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OMG SBC Workshop 2004

Extending CCM for real-time. Assessment on a UMTS protocol case study.

Vincent Seignole1, Virginie Watine1,

Sylvain Robert2, Ansgar Radermacher2,

Sébastien Gérard2, François Terrier2

2 CEA-LIST CEA Saclay 91191 Gif sur Yvette France

1 THALES/ALICE pilot programme THALES 91300 Massy France

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Context

RT and embedded systems sw-engineering via composition

motivations

Communication equipments less and less dedicated to specific functions

become multi-functions, multi-usage, need migration to modular design

partially open

may not be able to know completely at design time all functions they will execute

relevance of the component-based approach

provides systematic approach to deal with complexity and reuse

natural modularity of resulting system

ultimate goals (among others) :

have a library of components running on standard and common platform

allow to focus more independently on integration of components and on component programming

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Context (cont')

Component-based approach

on its way since some years in mainstream systems

needs adaptations to apply to real-time and embedded :

provision of real-time environment

richer, configurable inter-components interaction support

Topics for present talk:adaptation of Lw-CCM via:

introduction of an extensible CCM container able to support real-time features

introduction of a framework to integrate custom components interactions

confrontation with a communication system example

inspired from UMTS protocols layering at user equipment side

allows to assess and validate the developed approach.

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Presentation outline

Review of CORBA Component Model aspectsingredients, role, process

Presentation of CCM extension strategyi.e how it can be opened to handle some real-time aspectsand integrate connector notions to allow custom interactions

Confrontation with communication applicationbrief descriptionconnectors to capture and factorize recurring mechanisms for protocols inter-layers interactions

Conclusions and future work

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Review of CCM features

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Review of CCM features

a component modeldescription of components

provided/required services

events published/consumed

an execution environmentContainers

provide a templated usage of CORBAshield component developers from middleware technical concerns

underlying CORBA middleware support

a packaging modelsoftware-package formatself-descriptive : contents, dependencies

a deployment infrastructurecomponents assembly descriptors + runtime support

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CCM features review : development process / roles

different separated rolescomponent providerprovides implementationprovides packaged compnt

container providerprovides container with

adequate non-func support

integrator i.e provides assembly

description, configuration and use deployment tools to set up whole system.

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CCM extension strategy : on customizable interactions support

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Answering the need for CCM interactions improvement

Only basic interactions natively supported by CCMsynchronous method invocation(1,n) push-push typed events propagation

Wide variety of architectural styles in RT-systemsExamples are:pipes and filters

very classical in the communication protocols domain

variants of publish-subscribepush/pull, typed / untyped events

blackboard pattern

Goal: allow these kinds of architectural styles (and others) in CCM

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Answering the needs for CCM interaction improvements

ADLs presently introduce the concept of connectorsseparation between components and connectors

Components : functional entities

Connectors : interaction entities

We follow these ideas and introduce connectors in CCMto realize the interactions needed

behind well-defined interfaces, with suitable configuration mechanisms

in a reusable manneri.e do packaging of connectors

including : connector binaries, description, and code generation plugins for particular connectors that need it.

we are also lead to define new kinds of components' portswill correspond to “connector” interface for component role in a particular interaction.

goal : have a reusable Connector / Port types library.may be engineering domain specific (example : protocol domain)

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Analysis of main interaction patterns in real-time systems

Analysis :goal was to list main interaction mechanisms familiesBroad review of technologies

RTOSes, real-time middlewares, design platforms

classification follows spirit of paper :“Towards a taxonomy of software connectors”, by N.R Mehta, N. Medvidovic, S. Phadke, ICSE 2000but differs in some points

Results :Characterization of an interaction is done via:

the participants in the interaction (roles and cardinalities),

the type of service (communication or/and synchronization, ...)

QoS features (synchronicity, dynamic behavior, ...)

Three primitive connector typesMessage, Event, Procedure call

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Analysis of main interaction patterns in real-time systems

Characterization of a Message Connector (subsetted)

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CCM Extension strategy : custom interactions

Integration of connectors solution involvesprovision of description mechanisms for connectors for new kinds of ports, for usage of them

provision of framework for implementation of connectorsor for code generation plugins (in case of type-specialized conn.)

extension of CCM runtime to install / setup / configure bind connectors instances to components

extension of CCM assembly descriptors to let appear the connector notion in it and specify binding to components

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Impact on software engineering process

description of ports affected to components

done via extended IDL3 uses_port new keyword inside

“component” type declarations refers to Port collection

CIF is derived from port types definitions codegen

Assembly descriptordefines connector instances

in addition to comp instances

defines binding of components to connectors configuration of connectors

Connector instantiationadditional code generation step for “typed” connectors

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Illustration : basic Connectors Examples

Event Bus

Push/Pull pub-sub

publishers / subscribers

are associated to collection of char strings

event delivered to subscribers whose tag collection is a subset of the emitter one

Event channel plays particular role

Delayed procedure call

poll-based version

connector naturally parameterized by type of target interface

More relevant connectors examples in some of the next slides.

connector EvtBus { provides  PushInterface for_publishers; provides  PullInterface for_consumers;};

connector RPC_Poll<I> { provides trsf(I) for_poll_client; uses I for_server; };

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More insight on connectors for CCM (runtime setting)

connectors realizations are fragmentedsome fragments co-localized with components

implement interface between component and connector

some fragments in a priori unknown in advance placeexample : event channel

collaboration modalities between fragments is connector specific fragments created and configured via local factories

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Relations between components, ports, and connectors

Ports definitionscan be parameterizedrefer to IDL interfaces

not only syntactical contracts

Componentsdeclare Ports usage

Connectorsrefer to port typesor directly to interfaces if the mapping is 1 to 1

Exampleinterface PushEventInterface {  void push( in any evt );};

Port_type PushPublisher {  uses PushEventInterface pe;};

Port_type PushConsumer {   provides PushEventInterface pe;};

component C {  uses_port PushPublisher pub_port;};

component D {uses_port PushConsumer cons_port;

};

connector PushPushEvent { provides_port PushPublisher for_pubs; provides_port PushConsumer for_cons;};

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Extension of configuration and deployment for CCM

Extensions

of assembly file format (XML Schema)

for ability to refer to connector package

for ability to install, instantiate and configure connector instance

local fragments as well as non-localization constrained ones

configuration : per connector instance, or associated to particular components bound to connector instance

for ability to define bindings between components ports to connector slots (i.e between matching ports)

of CCM runtime

to support interaction with deployment tool

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CCM Extension strategy :how to build custom containers supporting real-time

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CCM Containers general facts

CCM containers: provide predefined lifecycle for components, some non-functional features (not the good ones for RT),come in four (fixed) categories

Extension approach :provide extensible container construction facilities to embed predefined / custom services in it

to apply on hosted componentsand obtain the container fitting the needs for particular application

how this is done : framework to develop container modules (interfaces to implement) container uses this framework base interfaces at particular points:

interceptioncreation of POA with particular policies, activation of servants services explicitly provided to component via context interface

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Extensible container overview / examples for services

Basic RT services as possible container ingredientsusage of an RT-CORBA ORB

with selected prio models / Policies (example : CLIENT_PROPAGATED)

Threadpools

scheduling mechanisms

clocks

time triggers

for periodic activities combined with associated defined priority

...

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Communication applicationSimplified UMTS radio protocol stack

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Overall description of use-case : UMTS protocol layers at UE

• Notions :• TTI: transmission time interval• TFC: transport format

combination• BO : buffer occupancy• SAP : service access point

in each TTI : - RLC submits info about BOs to MAC - MAC informs RLC of PDUs nb to submit to log channels SAPs - RLC submits PDUs to MAC - MAC processes RLC PDUs and submits transport blocks sets to PHY according to TFC selection - ...

Note: only uplink shown

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Connectors examples 1

Intent : capture recurring inter protocol layers interactions upper layers to RLC entity interaction in uplink

Message type connectorBuffering of RLC SDUsDiscarding policies for SDUs

based on : TTI index, if Buffer full, discard-timers

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Connectors examples 2

Intent : capture recurring inter-layers interactions

control of access to RACH (random access channel)

selection of TTI to initiate access on RACHdone randomly with probability P_iuntil success or nb_retries > N_maxadditional timing delay in case of Negative ACK by PHY

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Conclusions

Presented extensions of CCM to supportmultiple kinds of software interactionsextensible component framework construction supporting real-time

parts of this work are in progress

Assessment and validation of proposal:forthcoming with prototyping of presented radio-protocol stack use-case.Could lead to CCM extensions proposal in the future in case of good evaluation.

Acknowledgements:CARROLL-ICE : THALES/SC2 – CEA/LIST Collaborative project IST-Compare and ITEA-Merced European projects