design patterns in real-time systems

8/6/2019 Design Patterns in Real-Time Systems

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DESIGN PATTERNS FOR REAL-TIME

SYSTEMS

Neelakantam Pavani

M.Tech CSE

10B81D5810


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DESIGN PATTERNS

Each pattern describes a problem which

occurs over and over again in our

environment, and then describes the core ofthe solution to that problem, in such a way

that you can use this solution a million

times over, without ever doing it the same

way twice.

Christopher Alexander


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DESIGN PATTERNS BASICS

A design pattern is a generalized solution to a

commonly occurring problem.

Pattern hatching : The problem of identifying the

nature of the problem and examining thepatterns library for the best ones to apply.

Pattern mining : The identification and capture of

new patterns to add to the library.

Pattern instantiation: It is applying the patternto a particular collaboration to gain the benefits

of the pattern.


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PATTERNHATCHING LOCATING THE

RIGHT PATTERNS


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BASIC STRUCTURE OFDESIGN

PATTERNS

According to Gamma a pattern has four important

aspects:

Name : The name provides a handle or means

to reference the pattern. Purpose : The purpose provides the problem

context and the QoS aspects the pattern seeks to

optimize. The purpose identifies the kinds of

problem contexts where the pattern might be

particularly appropriate.

Solution : The Solution is the pattern itself.

Consequences : The Consequences are the set of

pros and cons of the use of the pattern


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REAL-TIME SYSTEMS

Areal-time system is a software system where thecorrect functioning of the system depends on theresults produced by the system and the time at whichthese results are produced.

Areal-time system is a system whosespecification includes both logical and temporal

correctness requirements.

Logical Correctness: Produces correct outputs.

Temporal Correctness: Produces outputs at the

right time.


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REAL-TIME SYSTEMS

Ahard real-time system is a system whoseoperation is incorrect if results are not producedaccording to the timing specification.

Asoft real-time system is a system whoseoperation is degraded if results are not producedaccording to the specified timing requirements.


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CHARACTERISTICS OF REAL-TIME SYSTEMS

Event-driven, reactive.

High cost of failure.

Concurrency/multiprogramming.

Stand-alone/continuous operation.

Reliability/fault-tolerance requirements.

Predictable behavior


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HARD & SOFT REAL-TIME SYSTEMS

Characteristic Hard Real-Time Soft Real-Time

Response Time Required Desired

Peak-loadperformance Predictable Degraded

Control of pace Environment Computer

Safety Often critical Non- critical

Size of data files Small/medium Large

Redundancy type Active Checkpoint-recovery

Data integrity Short-term Long-term

Error detection Autonomous User assisted


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REAL-TIME DESIGN PATTERNS

Priority Ceiling

Preemptive Multitasking

Proxy

MicroKernel

Watchdog


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PRIORITYCEILING PATTERN

The Priority Ceiling Pattern, or Priority Ceiling

Protocol (PCP) as it is sometimes called,

addresses both issues of bounding priority

inversion (and hence bounding blocking time)

and removal of deadlock.


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PRIORITYCEILING PATTERN STRUCTURE


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COLLABORATION ROLES

Abstract Thread

Concrete Thread

Mutex

Scheduler

Shared Resource

Task Control Block


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PREEMPTIVE MULTITASKING PATTERN

In Pre-emptive Multitasking, the tasks priority

and readiness are the determining factors that

need to be considered when queuing processes.

Priority based scheduling systems operate ineither static or dynamic mode.

In static mode, the tasks priority is set before

run-time, and cannot be changed while executing.

Dynamic mode is more widely used in real-timesystems, because it can be difficult to know

which tasks will be the most critical at any given

time, before that time.


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PREEMPTIVE MULTITASKING PATTERN

There are three common strategies to follow

when using the dynamic pattern :

Earliest Deadline Least Laxity

Maximum Urgency First


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MICROKERNEL PATTERN

The Microkernel architectural pattern applies to

software systems that must be able to adapt to

changing system requirements. It separates a

minimal functional core from extended

functionality and customer-specific parts.

The Microkernel pattern structures the software

architecture into a set of layers. This

organization has following advantages:

Portability

Reuse

Scaled-down applications


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MICROKERNEL PATTERN


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PROXYPATTERN

Intent

Provide a surrogate or placeholder for another object to

control access to it

Also Known AsSurrogate

There are situations in which a client does not or

can not reference an object directly, but wants to

still interact with the object

A proxy object can act as the intermediary

between the client and the target object


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PROXYPATTERN


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PROXYPATTERNAPPLICABILITY

Remote Proxy

Virtual Proxy

Protection Proxy

Smart Reference


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COLLABORATIONS & CONSEQUENCES

Proxy forwards request to RealSubject when

appropriate, depending on the kind of proxy.


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WATCHDOG PATTERN

Provide a very simplistic and low-cost solution to

detect and handle failures and faults during the

execution of safety-critical systems.

The watchdog is a low-cost solution for ensuring

the liveness of a component


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WATCHDOG PATTERN


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QUERIES

design patterns in real-time systems

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