DDS Efficiency and Extensibility

DDS OMG DDS OMG DDS OMG DDS OMG DDS OMG DDS OMG DDS OMG DDS

Efﬁciency and Extensibility
Angelo Corsaro, Ph.D.
[angelo.corsaro@prismtech.com]
Chief Technology Ofﬁcer DDS SIG Co-Chair
PrismTech Object Management Group (OMG)


./type-matters

© 2010 PrismTech. All Rights Reserved.


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

Statically Typed (ex.) Dynamically Typed (ex.)

F #


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)



T3
T1
T2 T2

T3

T4
? T1

T4

T1 T1

T3 T2

User Deﬁned Types Example:
T1 = RadarTrack T3 = FlightPlan
T1 T2 T3 T4
T2 = ClassiﬁcationInfo T4 = ...



T3
T1
T2 T2

Pub/Sub Types
T3 T1
octect sequence
T4 T4

T1 T1

T3
AMQP T2

User Deﬁned Types Example:
T1 T2 T3 T4



T3
T1
T2 T2
MapMessage

T3 ObjectMessage TextMessage T1

T4 StreamMessage T4
ByteMessage Pub/Sub Types
T1 T1

JMS
T3 T2

Example:



T3
T1 Pub/Sub Types == User Types!
T2 T2

T1 T1
T3 T1
T2 T3
T4 T3 T4
T4
T1 T1

DDS
T3 T2

Example:


./dds-types



Topic: the unit of Information
Topic Type

Track, UFO, AirCraft, ...
‣ Extensible and Evolvable
Unit of information Name
exchanged between
Publisher and Subscribers.
Type Topic QoS
‣ An association between a
unique name, a type and TrackedObject {
Reliability
a QoS setting Deadline,
Priority
Transient,
...
}


Topic: the unit of Information
Topic Type
‣ Extensible and evolvable Type

describing the data associated with
one or more Topics
struct Track {
long tid; //@key
‣ A key, made by one or more long x;
attributes long y;
};
‣ Annotations express attribute
properties

‣ Expressed in IDL, Java, XML, or UML


Writing Tracks
using 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 Tracks
using 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
Size for Serialized CoFlight Flight Data Plan

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

‣ Differences can be quite
extreme when dealing with see Esposito et al. OMG RTWS 2008

complex types


Encoding Impact on RTT
DDS RTT with different Serialization Format
see Esposito et al. OMG RTWS 2008

‣ Inefficient encoding can
easily dominate RTT

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


DDS is Very Efﬁcient!

‣ DDS implementations are
capable of delivering very
low and predictable

Latency (usec)
latencies

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

Size (bytes)


./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 Extensibility
Solution
‣ 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 { struct Track {
long tid; //@key long tid; //@key
long x, y; 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 Extensibility
Solution

‣ Simply extend the existing RadarTrack and properly
annotate new attributes

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



Track

Radar
Classifier

DDS
Display



Track
Classifier

JMS
Track

Radar
Classifier

DDS
Display



Track
Classifier

JMS
Track

Radar
REST
Classifier

DDS
Display

Web Browser

iPhone



Track
Classifier

JMS
Track

Radar
Classifier

DDS
Position
Working
Controller

WS-*

REST
Web Browser
Tuner

iPhone



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

DDS Efficiency and Extensibility

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