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Distributed Interactive Simulation
DIS is an IEEE standard for simulations, primarily virtual worlds
• Binary protocol: what’s standardized is the arrangement of bits in packets on the wire
• Mostly intended to push entities around in a 3D virtual world, so the most common packets relate to position, orientation, velocity, etc.
DIS: Entity State PDU
Entity State Protocol Data Unit/Packet:• PDU header: common to all PDUs• Entity ID: uniquely identifies an entity in
the world• Entity Type: describes the entity, ie tank,
plane, etc.• Position• Orientation• Velocity, acceleration, angular velocity,
etc.
DIS
Remember, what’s standardized is the arrangement of the bits in packets. There is no standardized API
Typically you convert a packet into a programming language object to work with, modify, read field values, and send
Information Representation
Programming Language
Object
DISBinary
Representation
Same information--position,orientation, etc--but in twodifferent representations, onecontained in a Java or C++object, one in a particular arrangement of bits in a packet
Other Representations
Why not expand this to multiple representations? For example, XML, Java Object Serialization format, SQL database, etc.
The DIS format is mostly useful as a standard for exchanging information with other simulations, but the drawback is that nobody outside DoD M&S understands the DIS format
If we had an XML representation of the data we could use bring the standard XML tools to bear for data transformation, data exchange, web services, etc.
This lets us exploit the information contained in DIS simulations in other contexts
Other Representations
Programming Language
Object
DISBinary
Representation
XMLRepresentation
Java ObjectSerialization
Representation
XML
What are the advantages of an XML representation?
• THE standard for data interchange• Useful for archiving• Lots of tools available for transforms, SQL
interchange, etcDisadvantages:• Verbose• Slow to parse• Exact format not standardized (no single
schema agreed upon for DIS)
Creating XML
The Open-DIS package at sourceforge.net is able to handle the process discussed here
https://sourceforge.net/projects/open-disIt uses a package from Sun called JAXB,
the Java API for XML Binding
JAXB
The process of “data binding” often comes up with XML. This is exactly the problem discussed earlier of having the same information in two different formats: XML and Java objects
You’ve got an XML document, and want to process the information in it. Often the easiest way to do this is to convert the XML document to Java objects, perform the computation, then convert the Java objects back to XML
Since this happens so often, it makes sense to have an automated framework to do this
JAXB
<Points><point x=“1.0” y=“2.0”/><point x=“3.0” y=“4.0”/></Points>You want to convert this to something like a list of
the following objectspublic class Point{ float x; float y;}
JAXB
JDK 1.6 includes JAXB in the standard distro. The easiest way to use it is via “annotations”. These look like Javadoc tags:
@XmlElement@XmlAttributeThey are essentially metainformation that
doesn’t change the meaning of the code, but are present so other tools can make use of the information
Using Annotations
The Cliff Notes version:• Have the object be Java Beans compliant• Use @XmlElement when you want to
marshal an instance variable that is an object
• Use @XmlAttribute when you want to marshal an instance variable that is a primitive type
• You can place the annotations immediately before the getX() methods
Marshalling to XML
JAXBContext context = JAXBContext.newInstance(Foo.class);
Marshaller marshaller = context.createMarshaller(); marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(fooInstance, new FileOutputStream(filename));
Unmarshalling from XML
Unmarshaller unmarshaller = context.createUnmarshaller();
PduContainer unmarshalledObject = (PduContainer)unmarshaller.unmarshal(new FileInputStream("somePdus.xml"));
In Open-DIS
There’s example PDU marshal and unmarshal code in edu.nps.moves.examples.MarshalExample.java
Example XML
See example XML fileNot always what you’d write by hand, but
that’s basically OK--the work saved by using the automated tool far outweighs this. Also, JAXB can be tweaked in ways I haven’t gotten around to fully exploiting yet
JAXB can also be used to generate an XML schema for the given Java classes, or generate Java classes from an XML schema
Efficient XML Interchange
Since XML is so cool, why not use it everywhere? Trying to use it in a network protocol as a drop-in
replacement for the DIS standard is problematic• XML takes a lot of space compared to a binary
representation• It takes a lot of time to parse compared to a
binary representationEXI is intended to address these issues. Whether it
can be used as a drop-in replacement for existing binary protocols is an open research question
EXI
XML is text-only and human (well, programmer) readable. If we relax this requirement we can get a more compact and faster to parse representation of the XML infoset
Exactly equivalent to the XML document, just in a different representation
The intent is to expand the use of XML to domains in which it could not otherwise be used: low bandwidth wireless, battery-powered devices with limited CPU, very high parsing speed requirements, etc.
EXI
Emerging World Wide Web Consortium standard
See http://www.w3.org/XML/EXI/The format document has entered “final
call” status, which is actually the first time many outsiders take a close look at it
EXI
Test encodings of sample DIS PDUs in XML format has shown that they are about the same size as the original IEEE binary PDUs
A “standard” ESPDU is 144 bytes long. EXI encoded XML representations are ~140 bytes long
Since EXI encodes numbers in a variable length format, the size of PDUs can vary depending on things like the number of significant digits in floating point numbers
EXI
Unknowns: parse speed compared to binaryThe standard XML parsing APIs only deal with
strings, while the DIS data is primarily numeric. What is needed is a typed API for XML, so that we can directly retrieve a floating point number rather than converting from a string
DIS binary is the standard, EXI…isn’t. But potentially future networking standards can be specified as XML documents from the start