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Efficient Event Update Mechanism for Client Subscription in EPICS Collaboration

This project aims to enhance the existing client-server interaction in EPICS by introducing a modular, server-side plug-in architecture for efficient event update subscriptions. The design considerations include compatibility with existing protocols, modularity for small systems, and API compatibility. The project is currently in the design phase and is expected to be operational by summer 2010.

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Efficient Event Update Mechanism for Client Subscription in EPICS Collaboration

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  1. Advanced Monitor/Subscription Mechanisms Ralph Lange EPICS Collaboration Meeting October 11, 2009

  2. Motivation • Things get fasterNot all clients want all updates • Wide use of timing/event systemsClients want to get updates only during “interesting” system states

  3. Current Limitations • Update rate is configured in the database One setup for all clients • Existing timing/event system driver and support • May cause records to process on event • May set time stamps from hardware No filtering of updates for unrelated records

  4. Design Considerations • Channel Access protocol compatibilityChanges create a lot of trouble • ModularitySmall systems need small footprint • API CompatibilityDon’t break existing 3rd party code

  5. Server-Side Plug-Ins Channel Access Client 2 Channel Access Client 1 Client Ethernet • Stackable modules Inserted between the event subscription and the event queue • Configuration and Instantiation Client uses JSON modifiers in PV name Plug-in parses its own configuration Event Task 2 Event Task 1 IOC Channel Access Server EventQueue EventQueue EventQueue One set of event queues per client ring buffer ring buffer ring buffer Record processing or field changes cause event updates Plug-In Layer Record Event Subscriptionfield 1flags Event Subscriptionfield 2flags Event Subscriptionfield 1flags Event Subscriptionfield 3flags MLISfield 1 field 2field 3…

  6. Possible Uses • Update rate limiting Current.VAL{”rate”:{”min”:0.1, ”max”:0.5}} • System state correlation Current.VAL{”state”:{”while”:”injecting”}} Timing/event system sets and resets state Plug-in passes only updates while in “injecting” state • Event value filtering Current.VAL{”avg”:3} • Array data buffering • …

  7. Project Status and Schedule • Currently in design phase • Work by Andrew Johnson and Ralph Lange • Part of EPICS Base 3.15 • Expected to be working summer 2010 • First plug-ins will include • Update rate limiter • Event-derived system state correlator

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