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High-Level Architecture

3,947 bytes added, 20:51, 13 January 2017
Federates
{{Official Core Development Effort}}
{{infobox subsystem
|description = Implementing support for the High Level Architecture to modularize FlightGear
|status = Under active development (2009-2016)
|developers = Mathias Fröhlich[http://sourceforge.net/p/flightgear/mailman/message/23723508/][http://sourceforge.net/p/flightgear/mailman/message/22914687/], James Turner[http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg39693.html], Stuart Buchanan[http://sourceforge.net/p/flightgear/mailman/message/34600516/], Richard Harrison [http://sourceforge.net/p/flightgear/mailman/message/34632142/]
| changelog =
* {{Git File History|project=flightgear|path=/src/Network/HLA}}
# It provides a very good framework to allow anyone to create components that interact with FlightGear using programming languages other than C/C++ (think Ada, Java, Python etc), which may be running in their own threads, and reside in separate binaries<ref>http://sourceforge.net/p/flightgear/mailman/message/34196458/</ref>, which will be also easier to debug/troubleshoot (think regression testing, i.e. running a self-contained subsystem in a dedicated gdb/valgrind session), without having to know how to modify/patch and rebuild FlightGear.
As A good overview of late 2015, Stuart has started work on re-architecting parts of FlightGear to use how the HLA, though this is expected to architecture works can be a multi-year projectfound here http://www. Anyone interested in the current status of development should subscribe to the Flightgear -devel mailing listpitch.se/images/files/tutorial/TheHLAtutorial.pdf.
{{FGCquote|1= For those looking for =Design== The planned overall design is as follows:* We intend to use OpenRTI as the underlying RTI. This is an open source IEEE 1516 standard RTI, written by Mathias Fröhlich and available from https://sourceforge.net/projects/openrti/.* Mathias has also written an open source toolkit to act as an interface library, with a summary working name of SimKit. This sits on top of the RTI and simplifies interfacing with the RTI massively. It also has excellent Python integration, making it very easy to write scripted Federates. Using [[SimKit]], the integration point is within the FlightGear code (rather than SimGear), in particular the code under $FG_SRC/Network/HLA. ==Federate Object Model== A key part of the design is writing the Federation Object Model (FOM), which defines the following PDF provides objects and updates that are published by the RTI. While it might at first glance seem a useful overviewgood idea to use the FOM to share the internal property tree across multiple federates, this is probably the wrong way to use HLA as the granularity is too low <ref>https: http//sourceforge.net/p/flightgear/mailman/message/34795859/</ref><ref>https://wwwsourceforge.pitchnet/p/flightgear/mailman/message/34803436/</ref><ref>https://sourceforge.senet/imagesp/flightgear/mailman/message/34794675/</ref> and it's likely to lead to synchonization issues. Instead, we'll need to make explicit decisions about the data models to communicate. The FOM is a set of XML filesin [https:/tutorial/TheHLAtutorialsourceforge.pdfnet/p/flightgear/fgdata/ci/next/tree/HLA/ fgdata/HLA/] The orientation data in the FOM (SGOrientationWGS84) are Quaternions ([https://en.wikipedia.org/wiki/Quaternion https://en.wikipedia.org/wiki/Quaternion]), and those are the i,j,k values. From memory, the orientation is relative to a earth-centered-frame, and there are functions in SimGear to convert to heading/pitch/roll in combination with the object's position.<ref> {{cite web |2url = http://forum.flightgear.org/viewtopic.php?p=280098#p280098 | title = <nowiki>Re: HLA </nowiki> | author = <nowiki>stuart</nowiki> | date = Mar 21st, 2016 | added = Mar 21st, 2016 | script_version = 0.25 }}</ref> <ref>{{cite web | url = http://sourceforge.net/p/flightgear/mailman/message/3463221434963798/ | title = <nowiki>Re: [Flightgear-devel] HLA developments</nowiki>
| author = <nowiki>Stuart Buchanan</nowiki>
| date = Nov 19thMar 24th, 20152016 | added = Nov 19thMar 24th, 20152016 | script_version = 0.2325
}}
}}</ref>
{{FGCquote|1= Depsite appearances moving Richard's [[Emesary]] system would also probably work quite well with the rendering HLA FOM to give us a seperate node/process is probably the easiest way for models to implement as the inter module dependencies aren't that strong, or at least those that are strong can be shipped as a block update at the end of the simulation framecommunicate with other models.|2= <ref> {{cite web | url = http://sourceforge.net/p/flightgear/mailman/message/3463187534985731/ | title = <nowiki>Re: [Flightgear-devel] HLA developmentsMessage passing interface for Nasal</nowiki>
| author = <nowiki>Richard Harrison</nowiki>
| date = Nov 19thApr 1st, 20152016 | added = Nov 19thApr 1st, 20152016 | script_version = 0.2325
}}
</ref>{{Appendix}}
{{FGCquote|1= I'm hoping that OpenRTI will provide the infrastructure for the data sharing so I don't need to worry about shared memory etc. As I mentioned previously, I'm planning to use an IEEE 1516 standard RTI, of which OpenRTI is a conveniently free implementation. AFAICT this is one of the main standards for distributed simulation communication (the other being DIS). I think that will address the communications side of the equation, and OpenRTI can work both as a local RTI and over a network. It's also got a robust logical time implementation, and the ability to clock the simulation.|2= {{cite web | url Federates= http://sourceforge.net/p/flightgear/mailman/message/34632142/ | title = <nowiki>Re: [Flightgear-devel] HLA developments</nowiki> | author = <nowiki>Stuart Buchanan</nowiki> | date = Nov 19th, 2015 | added = Nov 19th, 2015 | script_version = 0.23 }}}}
{{FGCquoteFederates can be written in most languages, but SimKit provides very good hooks for writing them in C++ and Python.|1= HLA is really intended For some Subsystems split off from the existing FlightGear source, it's fairly easy to operate at a much higher level of granularity create an executable with its own property tree<ref>https://sourceforge.net/p/flightgear/mailman/message/34632142/</ref> and pass have shared C++ code to map FOM objects around a distributed simulationto property values. The use However, this is an implementation detail -case I'm working on at the moment passes an AI instance whole point of HLA and the FOM is that it makes no assumptions about what Federates do with the data. Regarding weather, in a HLA/RTI context the way to do this would be to have a weather engine as an object RTI Federate. This would run any required weather simulation, and pass (with very) local weather conditions to each of the aircraft/tower/windsock in the simulation, plus publish positioninformation on clouds for use by visualiation engines.<ref>{{cite web |url = https://sourceforge.net/p/flightgear/mailman/message/35595300/ |title = <nowiki> Re: [Flightgear-devel] Traffic 2020: Towards a new Development Model for FlightGear AI Traffic </nowiki> |author = <nowiki> Stuart Buchanan </nowiki> |date = Jan 10th, velocity2017 |added = Jan 10th, accelerations, model 2017 |script_version = 0.40 }}</ref> ==Current Status ==Last updated: (13/01/2017) Stuart is cautiously optimistic that he may be able to provide the start of a HLA implementation quite soon, as one of the blocking factors may be displayed)resolved soon. How <ref>{{cite web |url = https://sourceforge.net/p/flightgear/mailman/message/35595300/ |title = <nowiki> Re: [Flightgear-devel] Traffic 2020: Towards a HLA Federate maps that data internally new Development Model for FlightGear AI Traffic </nowiki> |author = <nowiki> Stuart Buchanan </nowiki> |date = Jan 10th, 2017 |added = Jan 10th, 2017 |script_version = 0.40 }}</ref>  Currently, there is an implementation decisionsome very old HLA support in SimGear. There shouldn't This is very out of date and should be any race conditions because no assumptions are made about how ignored. Stuart has HLA support using the data latest OpenRTI and SimKit working on a local build, but is used or mappedwaiting for Mathias to officially publish his SimKit before he pushes his changes. At present So all of the following is local-only, I'm not thinking that but included here for information. The Flightgear core currently supports HLA will be used as a mechanism follows.* SimKit integration, reading the SimKit FOM and connecting to provide a shared property tree across multiple an OpenRTI RTI.* Instantiating MP AI objects so users can view objects published over the RTI by other Federates. I think that's This is currently somewhat unsatisfactory as it overloads the wrong way to use HLAMP code, where really these objects are more basic. Instead you make explicit decisions about * Pick up Environment Objects for this instance and use them to set the data models you will communicatelocal METAR string for weather generation. The  {{See also|FGTraffic}}We currently have the following other reason HLA isnFederates:* 't a good choice is ''fgogel''' - An AI model written in python the publishes over to do with simulation clockingthe RTI. I'm still getting my head around this properly Part of SimKit, but handy nevertheless!* '''fgtraffic''' - to run an AI Scenario externally to the overall model is one where FG Core* '''fgmetar''' - written in python that retrieves the simulation clock is advanced, closest METAR station for other published Federates and publishes the instance METAR for them to pick up. This could be expanded to provide [[FGPythonSys#Weather_Simulation|a general Weather Engine]]. Stuart now has the [[AI]] Manager working as an external binary, and publishing updates over the RTI which are displayed as MP Aircraft. Stuart has created a little python script that looks for objects with locations being published over the RTI and publishes changes the closest METAR to them, then modified FG to pick up the RTI that will appropriate METAR string for this instance and use it as the real-time input for weather generation.  The python script could be picked up by other instances fairly easily enhanced into a more substantial weather engine allowing a single coherent weather environment to be used across the simulation. Stuart also started work modifying [[FGViewer]] so that it works with SimKit. This is a fairly big deal and quite a bit of fairly dull plumbing work but would allow us at least have a view of the next clock advancesimulation entirely separate from the running simulation.|2= <ref> {{cite web | url = http://sourceforge.net/p/flightgear/mailman/message/3479585934877355/ | title = <nowiki>Re: [Flightgear-devel] Designing a thread-safe property tree API (was Re: A FGPythonSys implementation: ...)HLA Status Update 23/2/2016</nowiki>
| author = <nowiki>Stuart Buchanan</nowiki>
| date = Jan 26thFeb 23rd, 2016 | added = Jan 26thFeb 23rd, 2016 | script_version = 0.2325
}}
}}</ref>
{{FGCquote|1= For now I've only added the code but not used it anywhere. This Separately, Erik has the advantage been doing some preparatory work suitable for supporting HLA in JSBSim by adopting so called PropertyObjects to hopefully get rid of getting tied properties (compilerhttp://sourceforge.net/p/jsbsim/mailman/message/34720784/) errors early before new functionality gets added. Most of the code remains the same, even the Tie functions are still there and for stand alone programs they are perfectly fine.
But JSBSim will be used in a HLA environment soon because that's where FlightGear is heading and for that the new code is useful (accessing tied properties from multiple sources).|2= {{cite web | url = http://sourceforge.net/p/jsbsim/mailman/message/34720784/ | title = <nowiki>[Flightgear-devel] (JSBSim) Standalone Properties</nowiki> | author = <nowiki>Erik Hofman<references/nowiki> | date = Dec 25th, 2015 }}}}
== Related ==
For additional information, please see:
* [[Technical_Reports#A_New_Architecture_for_FlightGear_Flight_Simulator]]
* [[Decoupling the AI Traffic System]] (currently work-in progress, highlights the limitations of the current AI system related to distributed/multi-instance and multiplayer usage)* [[FlightGear HLA support (High Level Architecture)]](outdated, documenting the original HLA infrastructure code)
* [[FlightGear CIGI Support (Common Image Generator Interface)]].
* [[Modularizing, parallelizing and distributing FlightGear]] (background info/discussions pre-dating HLA adoption in FlightGear)* [[Distributed Interactive Simulation]] (DIS specific review of the FlightGear multiplayer system/protocol and its limitations, which can be addressed via HLA)
== External links ==
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