Make your Systems Work Together

A First Look at the HLA Evolved Web Service API

ABSTRACT: A new API is being added to the IEEE 1516 HLA standard for simulation interoperability. The new API is based on Web Services, an implementation of the Service Oriented Architecture. This paper provides a brief technical introduction and an overview from a strategic and architectural viewpoint.

A Web Service is a service that can be accessed from another system using an http request and response, similar to fetching a web page. The request and response is described using XML. A service can be described using the Web Service Description Language (WSDL). This is currently the predominant implementation of the Service Oriented Architecture.

The capabilities of HLA and Web Services may at first seem to overlap. A closer look reveals some fundamental differences:

  • Web Services provides a loosely coupled mechanism for performing coarse-grained services with modest performance over both LAN and WAN. Services are supposed to be stateless. It is based on technologies that are familiar to most enterprises. A wide range of supporting vendors and software is available.
  • HLA provides extremely high performance and scalability for interactions and information exchange in a shared, complex state (scenario). It also provides unique capabilities for synchronizing the data exchange between systems using scenario time (as opposed to wall clock time).

    The Web Service API for HLA combines the best of these two worlds, bringing the above capabilities to a shared approach. Simulation system functionality may now be accessed in three ways:
  • Web Services systems can access the functionality of a federation on a coarse-grained service level using Web Services.
  • Web Services systems that need a closer interaction, synchronization and more efficient access to the shard scenario can use the HLA Web Service API to fully participate as federates in the federation.
  • Federates requiring the highest level of performance and scalability can use the C++ or Java API.

    The Web Service API will allow simulation developers to reap the benefits of both the Web Service and the HLA technologies. As the Service Oriented Architecture evolves beyond the current http and request-response implementation these two technologies can be expected to blend, potentially into new concepts and interoperability
    architectures.

Authors: Björn Möller, Clarence Dahlin
Publication: Proceedings of 2005 Euro Simulation Interoperability Workshop, 06E-SIW-061, Simulation Interoperability Standards Organization, June 2006.

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