presentation 3: designing distributed objects. ingeniørhøjskolen i Århus slide 2 af 32 outline...

Presentation 3:Designing Distributed Objects

Ingeniørhøjskolen i ÅrhusSlide 2 af 32

Outline

• Assumed students are knowledgeable about OOP principles and UML …

• … but a ultra short introduction to some of the UML Representations for Distributed Object Design used by Emmerich will be given– to make sure you understand Emmerich– because you will need to use this in your assignment!

• Local vs. Distributed Objects– How do local and distributed objects differ

Evolution of Object Technology


A Brief History of Objects

Simula-67Information Hiding

Smalltalk

C++Eiffel

Java

CORBACOM

OOAD

UML

Sun ONC

DCE

Programming LanguagesDistributed Systems Software Engineering

Time

1970

1980

1990

DCOMSOAP

UML Representations for Distributed Object Design


Use Case Diagrams !

TrainerPlan Training

Schedule

ManagerNational Soccer

Association

Determine LeagueGames

PlayerPrint Personal

Schedule

actor

use case

system boundary

Emmerich does not use activation arrow

system

need to implement!


Sequence Diagrams – dynamic models

BvB:Team Teddy:Player Stefan:Player

bookGoaliesbook()

book()

:Output

printTrainingPlan

Tony:Trainer

asynchronousmessage

(remote) procedure

call

lifelineobjectactivation

synchronousMessage

ObjectName Type

objectdeletion

Name is purecoincidence


Class Diagrams – static models !

Team

-name:string

+bookGoalies()

coaches 1..*

1..*Player

-name:string-role:Position-Number:int+book()

+transfer(p:Player)

Club

-noOfMembers:int-location:Address

Trainer

-name:string

Organization

#name:string

works for

1 1..*uses

plays in

1 11..16

has1

*

privatepublic

protected

class nameattributes

operations

association

generalizationaggregation

composition

dependency

cardinality

need to implement!


Packages - decoupling

SoccerClubs

LeagueManagement

NationalTeam

Middleware

Collections of Classes that workClose together, but Decoupled from otherPackages

need to implement!


Object Diagrams – instances relationships

name = “Tottenham Hotspurs”

spurs:Team

stefan:Player

name = “Stefan Freund” role = DefenderNumber=4

name=“Jürgen Klinsmann”role=ForwardNumber = 18

klinsi:Player

Object

AttributeValue

Object TypeObject Name

AttributeName

Link


in team

State Diagrams – instances state and transitions

healthy

in match

injured

/injury

/recovered

reserve

playing /swap/swap

/exercise

freetraining

/finish

[!in_squad]/play

[in_squad]/play

/finish

state compositestate

indicatesdefaultstate

transition

action during transition

condition


Parallel State Composition

in team

on transfer list

Player

happy

/unsatisfaction

/resign

/sign_contract

concurrentthreads

finalstate


Object

• Has a unique identifier.• May have many different references that refer to

the object.• Has a set of attributes whose names denote

values.• References may denote

– equal objects– identical objects

• Is encapsulated by operations.• May raise particular exceptions.


Sample Object

klinsi:Player

name = “Jürgen Klinsmann”role = ForwardNumber = 18


Attributes

• Attributes have a name and a type.• Type can be an object type or a non-object type.• Attributes may or may not be modifiable by other

components.• Attributes correspond to one or two operations

(set/get).


Exceptions

• Service requests in a distributed system may not be properly executed.

• Exceptions are used to explain reason of failure to requester of operation execution.

• Operation execution failures may be – generic or – specific.

• Specific failures may be explained in specific exceptions.


Operations

• Operations have a signature that consists of– a name, – a list of in, out, or inout parameters,– a return value type, and– a list of exceptions that the operation can raise.


Operation Execution Requests

• A client object can request an operation execution from a server object.

• Operation request is expressed by sending a message (operation name) to server object.

• Server objects are identified by object references.• Clients have to react to exceptions that the

operation may raise.

Local versus Distributed Objects


Motivation

• You all have experience with designing local objects that reside in the run-time environment of an OO programming language such as:– C++– C#– Java

• Designing distributed objects is different!• In the next section we will

– Explain the differences.– Try to avoid some serious pitfalls


Local vs. distributed Objects

• Differences between local and distributed objects in:– References (to objects)– Activation/Deactivation– Migration– Persistence– Latency of Requests– Concurrency– Communication– Security


Object Lifecycle• If we look at local vs distributed objects lifecycles

we see that:– OOPL objects reside in one virtual machine.

• From creation to deletion– Distributed objects might be created on a different machines

• Not possible to use OOPL’s creation operators • Need to design features for it• Should be independent of server location

– Distributed objects might be copied or moved (migrated) from one machine to another

• To avoid overloading of a server• This will not happen with local objects

– Deletion by garbage collection does not work in a distributed setting.• To difficult to maintain referential integrity

– Lifecycle needs attention during the design of distributed objects.

• We will visit Life Cycle closer in Chapter 9


Object References

• References to objects in OOPL are usually pointers to memory addresses– sometimes pointers can be turned into references (C++)– sometimes they cannot (Smalltalk,Java)

• References to distributed objects are more complex– Location information (which server, how to communicate)– Security information (now not anymore a protected process)– References to object types (might differ – which server is in control

– adminstrators forgetting to synchronize) References to distributed objects are bigger (e.g 40 bytes

with Orbix small footprint) – As opposed to 4 bytes for 32 bit references on OOPL VM’s and possible up 400 if security is tightly integrated To virtual memory consuming for clients holding references


Latency of Requests

• Performing a local method call requires a couple of hundred nanoseconds.

• An object request requires between 0.1 and 10 milliseconds. – And possible more depending on the network– And the size of the objects

Interfaces of distributed objects need to be designed in a way that – operations perform coarse-grained tasks– do not have to be requested frequently

• Therefore:– Be vary of turning everything into objects– Use Façade patterns instead – and decouple


Activation/Deactivation

• Objects in OOPL are in virtual memory between creation and destruction.

• This might be inappropriate for distributed objects– They are bigger than local objects (take up more space)– Combined with sheer number of objects …– Results in problems with capacity of server (virtual memory)– Also: objects might not be used for a long time– And: some hosts might have to be shut down without stopping all

applications

• Distributed object implementations are– brought into main memory (activation)– discarded from main memory (deactivation)


Activation/Deactivation (cont’d)

BvB:Team

bookGoalies

Tony:Trainer

objectactivated

objectDeactivation-Might need to serialize to maintain state


Activation/Deactivation (cont’d)

• Several questions arise– Explicit vs. implicit activation (transparent for the programmer)– When to deactivate objects

• Need to persist the state

– Association between objects and processes– How to treat concurrent requests

• Even across servers

• Who decides answers to these questions?– Designer– Programmer– Administrator– Middleware – but designer must provide persistence facilities


Persistence

• Stateless vs. statefull objects• Statefull objects have to save their state between

– object deactivation and– object activation

onto persistent storage• Can be achieved by

– externalization into file system– mapping to relational database– object database

• To be considered during object design• Statefull objects are more expensive to make redundant

– Need to replicate more between servers• More of this in chapter 10


Parallelism

• Execution of OOPL objects is often – Sequential in a process– concurrent (with multi-threading or multi processor)

• Distributed objects execute in (true) parallel– Same object might reside on more than one computer– How to maintain concurrency across computers?– See Chapter 11 ; ) or the Concurrency Course


Communication

• Method invocations of OOPL objects are synchronous– Because it is fast and performant– But it does block the calling object

• Though not for long

• Alternatives for distributed objects:– synchronous requests

• Nice – but not always performant due to latency• Blocks the calling object for to long

– oneway requests– deferred synchronous requests– asynchronous requests

• Who decides on request– Designer of server?– Designer of client?

• More on this in Chapter 7


Failures

• Distributed object requests are more likely to fail than local method calls– More points of failures

• Server(s)• Client• Network

• Different request reliabilities are available for distributed objects– Exactly-once semantics to demanding on latency– Instead At-most-once

• – only tries once – but inform of failing (Exception)

• Clients have an obligation to validate that servers have executed request– Meaning: has to check for exceptions– Need to use transactions (all or nothing) – with roll back capabilities

• More on this in Chapter 7


Security

• Security in OO applications can be dealt with at session level– Only need to authenticate the user at process/session start

• OOPL Objects do not have to be written in a particular way.– Security lies in one process (maybe more tasks)

• For distributed objects:– Who is requesting an operation execution?– How can we know that subject is who it claims to be?– How do we decide whether or not to grant that subject the right to

execute the service?• We need to use secure connections and object level security

– How can we prove that we have delivered a service so as to make the requester pay

– Answers in chapter 12 – and the Course dDistSik

presentation 3: designing distributed objects. ingeniørhøjskolen i Århus slide 2 af 32 outline...

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