Real-time Publish/subscribe ECE Expert Topic Lizhong Cao Milenko Petrovic March 6 th,2003

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)

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.

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)

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

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

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

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

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

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

P/S Model for Real-Time Databases 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

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

Network Data Delivery Service Pardo-Castellote and Schneider (ICRA'94) Subject-based, soft real-time time data updates received persistence accept only updates of higher strength accept any update time consumer notified min. separation no notifications deadline new updates only always notify at deadline Producer Consumer

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

Open Issues Content-based real-time P/S Hard real-time P/S Real-time distributed P/S

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