dds efficiency and extensibility

Chief Technology OfficerPrismTech

DDS SIG Co-ChairObject Management Group (OMG)

Angelo Corsaro, Ph.D.[angelo.corsaro@prismtech.com]

Efficiency and Extensibility

./type-matters

The Type Systems Debate

‣ The debate around type Programming Languages Type Systems has animated computer scientist over several decades and is far from being resolved

‣ New Programming Languages are continuing to appear on both sides of the fence

F#Statically Typed (ex.) Dynamically Typed (ex.)

Are you a Type-Phobic Type?

‣ Proponents of Dynamically Typed Programming Languages advocate against strong typing in favor of reduced verbosity and added flexibility

Yet... Are they so sure that a type system always gets between you and what you are trying to achieve?

Strongly-Typed Type-Systems

‣ Well designed type-systems don’t add unnecessary verbosity to your application since use sophisticated inference to derive types

‣ Allow the detection of many errors at compile time, thus improving productivity, safety, code quality and reducing the potential for run-time errors

‣ Enable the generation of more efficient code

Types and Distributed Systems

‣ The “Type System debate” has not spared distributed systems

‣ Today we can find different technologies taking different approaches: from completely untyped to strongly typed

‣ The differences are in this case more complex to organize:

‣ Some technologies support only one type, being a string or an array of bytes (e.g. AMQP)‣ Other technologies support some predefined types (e.g JMS)‣ Other technologies allow the definition of user-specified types (e.g DDS)

T1 T2 T3 T4

User Defined Types Example: T1 = RadarTrack T3 = FlightPlanT2 = ClassificationInfo T4 = ...

octect sequence

T1 T2 T3 T4

User Defined Types

Pub/Sub Types

Example: T1 = RadarTrack T3 = FlightPlanT2 = ClassificationInfo T4 = ...

ObjectMessage TextMessage

ByteMessage

StreamMessage

MapMessage

Pub/Sub Types

Pub/Sub Types == User Types!

./dds-types

Topic: the unit of Information

Topic Type

‣ Extensible and Evolvable Unit of information exchanged between Publisher and Subscribers.

‣ An association between a unique name, a type and a QoS setting

TrackedObject

Track, UFO, AirCraft, ...

{ Reliability Deadline, Priority Transient, ...}

TopicType QoS

Topic: the unit of InformationTopic Type

‣ Extensible and evolvable Type describing the data associated with one or more Topics

‣ A key, made by one or more attributes

‣ Annotations express attribute properties

‣ Expressed in IDL, Java, XML, or UML

struct Track { long tid; //@key long x; long y;};

Writing Tracksusing dds::core; using dds::domain; using dds::pub; using dds::topic;

DomainId id = 0;

DomainParticipant dp = theParticipantFactory().create_participant(id);

Publisher pub = dp.create_publisher();

Topic<RadarTrack> topic = dp.create_topic("RadarTrackTopic");DataWriter<RadarTrack> dw = pub.create_datawriter();

RadarTrack t("T101", 100, 200);

dw.write(t);

Reading Tracksusing dds::core; using dds::domain; using dds::pub; using dds::topic;

DomainId id = 0;DomainParticipant dp(id);

DomainParticipant dp = theParticipantFactory().create_participant();

Subscriber sub = dp.create_subscriber();

Topic<RadarTrack> topic = dp.create_topic("RadarTrackTopic");DataReader<RadarTrack> reader = sub.create_datareader();

std::vector<RadarTrack> t(max_size);std::vector<SampleInfo> i(max_size);

reader.read(t.begin(), i.begin(), max_size);

./efficiency

Encoding Efficiency

‣ DDS serialization is several times more efficient than JSON, XML and YAML

‣ Differences can be quite extreme when dealing with complex types

Size for Serialized CoFlight Flight Data Plan

see Esposito et al. OMG RTWS 2008

Encoding Impact on RTT

see Esposito et al. OMG RTWS 2008

DDS RTT with different Serialization Format

‣ Inefficient encoding can easily dominate RTT

‣ DDS native encoding is the most efficient among extensible encodings!

Size (bytes)

‣ DDS implementations are capable of delivering very low and predictable latencies

‣ No other Pub/Sub technology can approach similar level of performance!

DDS is Very Efficient!

./extensibility

Type Extensibility

Problem

‣ What if we need to extend the RadarTrack type to add the derivative on the x and y direction?

‣ How can we do this with no impact on running applications?

Type ExtensibilitySolution‣ Simply extend the existing RadarTrack and properly annotate new

attributes

‣ Existing application will detect that the new type is an extension and will “project/extend” it to the known type

‣ Type “projection/extensions” can be controlled via QoS

struct Track { long tid; //@key long x, y;};

struct Track { long tid; //@key long x, y; long dx, dy;};

Type Extensibility

Problem

‣ OK, that was good. But now I need to add an optional plot to my track

‣ How can I add an optional field? Furthermore, I don’t want the plot to be copied into the topic, I’d like it to be a reference. Can I do this to limit the number of copies and reduce he memory utilization?

Type ExtensibilitySolution

‣ Simply extend the existing RadarTrack and properly annotate new attributes

struct Track { long tid; //@key long x, y; long dx, dy;};

struct Track { long tid; //@key long x, y; long dx, dy; sequence<octet> plot; //@optional @shared};

In Summary

‣ DDS provides a strongly typed extensible type system

‣ Types can be evolved and extended without breaking backward compatibility nor loosing type-safety

‣ Type “Projection/Promotions” can be fully controlled via QoS

‣ This flexibility does not come at the cost of introducing the time+space overhead typical of XML-based approaches

DDS provides extensibility without compromising efficiency!

./coming-soon

./web-enabled

TrackClassifier

Display

DDSJMS

TrackClassifier

Display

DDSJMS REST

TrackClassifier

Display

Web Browser

iPhone

DDSJMS

ControllerWorkingPosition

Web Browser

iPhone

TrackClassifier

Web-Enabled DDS

Standardize access to the DDS Global Data Space via:

‣ RESTful Web Services

‣ W3C Web Services

‣ RSS

‣ Atom

‣ XMPP

./demo

./security

Secure DDS

‣ An RFP is close to be finalized for standardizing a security solution for DDS

‣ Key use cases taken into consideration come from Military and Civilian systems

Concluding Remarks

‣ DDS is a standard that was designed for time/space efficiency as well as real-time predictability

‣ DDS provides support for end-to-end type propagation and type checking with a very powerful type extensibility mechanism

‣ Upcoming standards for DDS are coming to address integration with Web/Internet technologies as well as security

Learn More

‣ Data Distribution Service for Real-Time Systems

‣ Extensible and Dynamic Topic Types Specification

‣ Web-Enabled DDS RFP

http://www.omgwiki.org/dds/content/page/specifications

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