Real-time Publish/subscribe

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Real-time Publish/subscribe

• ECE 1770-- Expert Topic

Lizhong Cao Milenko Petrovic March 6th,2003
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What Is Real-time?

• Real-time vs. Non-real-time
• Hard real-time vs. Soft real-time
• Fundamental factors
• Time constraints (deadline)
• Timeliness optimality
• Sequencing
• Predictability
• Traditional real-time (static)
• Next generation real-time (adaptive, dynamic)

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Distributed Computing System

• Asynchronous and synchronous models
• Bounds/no bounds on execution and transmission
  latency, clock drift rate
• Real-time distributed system is partial
  synchronous at least.
• Multi-node behaviors
• Including control flow (RPC/RMI), data flow (
  pub/sub) , asynchronous message-passing, mobile
  code, virtual shared memory
• Multi-node properties maintained integrity of
  behavior, detection and handling of distributed
  events, timeliness, security and fault management.

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Real-time Distributed

• Horizontal real-time (end to end timeliness)
• Multi-node sequencing categories
• Using a consistent sequencing policy (alpha,
  RT-CORBA 1 / 2)
• Interact to perform sequencing for all nodes
  (improved timeliness, but higher costs)
• One or more level meta resource sequencer above
  node resource sequencer (hierarchical)
• Priority scheduling
• Fixed priority (real-time CORBA 1)
• Dynamic scheduling ( real-time CORBA 2)

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Real-time CORBA

• RT-CORBA end-to-end predictability,
  fixed/dynamic scheduling, vertically and
  horizontally integrated and managed by ORB

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Real-time Java DRTSJ/RMI

• Distributed real-time specification for java
  (DRTSJ)
• Supporting predictability of end-to-end
  timeliness for sequentially trans-node behaviors
  in dynamic distributed object systems.
• Based on alpha distributed real-time OS
• Distributed real-time RMI
• Arbitrary scheduling disciplines, global physical
  time, crash failures only, RT-JRMP and IIOP

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Real-time Pub/sub

• Pub/sub is ideal for real-time data distribution
• Many-to-many communication
• Distributing time-critical information
• No request traffic, direct event-driven transfer
• Anonymous communication structures code
• Maps well to real-time needs
• Streaming signals
• Status updates
• Event-driven commands

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Real-time Pub/sub Model

• RTPB model by RTI
• Publisher
• Fast, robust, re-configurable
• Supports fail-over and migration
• Subscriber
• Flow-controlled, timely delivery of each data
  stream
• Service
• Fault-tolerant request and reply
• Requests have deterministic best then first
  semantics
• Tradeoff between speed and quality of response
• Reliable and determinism tradeoff

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Existing Real-Time P/S

• Allow timing constraints in P/S model
• Once events and subscriptions have timing
  information, scheduling is possible
• Running time of matching algorithms is usually
  not predictable can do soft real-time only
• Subject-based P/S simple and fast easier to
  bound matching time can do hard-real time
• Centralized P/S only

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P/S Systems for Multimedia

• First papers on multimedia and P/S (ICDCSW'02)
• Focus on using existing (non-real-time) P/S to
  improve soft real-time applications
• Elvin Real-Time Protocol
• Allows TV channel customization without
  intervention from the user
• Content Analysis using MBus
• Content analysis is time-sensitive and
  computationally expensive
• Uses P/S to build distributed content-analysis
  prototype

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P/S Model for Real-TimeDatabases

• Active real-time databases (ARTDB) use ECA
  (event-condition-action) rules with timing
  constraints for real-time transactions (ARTDB'95)
• ECA Example If IBM stock price declines by 10
  within any 10min interval, then notify me and
  send a sell order for 1000 IBM shares within 2sec
• Real-time update propagation/cache update
  (Distributed and Parallel Databases 9(1), 2001)
• Keep client-side cache up to date
• Introduce notification deadline for each
  subscription

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Real-Time P/S Middleware

• ISIS-like toolkit for distributed real-time
  systems with focus on analyzability and
  predictability (RTAS'95)
• RTPS Protocol/NDDS (Stanford, RTI Inc. )
• (Soft) Real-time publish subscribe protocol
  (Informational RFC from IETF)
• CORBA-based real-time P/S (JSS'02)
• For developing distributed and embedded
  applications using COTS platforms

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Network Data Delivery Service

• Pardo-Castellote and Schneider (ICRA'94)
• Subject-based, soft real-time

always notify at deadline
accept only updates of higher strength
Consumer
time
new updates only
no notifications
persistence
accept any update
data updates received
time
min. separation
Producer
consumer notified
deadline
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CORBA-based real-time P/S

• COTS components
• Based on TAO
• Content-based P/S
• Allows federated brokers
• Timeliness ensured using staged matching

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Open Issues

• Content-based real-time P/S
• Hard real-time P/S
• Real-time distributed P/S

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Thank You