NDDS: The Real-Time Publish Subscribe Middleware

Network Data Delivery ServiceAn Efficient Real-Time Application Communications PlatformPresented By: Chris Lee

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NDDS: Outline

➢Real-Time Data Flow Semantics➢Communication Architectures➢The Publish-Subscribe Model➢The Real-Time Publish-Subscribe Model➢NDDS Introduction➢Performance

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Real-Time Data Flow Semantics

➢Understanding Flow Types Critical to RT➢ Signals (change constantly)

➢ Time-critical: Updates are useless if old.➢ Idempotent: Repeated updates are acceptable.➢ Last-is-best: Latest information is more

important than retrying missed samples➢ Commands (sequential instructions)

➢ Status (gives state or goal)

➢ Events (synchronize asynchronous operations)

➢ Requests (specific request for data)

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Properties of Signals

➢Signals➢ Rapid in nature➢ Short persistance➢ High bandwidth➢ Most recent value is more important

➢Example➢ Water-level sensor for pumps

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Properties of Commands

➢Commands➢ Delivered in-order➢ Delivered once and only once➢ Often not time-critical

➢Example➢ Open a pump valve by 10%

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Properties of Status

➢Status➢ Usually persists for some time interval➢ Single delivery is usually not critical➢ May not be time critical or reliable

➢Example➢ Tank low status

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Properties of Request

➢Request➢ Could be blocking or non-blocking➢ Not uniform, each request different

➢Example➢ Request a pumps value setting

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Communication Architectures

➢Three Major Architectures➢ Client-Server Architectures

➢ All data flows through the server to the clients.➢ Could become bottleneck of communication.➢ Expensive point of failure.

➢ Object Brokers➢ Relies on broker to route requests➢ Cumbersome for multi-to-multi communicaitos➢ Little support for deterministic delivery

➢ Publish-Subscribe (Next slide)

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The Publish-Subscribe Model

➢Publish-Subscribe Model➢ Simplifies one-to-many data-distribution➢ Publishers send data anonymously➢ Subscribers receive data anonymously

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Publish-Subscribe Advantages

➢Publish-Subscribe are best-suited to distributed applications because...➢ Applications are modular and scalable

➢ Easy to manage regardsless of scale➢ Data subscribed by name ➢ Easy redundancy*➢ Efficient bandwidth utilization

➢Not well suited for...➢ Sporadic request/response traffic

*When multiple publishers and subscribers can be supported, this allows programs to be replicated and moved transparently.

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Characteristics of P-S

➢Characteristics of Publish-Subscribe➢ Distict declaration and delivery

➢ Publisher declares intent to publish.➢ Subscriber declares interest.➢ Publisher sends a publication issue.

➢ Named publications➢ Many-to-many communications support➢ Event-driven transfer

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PS Middleware

➢The Publish-Subscribe Middleware➢ Maintains a database that maps publishers to subscribers forming logical data channels

➢ Marshals/Demarshals data from pub to sub➢ Delivers data when published

Application

ApplicationProgramming

Publish-subscribeMiddleware

Operating SystemNetwork Interface

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Goals of Real Time PS

➢Goals of Real Time Publish-Subscribe➢ Delivery timing control➢ Reliability control➢ Request-reply semantics*➢ Flexible delivery bandwidth➢ Fault tolerance**➢ Selective degradation➢ Robustness➢ Dynamic scalability➢ Efficiency

* Does not fit will into PS semantics.** Could Use “hot standby” publishers and subscribers.

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The RTPS Model

➢The Real-Time Publish-Subscribe Model➢ Extends the PS model for timing-control and client-server communication

➢ Publication Parameters➢ Subscription Parameters➢ Reliability and Time-Determinism➢ Client-Server Requests

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Publication Parameters

➢Publication Parameters➢ Four parameters:

➢ Topic – label that identifies each data flow.➢ Type – describes the data format.➢ Strength – indicates publisher's weight relative

to other publishers of the same topic.➢ Persistence – indicates how long a publication

is valid.

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Subscription Parameters

➢Subscription Parameters➢ 4 parameters

➢ Topic & Type (same as publication properties)➢ Minimum separation

➢ Interval were no new issues are accepted.➢ Deadline

➢ Max time a subscriber should wait for the next issue.

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Reliability and Time-Determinism

➢Reliability and Time-Determinism➢ Publishers and Subscribers buffer issues➢ Ensures that subscribers receive issues in the proper sequence

➢ Uses issue sequence numbers➢ Subscribers request missing issues➢ Issues removed from buffer when all subscribers receive the issue

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Client-Server Requests

➢Client-Server Requests➢ Breaks the PS model; two-way traffic➢ RTPS makes requests consistant with PS model by implementing requests via a named services.

➢ Has separate data types for request and reply➢ Has best-then-first semantics ( Next Slide )

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Best Then First Semantics

➢Best Then First Semantics➢ Client specifies a minimum wait and timeout for a reply

➢ Client will choose “strongest” reply received by the first decision time, unless no reply has come in which the client will choose the first reply or timeout.

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NDDS Introduction

➢Network Data Delivery Service➢ Middleware for distributed, real-time application development

➢ Uses publish-subscribe to:➢ Handle one-to-many and many-to-many data

distribution.➢ Meet RT and reliability objective though

customizable middleware.➢ Provide application-transparent fault tolerance

and system robustness

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NDDS Outline

➢NDDS Outline➢ Architectural Overview

➢ Middleware layers➢ Symmetric Design

➢ RTPS Enhancements➢ Patterned subscriptions➢ Customizing reliability requirements➢ Multicast support

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NDDS Architectural Overview

➢Architectural Overview➢ Library

➢ Linked into application➢ Database

➢ Keeps information about nodes➢ Tasks

➢ Performs message addressing➢ Marshaling/Demarshaling➢ Transports

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Symmetric Design

➢Symmetric Design➢ NDDS maintains a database at each node

➢ Robust: No “special” or “central” nodes that can fail

➢ Nodes can join and leave at any time

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RTPS Enhancements

➢RTPS Enhancements➢ Pattern subscriptions

➢ Allows subscriber to receive a set of related publications without subscribing to each one

➢ Customizing reliability requirements➢ Reliability vs. Determinism controlled by

programmer➢ Multicast support

➢ Provides flexibility for high bandwidth data (at a cost of latency)

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NDDS Performance

LatencyTotal end-to-end delay form publisher to subscriber for different publication issue sizes.

Found to be largely independent of packet loss rates.

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Maximum Rate and Throughput

ThroughputMaximum issue rate and corresponding publisher-to-subscriber thoughtput for different issue sizes.

Heavily dependent on sample size and acceptable packet loss.

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References

➢ G. Pardo-Castellote, S. Schneider, and M. Hamilton, “NDDS: The Real-Time Publish-Subscriber Middleware”, Real-Time Innovations, Inc., August 1999.

➢ G Parso-Castellote, S. Thiebaut, M. Hamilton, H. Choi, “Real-Time Publish-Subscribe Protocol for Distributed Real-Time Applications”, Real-Time Innovations, Inc., September 2001.

➢ Bob Kindel, “Publish-Subscribe Middleware for Real-Time Applications”, PowerPoint presentation, July 2002. http://www.opengroup.org/boston-jul02/uploads/ 40_1028305308__qos_bos_real-time_bob_kindel.pdf