Distributed Objects

1
Distributed Objects

• Objective
• Understand the difference between local and
  distributed objects and how to correctly use them
• Outline
• Object Model
• Local versus Distributed Objects

2
Object Types

• Object types specify the common characteristic of
  similar objects
• Object types define a contract that binds the
  interaction between client and server objects
• Object types are specified through interfaces
  that determine the operations that clients can
  request.
• Operation visibility public, private, etc.
• Signature of an operation name, list of formal
  parameters, and the result with type information
• Non-Object Types
• Atomic types boolean, char, int, etc,
• Values
• No identity
• Cannot be referenced

3
Objects

• Object
• Unique identifier
• Multiple references
• Attributes
• public
• Private
• Name and a type
• Class or static variables (not for distributed
  objects)
• Operations
• Object Identity vs Equality
• Identical ? equal
• Equal !? identical

4
Requests

• An object request is made by a client object in
  order to request execution of an operation from a
  server object.
• Object reference
• Operation
• List of actual parameters
• Request vs method invocation
• Resolve data heterogeneity
• Synchronization between client and server objects
• Communication through network

5
Exceptions

• Exceptions are a mechanism for notifying clients
  about failures that occur during the execution of
  an object request.
• Data structures carrying details about failures
• Part of the contract between client and sever
  objects
• Raised by a server object
• Raised by middleware (system exception)
• Transferred via network from server to client

6
Types and Distributed Objects

• Attributes, operations and exceptions are
  properties objects may export to other objects.
• Multiple objects may export the same properties.
• Only define the properties once!
• Attributes and operations, and exceptions are
  defined in object types.

7
Attributes

• Attributes have a name and a type.
• Type can be an object type or a non-object type.
• Attributes are readable by other components.
• Attributes may or may not be modifiable by other
  components.
• Attributes correspond to one or two operations
  (set/get).

8
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.

9
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.

10
Object Type Example

ltltinterfacegtgt Player
-namestring -rolePosition -Numberint
void book(in Date d) raises (AlreadyBooked)
11
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.

12
Subtyping

• Properties shared by several types should be
  defined only once.
• Object types are organised in a type hierarchy.
• Subtypes inherit attributes, exceptions and
  operations from their supertypes.
• Subtypes can add more specific properties.
• Subtypes can redefine inherited properties.

13
Multiple Inheritance

• Means that one object type can be subtype of more
  than one super type
• Not supported by all middleware
• May lead to ambiguities

14
Multiple Inheritance Example

ltltinterfacegtgt Player
ltltinterfacegtgt Trainer
-namestring -rolePosition -Numberint
-salaryint
void book(in Date d) raises (AlreadyBooked) nex
t_game()Date
next_game()Date
ltltinterfacegtgt PlayerTrainer
15
Polymorphism

• Object models may be statically typed.
• Static type of a variable restricts the dynamic
  type of objects that can be assigned to it.
• Polymorphism denotes the possibility of
  assignments of objects that are instances of the
  static type and all its subtypes.

16
Polymorphism Example

chelseaTeam
name Chelsea
vPlayerTrainer
dPlayer
name Gianluca Vialli role Forward Number
10 salary1000000
name Marcel Desailly roleDefender Number5
zPlayer
name Gianfranco Zola roleForward Number3
17
Motivation

• Many will have experience with designing local
  objects that reside in the run-time environment
  of an OO programming lang.
• Designing distributed objects is different!
• Explain the differences.
• Avoid some serious pitfalls

18
Local vs. distributed Objects

• References
• Activation/Deactivation
• Migration
• Persistence
• Latency of Requests
• Concurrency
• Communication
• Security
• Several Pitfalls are lurking here

19
Object Lifecycle

• OOPL objects reside in one virtual machine.
• Distributed objects might be created on a
  different machine.
• Distributed objects might be copied or moved
  (migrated) from one machine to another.
• Deletion by garbage collection does not work in a
  distributed setting.
• Lifecycle needs attention during the design of
  distributed objects.

20
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
• Security information
• References to object types
• References to distributed objects are bigger (e.g
  40 bytes with Orbix).

21
Latency of Requests

• Performing a local method call requires a couple
  of hundred nanoseconds.
• An object request requires between 0.1 and 10
  milliseconds.
• Interfaces of distributed objects need to be
  designed in a way that
• operations perform coarse-grained tasks
• do not have to be requested frequently

22
Example Iteration over a Sequence

• Java

• Distributed Objects

23
Activation/Deactivation

• Objects in OOPL are in virtual memory between
  creation and destruction.
• This might be inappropriate for distributed
  objects
• sheer number of objects
• objects might not be used for a long time
• 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)

24
Activation/Deactivation (contd)
BvBTeam
TonyTrainer
bookGoalies
object activated
object deactivation
25
Activation/Deactivation (contd)

• Several questions arise
• Repository for implementations
• Association between objects and processes
• Explicit vs. implicit activation
• When to deactivate objects
• How to treat concurrent requests
• Who decides answers to these questions?
• Designer
• Programmer
• Administrator
• How to document decisions?

26
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

27
Parallelism

• Execution of OOPL objects is often
• sequential
• concurrent (with multi-threading)
• Distributed objects execute in parallel
• Can be used to accelerate computations

28
Communication

• Method invocations of OOPL objects are
  synchronous
• Alternatives for distributed objects
• synchronous requests
• oneway requests
• deferred synchronous requests
• asynchronous requests
• Who decides on request
• Designer of server?
• Designer of client?
• How documented?

29
Failures

• Distributed object requests are more likely to
  fail than local method calls
• Different request reliabilities are available for
  distributed objects
• Clients have an obligation to validate that
  servers have executed request

30
Security

• Security in OO applications can be dealt with at
  session level.
• OOPL Objects do not have to be written in a
  particular way.
• 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?
• How can we prove that we have delivered a service
  so as to make the requester pay

31
Key Points

• Distributed objects evolved from research and
  development in object-oriented programming
  languages and distribution middleware
• The Unified Modeling Language can be used to
  design distributed objects
• Meta object models determine the characteristics
  of distributed objects
• Designers need to be aware of differences between
  local and distributed objects