data distribution service (dds)...

© 2006 RTI and THALES - All rights Reserved 1

Data Distribution Service (DDS)Introduction

Space Focus Day

26 September 2006

Anaheim CA

dds/2006-09-02

Gerardo Pardo-Castellote – RTI

[email protected]

26 September 2006

© 2006 RTI - All rights Reserved 2

The DDS Standard

Data Distribution Service for Real-Time Systems

� Adopted in June 2003

� Finalized in June 2004

� Revised June 2005

� Joint submission (RTI, THALES, OIS)

� Specification of API required to facilitate the Data-Centric Publish-Subscribe communication environment for real-time distributed systems.

3

EU Air Traffic Management

RETF (USA)Train Communications

Tokyo JapanTraffic Control

Army FutureCombat System

Boeing AWACSProgram

US Navy, DD(X)LCS, SSDS, LPD-17

and 13 Navies

DDS Adoption


DDS mandated for data-distribution

DISR (formerly JTA)

� DoD Information TechnologyStandards Registry

US Navy Open Architecture

FCS SOSCOE

� Future Combat System –System of System CommonOperating Environment

In Progress

� RETF

� Railroad Electronics Task Force

� UK MOD

� Advocating Open Systems


DDS

Point-to-PointTelephone, TCPSimple, high-bandwidthLeads to stove-pipe systems

Client-ServerFile systems, Database, RPC, CORBA, DCOMGood if information is naturally centralizedSingle point failure, performance bottlenecks

Publish/Subscribe MessagingMagazines, Newspaper, TVExcels at many-to-many communicationExcels at distributing time-critical information

Middleware Information Models

Replicated DataLibraries, Distributed databasesExcels at data-mining and analysis


DDS Enables Global Data

InstanceInstance

Instance 2Instance 2

InstanceInstance

Data Writer1

Data Writer1

Data Writer2

Data Writer2



Data ReaderData Reader

Data Reader

Data ReaderData Writer

Topic “Square”Topic “ Topic “Topic “Circle”

Address in Global Data Space = (DomainId, Topic, Key)

� Each topic corresponds to a multiple data instances

� Each DataWriter can write to multiple instances of a single topic

� Multiple DataWriters may write to the same instance

� Each DataReader can read from multiple instances of a single topic

� Multiple DataReaders may read from the same instances


DataReader“Circle”

DomainParticipant

DataWriter

“Circle”

DomainParticipant

DDS Communications Model

Publisher declares information it has and specifies the Topic

� and the offered QoS contract

� and an associated listener to be alerted of any significant status changes

Subscriber declares information it wants and specifies the Topic

� and the requested QoS contract

� and an associated listener to be alerted of any significant status changes

DDS automatically discovers publishers and subscribers

� DDS ensures QoS matching and alerts of inconsistencies

Failed to produce

data

Listener

Offered

QoS Listener

Failed to get data

Offered

QoS


DDS: Quality of Service

TRANSPORT PRIORITYCONTENT FILTERS

PRESENTATIONLIFESPAN

DESTINATION ORDERENTITY FACTORY

LATENCY BUDGETDEADLINE

LIVELINESSTIME BASED FILTER

OWNERSHIP STRENGTHRELIABILITY

OWNERSHIPRESOURCE LIMITS

PARTITIONWRITER DATA LIFECYCLE

GROUP DATAREADER DATA LIFECYCLE

TOPIC DATAHISTORY

USER DATADURABILITY

QoS PolicyQoS Policy

Vol

atili

tyU

ser QoS

Del

iver

yP

resentationR

edundancyIn

fras

truc

ture

Transport


What makes DDS different?

Data-centricity

� High level of data abstraction: Domain, Topic, Key

� Proven scalable model for RT systems

� “Smart” services such as:

� Ownership, ContentFilteredTopics, KeepLast History

� Built-in support for redundancy

� Automatic discovery

� Directly supports state propagation/caching

Configurability by QoS

� Wide range of applicability: Enterprise to real-time

� P2P infrastructure:

� High-performance and scalability

� Fault-tolerance

� Scalability

� Subsumes message-oriented and data-centric

Object models are built as local cache


Data-Distribution and Real-Time

Non-real-time Soft real-time Hard real-time Extreme real-time

Java/RMIJava/JMS

CORBA

MPI

Java RTSJ (soft RT) RTSJ (hard RT)

Web Services

Mes

sag

ing

Tec

hn

olo

gie

s an

d S

tan

dar

ds

Mes

sag

ing

Tec

hn

olo

gie

s an

d S

tan

dar

ds

Data Distribution Service / DDS

RT CORBA

Adapted from NSWC-DD OA Documentation

Enterprise BusinessLogic Integration

Real-TimeTacticalInformation


DDS/DCPS

Provides a “Global Data Space” that is accessible to all interested applications.

� Data objects addressed by DpmainId, Topic and Key

� Subscriptions are decoupled from Publications

� Contracts established by means of QoS

� Automatic discovery and configuration

DistributedNode

Global Data Space

DistributedNode

DistributedNode

DistributedNode

DistributedNode

DistributedNode

P

PPP

P

P

X

P

P


Example without Keys

When not using keys:

� Each Topic corresponds to a single data instance.

� A DataWriter associated with a Topic can write to the instance corresponding to That topic.

� Multiple DataWriters may write to the same instance.

� A DataReader specifies the Topic (instance) it wants to receive updates from.

Topic “green” Topic “orange”

InstanceDW 1

DW 2

DW 3

Instance

DR 1

DR 2

write

write

write

read

read

DR 3read


Example with Keys

Address in Global Data Space = (DomainId,Topic, Key)

� Each Topic corresponds to multiple data instances

� Each DataWriter can write to multiple instances of a single Topic

� Multiple DataWriters may write to the same instance

� Each DataReader can receive updates from multiple instances of a single Topic

� Multiple DataReaders may read from the same instances

DW 1

DW 2

DW 3

DR 1

DR 2

write

write

write

read

read

DR 3read

Instance

Instance

Instance

Instance

write read


Domain Separation

Instance

DW 3

DW 4

DW 5Instance

DR 4

DR 5

DR 6

Topic “green” Topic “orange”

InstanceDW 1

DW 2Instance

DR 1

DR 2

DR 3

Domain2

Domain1

2

1

1

2

DomainParticipant Node


Example: Publication

// Entities creationPublisher publisher = domain->create_publisher(

publisher_qos, publisher_listener);

Topic topic = domain->create_topic(“Track”, “TrackStruct”,topic_qos, topic_listener);

DataWriter writer = publisher->create_datawriter( topic, writer_qos, writer_listener);

TrackStructDataWriter twriter = TrackStructDataWriter::narrow(writer);

TrackStruct my_track;// (Repeat each time data needs to be written)twriter->write(&my_track);


Example: Subscription

// Entities creationSubscriber subscriber = domain->create_subscriber(

subscriber_qos, subscriber_listener);

Topic topic = domain->create_topic(“Track”, “TrackStruct”, topic_qos, topic_listener);

DataReader reader = subscriber->create_datareader( topic, reader_qos, reader_listener);

// Use listener-based or wait-based access


How to Get Data? (Listener-Based)

// Listener creation and attachmentListener listener = new MyListener();reader->set_listener(listener);

// Listener codeMyListener::on_data_available( DataReader reader ){

TrackStructSeq received_data;SampleInfoSeq sample_info;TrackStructDataReader treader =

TrackStructDataReader::narrow(reader);

treader->take( &received_data, &sample_info, …)

// Use received_data}


QoS Contract “Request / Offered”

QoS Request / Offered: Ensure that compatible QoS parameters are set.

QoS:DurabilityQoS:PresentationQoS:DeadlineQoS:Latency_BudgetQoS:OwnershipQoS:LivelinessQoS:Reliability

Offered

QoSRequested

QoS

X

QoS not compatible

Communication not established

Topic

Subscriber

DataReader

DataWriter

Publisher

Topic


QoS: RELIABILITY

DataWriter

R

BEST_EFFORTSample delivery is not guaranteed

TopicBE

DataReader

BE

SubscriberPublisher

DataWriter

BE

Publisher

TopicR

DataReader

R

SS S S

Missed samples

S7 S5S6 S4 S3S2 S1

RELIABLESample delivery is

guaranteed

S7

S5S6

S4S3S2S1

history


QoS: HISTORY – Last x or All

DataWriter

Keep All

KEEP_LAST: “depth”integer for the number of

samples to keep at any one time

DataReaderKeep all

Publisher Subscriber

Topic

S5S7 S2S4 S1

KEEP_ALL:Publisher : keep all until deliveredSubscriber : keep each sample until the application processes that instance

S7

S5S6

S4S3S2S1

S7

S5S6

S4S3 Data

ReaderKeepLast4

Subscriber

S7

S5S6

S4

S3S6

DataWriter

KeepLast 2

Publisher

S7S6

Topic

S7 S6 S5 S4 S3


QoS: DEADLINE

Topic

Publisher

DataWriter

Subscriber

DataReader

DEADLINE “deadline period”

deadline

Commits to provide data each deadline period.

Expects data every deadline period.

S X S S S S S

Listener

Failed to get data


QoS: LIVELINESS – Type, Duration

Domain Participant

DataWriter

Topic

Publisher

LP S LP LP

lease_duration

X

DataReader

Subscriber

Listener

Liveliness Message

Domain Participant

"type"• AUTOMATIC = Infrastructure Managed• MANUAL = Application Managed

Failed to renew lease


QoS: TIME_BASED_FILTER

Domain Participant

DataWriter

Topic

Publisher

SS S S S

minimum separation

DataReader

Subscriber

Data Samples

“minimum_separation”: Data Reader does not want to receive data faster than the min_separation time

SS

Discardedsamples


DDS Subscription ( ContentFilteredTopic)

Content Filtered Topic

“Filter Expression ”Ex. Value > 260

Value = 249

Topic Instances in Domain

Instance 1

Value = 230Instance 2






The Filter Expression and Expression Params will determine which instances of the Topic will be received by the subscriber.

Topic

optionaloptional


QoS: OWNERSHIP_STRENGTH

DataWriter

Topic

DataReader

Subscriber

Domain Participant

Publisher

DataWriter

Publisher

S=1 S=4 S=4

Note: Only applies to Topics with OWNERSHIP=Exclus ive

OWNERSHIP_STRENGTH: Specifies which writer is allowed to update the val ues of data-objects

S


QoS: LATENCY_BUDGET

� Intended to provide time-critical information to the publisher for framework tuning where possible.

� Will not prevent data transmission and/or receipt.

Physical I/F

Transport

DDS

Application

Publication

t1

t2

Physical I/F

Transport

DDS

Application

Subscription

t3

Latency = t1 + t2 + t3


QoS: RESOURCE_LIMITS

� Specifies the resources that the Service can consume to meet requested QoS

Domain Participant

Topic

123

12

4

123

DataReader

Subscriber

max_samples: max # data samples for a single DW or DR, across all instances

max_instances: max # instances for a single DW or DRmax_samples_per

_instance: max # data samples per instance

……


QoS: USER_DATA

� User-defined portion of Topic metadata

� Example of use: Security Authentication

Entity:

Domain Participant (user_data)DataReader (user_data)DataWriter (user_data)

DDS Node

ignore_participant()ignore_publication()ignore_subscription()ignore_topic()

USER_DATA can be used to authenticate an origination entity.

Remote Application

Authenticate Origin

DDS Node

Accept Entities

yes

no

Note: USER_DATA is contained within the DDS metadata.


QoS: PARTITION

Publisher

Domain Participant

TopicA

TopicB

Subscriber

DataWriter

DataWriter

DataReader

DataReader

TopicC

DataWriter

DataReader

PartitionU,Z

DataReader

Logical “namespace” for topics

** Partition string namesmust match between publisher and subscriber

Subscriber

PartitionX,Y

PartitionU,W


QoS: DURABILITY

Topic

Publisher

Domain Participant

DataWriter

# saved in Transient affected by QoS: History and QoS: Resource_Limits

Publisher

Domain Participant

DataWriter

LocalMemory

Durability =Transient

Durability =Volatile

Durability Kind:VOLATILE – No Instance History SavedTRANSIENT – History Saved in Local MemoryPERSISTENT – History Saved in Permanent storage

Perm.Storage

Durability = Persistent

Determines if/how instances of a topic are saved.


DistributedNodeGlobal Data

Space

DistributedNode

DistributedNode

PPP

P

DDS-DCPS Summary

DDS targets applications that need to distribute data in a real-time environment

DDS is highly configurable by QoS settings

DDS provides a shared “global data space”

� Any application can publish data it has

� Any application can subscribe to data it needs

� Automatic discovery

� Facilities for fault tolerance

� Heterogeneous systems easily accommodated

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