifier is under development by DMSO to insure that these RTIs are compliant with the HLA Interface Specification. Developers interested in having their RTI applications tested by this tool may submit their requests through the HLA Help Desk at {hla@msis.dmso.mil}. HLA education and training programs have also been expanded in support of HLA implementations. Regional Training programs offers a comprehensive introduction to the HLA and is offered monthly. Additionally, Focused Training provides half day sessions as adjuncts to Regional and Special request events and provides an in-depth look into the Federation Development and Execution Process (FEDEP), Data Distribution Management (DDM), Adapting Your Simulation to Use HLA, Federation Compliance Testing, and HLA Federation Development and Execution Process (FEDEP) and Supporting Tools. Special training is available to specific organizations based on specific requests. Tutorials are also presented at simulation-related workshops, conferences and symposia. For the developers, there is a Hands-on Practicums offered twice a month that provides implementer's-level training in the use of the HLA. Additionally, CDs covering a variety of subjects are also available through the HLA website {http://hla.dmso.mil}. Help Desk support has also been expanded. In addition to the HLA Help Desk {hla@msis.dmso.mil} and the RTI help desk {RTI13_support@msis.dmso.mil} each of the runtime tools has Help Desk support. Information on these Help Desks is available in the documentation downloaded with the tools. Finally, HLA compliance testing has been expanded to support both classified and unclassified tests. For more information on compliance testing contact {hlatest@msis.dmso.mil} and participate in the Simulation Interoperability Workshop Testing Forum. In this first article we have tried to provide a broad overview of the HLA and the infrastructure in place to support programs transitioning to it. More specifics and updates will be provided in future editions of this web-based publication. In the interim, however, let me suggest that you subscribe to HLA Online. This is a moderated reflector that is available to the M&S community. We use it to make announcements to the community at large including software release information and other important developments. You can subscribe to HLA Online through the HLA website {http://hla.dmso.mil}.
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