Persistence Service

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DCOM PERSISTENCESERVICE

ABISHA.N, DIVYA.R


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DCOM Services:

1. Persistence Service

2. Queuing Service

3. Transaction Service

4. Security Service

5. Clustering Service

Major Services offered by DCOM are,


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PERSISTENCE

Storing an Objects state permanently.

Persistence is the ability of an object state to survive

termination of the process in which the object executes.

Persistence is transparent to client

Persistent state service hide the details of data storesfrom server objects


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AIMS OF PERSISTENT SYSTEM

1. Illustration of perpetuity which gives an illusion that the

software system is continuously active.

>>This is done by storing the state of the

private member variables of an objects and then

retrieving their values when needed.

1. Allow a system to work with a large collection of data

than that could be held by the system memory.

2. Share data between multiple processes.


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ACHIEVEING PERSISTENCE

To achieve persistence, an object should perform the

following:

Implement the IPersistFile interface.

Read its data at the start of each method.

Write data to Storage whenever the state of theobject changes. It is done without the

acknowledgement of the client.


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REQUIREMENTS FOR PERSISTENT

SERVICES

1) Object-oriented databases

2) File Systems

3) Structured storage

FILE SYSTEMS:

In file systems, each bit has a unique

address.

Files that store data are identified by their

pathname.


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Structured storage

Structured hierarchy with the root file having the sub-

storages.

2 methods:

Smart pointers:

>> Includes Object ID along with the

address.

load store


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Structured Storage(CONT)

Steps in storing objects to Structured storage:

1. Construct object(Objects added to

Persistent object registry)

2. load and store cycle(Object utilize smart

pointers to restore pointer values)


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ELEMENTS OF STRUCTURED

STORAGE

TYPES OFSTORAGE

STREAM

STORAGE

STREAM : DISK STRUCTURE.

STORAGE : DIRECTORY STRUCTURE.


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STREAM OBJECT

Provides ISTREAM interface.

Equivalent to single disk file.

Can have internal structure.


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STORAGE OBJECT

Provides ISTORAGE interface.

Equivalent to directory structure.

Can have any number of sub-storages and

stream.


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MODES OF ACCESS

DIRECT

MODE

TRANSACTE

D MODE

Changes made are

immediate and

permanent

Changes are

buffered so that

they may be

saved/committedwhen modifications

are complete


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MONIKERS

The file names is used to identify an object.

However, file names are unintelligent. Intelligent can bebrought about by encapsulating the information of

working with the names within the name itself.

Here, the name becomes an object that implementsname- related interfaces. These objects are called

monikers.

Each individual moniker object maintains its own namedata that identifies some other particular object or

operation.


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IMONIKER

A moniker is simply an object that supports IMONIKERinterface.

IMONIKER interface includes IPersistStream interface.

PERSISTENT FORM OF MONIKERS:

Can be saved to and loaded from streams.

Also helps to provide transparency to clients.


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OPERATIONS OF IMONKER

1. BINDING:

Binding to the object that it points.

Binding function in IMONKER takes as aparameter the interface identifier by which theclient communicates with object.

Used to locate the object.

Expensive Binding: Avoids repeated Connections.


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2. REDUCTION: Re-writes itself into another equivalent MONIKER.

3. DISPLAY NAME: Equivalent to file name of an object.

OPERATIONS OF

IMONKER(cont)

RUNNING OBJECT TABLE:Instantiated COM object register themselves and

their MONIKER in the table, called Running object table.


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INTERFACES

COM allows read and write itself to storage by providing

additional interfaces.

3 important interfaces are:

IPersistStorage

IPersistStream

IPersistFile


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Monikers in Detail

Object or Component that refers to specific

instance of another object.

Name which uniquely identifies an COM object

Persistent Object & Naming Object 1:1

IMoniker Interface is a Framework

Co Create Instance cant be used


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Implementations of Monikers

File Monikers: Wrapper for file path name

Item Monikers: Object within another Object

Pointer Monikers: Object Exist only in Running

StateAnti-Moniker: Inverse of A moniker

Composite Monikers: Composed of OtherMonikers

Class Monikers: Wraps CLSID

URL Monikers: Manages URL

Custom Moniker: Object Implements IMoniker

Interface


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IMoniker Interface

Functionalities

BindToObject

BindToStorage

GetDisplayName

ParseDisplayName

IsSystemMoniker

Reduce

RelativePathTo

CompositeWith

Enum

IsEqual

Hash

IsRunning

Inverse

CommonPrefixTo


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Inheritance Hierarchy

IUnknown

IPersistIPersistStream

IMoniker


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IPersist

Main Interface

Base for Persistent Framework

Some Specific Interfaces:

IPersistFile

IPersistStorage

IPersistSttream

IPersistStreamInit


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IPersistFile

Reads/Writes information into a Completely

Separate File outside structured storage.

Methods:

1.Load

2.Save

3.IsDirty

4.SaveCompleted

5.GetCurFile


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IPersistentStorage

Reads/Writes information in a storage

hierarchy

1.Load

2.Save

3.IsDirty

4.SaveCompleted

5.InitNew

6.HandsOffStorage


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IPersistStream

Reads/Write Information in a single stream

Basic Model and provides semantics of simple

load and save

1.Load

2.Save

3.IsDirty

4.GetSizeMax


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IPersistStreamInit

Improves the functionality of IPersistStream

Initialization function is added


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Real Time Examples

a moniker is created for a query, the

programmer can reuse the query simply by

calling the moniker in the code.

Transactions made in Ecommerce
http://searchsqlserver.techtarget.com/definition/queryhttp://searchsqlserver.techtarget.com/definition/query

Persistence Service

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