DESIGNING A PERSISTENCE

FRAMEWORK WITH PATTERNS

The Problem: Persistent Objects

persistent object An object that can survive the process or thread

that created it. A persistent object exists until it is explicitly deleted

ProductDescription Storage Mechanisms and Persistent ObjectsObject databases Relational databases others

The Solution: A Persistence Service from a Persistence FrameworkThe framework should provide functions such as:store and retrieve objects in a persistent

storage mechanismcommit and rollback transactions

persistence framework general-purpose, reusable, and extendable set of

types that provides functionality to support persistent objects

framework A set of collaborating abstract and concrete

classes that may be used as a template to solve a related family of problems. It is usually extended via subclassing for application-specific behavior

Key Ideas MappingObject identity Database mapper Materialization and dematerialization CachesTransaction state of object Transaction operations Lazy materialization Virtual proxies

The Representing Objects as Tables

patternHow do you map an object to a record or relational database schema?

The Representing Objects as Tables pattern

Manufacturer

namecity...

...

name city

Now&Zen Mumbai

MANUFACTURER TABLE: Manufacturer

name = Now&Zencity = Mumbai

CelestialShortening San Ramon

UML Data Modeling Profile

«Table»ProductDescription

«PK» OID : char(16)Description : varchar(100) ...«FK» Manu _OID : char(16)

«Table»Manufacturer

«PK» OID : char(16)Name : varchar(100) City : varchar(50)

*1

aggregate signifies a referential constraint: a ProductDescription row can't exist without a related Manufacturer row

PK - primary keyFK - foreign key

对关系的存储设计

对泛化的存储设计

Method 1

Method 2a

Method 2b

object identifier (OID)pattern

OID

xyz123

abc345

This is a simplified design. In reality, the OID may be placed in a Proxy class.

primary key

Manufacturer

citynameoid : OID...

...

name city

Now&Zen Mumbai

MANUFACTURER TABLE: Manufacturer

city = Mumbainame = Now&Zenoid = xyz123

CelestialShortening San Ramon

Accessing a Persistence Service with a Facade

1PersistenceFacade

...

getInstance() : PersistenceFacade

get( OID, Class ) : Objectput( OID, Object )...

: DBProductsAdapter 1: PersistenceFacade

pd = get(...)

// example use of the facade

OID oid = new OID("XYZ123");ProductDescription pd = (ProductDescription) PersistenceFacade.getInstance().get( oid, ProductDescription.class );

Mapping Objects: Database Mapper or

Database Broker PatternWho should be responsible for materialization and dematerialization of objects (for example, a ProductDescription) from a persistent store?

The PersistenceFacade—as true of all facades—does not do the work itself, but delegates requests to subsystem objects.direct mapping

persistent object class itself indirect mapping

Database Broker patternDatabase Mapper pattern

Metadata-Based Mappers

class PersistenceFacade{ / / . . . public Object get( OID oid, Class persistenceClass ) { // an IMapper is keyed by the Class of the persistent object IMapper mapper = (IMapper) mappers.get( persistenceClass ); // delegate return mapper.get( oid ); } //... }usage:

(Manufacturer) PersistenceFacade.getInstance(). get( manuOID, Manufacturer.class) );

each mapper gets and puts objects in its own unique way , depending on the kind of data store and format

1PersistenceFacade

getInstance () : PersistenceFacade

get ( OID , Class ) : Objectput ( OID , Object )...

ProductSpecificationRDBMapper

...

get ( OID ) : Objectput ( OID , Object )...

ProductSpecificationFlatFileMapper

...

get ( OID ) : Objectput ( OID , Object )...

ManufacturerRDBMapper

...

get ( OID ) : Objectput ( OID , Object )...

note that the Class as a parameter is no longer needed in this version of get , as the class is "hardwired " for a particular persistent type

1

«interface»IMapper

get (OID ) : Objectput ( OID , Object )...

Class

UML notation : This is a qualified assocation . It means :

1 . There is a 1 -M association from PersistenceFacade to IMapper objects .2 . With a key of type Class , an IMapper is found (e .g ., via a HashMap lookup )

Template Method Pattern

GUIComponent

update()

repaint()

MyExcellentButton

repaint()

// this is the template method// its algorithm is the unvarying part

public void update(){ clearBackground();

// this is the hook method // it is the varying part repaint();}

hook method

- varying part- overriden in subclass-may be abstract, or have a default implementation

hook method overriden

- fills in the varying part of the algorithm

HOLLYWOOD PRINCIPLE: Don't call us, we'll call you

Note that the MyExcellentButton--repaint method is called from the inherited superclass update method. This is typical in plugging into a framework class.

FRAMEWORK class

OUR class

template method

hook method

Framework Design with the Template Method Pattern if (object in cache) return it else create the object from its representation in

storage save object in cache return it

AbstractPersistenceMapper

+ get ( OID ) : Object {leaf }

# getObjectFromStorage (OID ) : Object {abstract }...

«interface»IMapper

get (OID ) : Objectput ( OID , Object )...

// template methodpublic final Object get ( OID oid ){obj := cachedObjects .get (oid ); if (obj == null ) { // hook method obj = getObjectFromStorage ( oid );

cachedObjects .put ( oid , obj ); }return obj ;} HOOK

TEMPLATE

How to use the Framework

ProductDescriptionRDBMapper

# getObjectFromStorage(OID) : Object

AbstractPersistenceMapper

+ get( OID) : Object {leaf}

# getObjectFromStorage(OID) : Object {abstract}...

// template methodpublic final Object get( OID oid ){obj := cachedObjects.get(oid); if (obj == null ) { // hook method obj = getObjectFromStorage( oid );

cachedObjects.put( oid, obj ) }return obj}

// hook method overrideprotected Object getObjectFromStorage( OID oid ){String key = oid.toString();dbRec = SQL execution result of: "Select * from PROD_DESC where key =" + key

ProductDescription pd = new ProductDescription();pd.setOID( oid );pd.setPrice( dbRec.getColumn("PRICE") );pd.setItemID( dbRec.getColumn("ITEM_ID") );pd.setDescrip( dbRec.getColumn("DESC") );

return pd;}

IMapper

Further factoring out the varying and unvarying parts of the algorithm.

Final Framework

1

«interface»IMapper

get (OID ) : Objectput ( OID , Object )...

Class

1+ PersistenceFacade

getInstance () : PersistenceFacade

get ( OID , Class ) : Objectput ( OID , Object )...

AbstractPersistenceMapper

+ get ( OID ) : Object {leaf }# getObjectFromStorage (OID ) : Object ...

AbstractRDBMapper

+ AbstractRDBMapper (tableName )# getObjectFromStorage (OID ) : Object {leaf }# getObjectFromRecord (OID , DBRecord ) : Object- getDBRecord (OID ) : DBRecord

Persistence

NextGen Persistence

ProductDescriptionRDBMapper

+ ProductDescriptionRDBMapper (tableName )# getObjectFromRecord (OID , DBRecord ) : Object

ProductDescriptionFileWithXMLMapper

# getObjectFromStorage (OID ) : Object

SaleRDBMapper

...# getObjectFromRecord (OID , DBRecord ) : Object

ProductDescriptionInMemoryTestDataMapper

# getObjectFromStorage (OID ) : Object

AbstractPersistenceMapper

+ get( OID) : Object {leaf, guarded}...

// Javapublic final synchronized Object get( OID oid ){ ... }

{guarded} means a "synchronized" method; that is, only 1 thread may execute at a time within the family of guarded methods of this object.

IMapper

Configuring Mappers

class MapperFactory{ public IMapper getProductSpecificationMapper(){...} public IMapper getSaleMapper() {...}}

class MapperFactory{

public Map getAllMappers( ) {...}}

class PersistenceFacade{ private java.util.Map mappers = MapperFactory.getlnstance( ).getAllMappers( ); }

ProductDescriptionRDBMapper

# getObjectFromStorage(OID) : Object

AbstractPersistenceMapper

+ get( OID) : Object {leaf}

# getObjectFromStorage(OID) : Object {abstract}...

// template methodpublic final Object get( OID oid ){obj := cachedObjects.get(oid); if (obj == null ) { // hook method obj = getObjectFromStorage( oid );

cachedObjects.put( oid, obj ) }return obj}

// hook method overrideprotected Object getObjectFromStorage( OID oid ){String key = oid.toString();dbRec = SQL execution result of: "Select * from PROD_DESC where key =" + key

ProductDescription pd = new ProductDescription();pd.setOID( oid );pd.setPrice( dbRec.getColumn("PRICE") );pd.setItemID( dbRec.getColumn("ITEM_ID") );pd.setDescrip( dbRec.getColumn("DESC") );

return pd;}

IMapper

class ProductSpecificationRDBMapper extends …{// hook method overrideprotected Object getObjectFromStorage( OID oid ){

String key = oid.toString();dbRec = SQL execution result of: "Select * from PROD_SPEC where key =" +

key

ProductSpecification ps = new ProductSpecification();

ps.setOID( oid );ps.setPrice( dbRec.getColumn("PRICE") );ps.setItemID( dbRec.getColumn("ITEM_ID") );ps.setDescrip( dbRec.getColumn("DESC") );return ps;

}}

class RDBOperations{ public ResultSet getProductDescriptionData( OID oid ) {...} public ResultSet getSaleData( OID oid ) {...} ...}

class ProductDescriptionRDBMapper extends AbstractPersistenceMapper{

protected Object getObjectFromStorage( OID oid ){ ResultSet rs = RDBOperations.getInstance().getProductDescriptionData( oid );

ProductDescription ps = new ProductDescription(); ps.setPrice( rs.getDouble( "PRICE" ) ); ps.setOID( oid ); return ps;}

Pattern: Cache Management

to maintain materialized objects in a local cache to improve performance (materialization is relatively slow) and support transaction management operations such as a commit. When objects are materialized, they are placed in the cache, with their OID as the key. Subsequent requests to the mapper for an object will cause the mapper to first search the cache, thus avoiding unnecessary materialization

Transactional States and the State Pattern

Persistent objects can be inserted, deleted, or modified. Operating on a persistent object (for example, modifying it) does not cause an immediate database update; rather, an explicit commit operation must be performed.

OldClean OldDirty

OldDelete

commit / delete

delete

New

[ from DB ]

[new (not from DB )]

save

commit / update

delete

rollback / reload

rollback / reloadcommit / insert

State chart : PersistentObject

Legend :New--newly created ; not in DBOld--retrieved from DBClean --unmodifiedDirty --modified

Deleted

PersistenceDomain

ProductDescription

...

PersistentObject

oid : OIDtimeStamp: DateTime

commit()delete()rollback()save()...

GoF State pattern Context/ProblemAn object's behavior is dependent on its state, and its methods contain case logic reflecting conditional state-dependent actions. Is there an alternative to conditional logic?SolutionCreate state classes for each state, implementing a common interface. Delegate state-dependent operations from the context object to its current state object. Ensure the context object always points to a state object reflecting its current state.

PersistentObject

oid : OIDstate : PObjectState

commit()delete()rollback()save()setState(PObjectState)...

PObjectState

commit(obj : PersistentObject)delete(obj : PersistentObject)rollback(obj : PersistentObject)save(obj : PersistentObject)

OldDirtyState

commit(...)delete(...)rollback(...)

1

OldCleanState

delete(...)save(...)

NewState

commit(...)

OldDeleteState

commit(...)rollback(...)

ProductSpecification

...

...

Sale

...

...

*

{ state.delete( this ) }

{ // default no-op // bodies for // each method }

{ // deleteobj.setState( OldDeleteState.getInstance() ) }

{ // saveobj.setState( OldDirtyState.getInstance() ) }

{ // rollbackPersistenceFacade.getInstance().reload( obj )obj.setState( OldCleanState.getInstance() ) }

{ // commitPersistenceFacade.getInstance().update( obj )obj.setState( OldCleanState.getInstance() ) }

{ state.rollback( this ) } { state.commit( this ) }{ state.save( this ) }

{ // commitPersistenceFacade.getInstance().insert( obj )obj.setState( OldCleanState.getInstance() ) }

{ // commitPersistenceFacade.getInstance().delete( obj )obj.setState( DeletedState.getInstance() ) }

Designing a Transaction with the Command Pattern

Ordering the database tasks

Table A: caseNo StudentNo

Health

Table B: StudentNo StudentName

Inseart a record (“05001”,”wang”) to Bupdate A ("001","05001"),

Command

Context/ProblemHow to handle requests or tasks that need functions such as sorting (prioritizing), queueing, delaying, logging, or undoing?SolutionMake each task a class that implements a common interface

actions become objects, and thus can be sorted, logged, queued, and so forth.

«interface»ICommand

execute( )undo()

DBInsertCommand

execute()

DBUpdateCommand

execute()

DBDeleteCommand

execute()

Transaction

commands : List

commit()addDelete(obj: PersistentObject)addInsert( obj: PersistentObject)addUpdate( obj: PersistentObject)sort()...

1..*

DBCommand

object : PersistentObject

execute() { abstract}undo() {leaf}

undo is a no-op for this example, but a more complex solution adds a polymorphic undo to each subclass which uniquely knows how to undo an operation

PersistentObject

commit()...1{

commands.add( new DBUpdateCommand(obj) );}

use SortStrategy objects to allow different sort algorithms to order the

Commands

perhaps simply object.commit()but each Command can perform its own unique actions

{sort()for each ICommand cmd

cmd.execute() }

Lazy Materialization with a Virtual Proxy

ManufacturerProxy

realSubject : IManufacturer

- getRealSubject() : IManufacturer

+ getAddress()...

Manufacturer

address

getAddress()...

«interface»IManufacturer

getAddress()...

Proxy-for 1

realSubject

{return getRealSubject().getAddress()}

ProductSpecification

manufacturer : IManufacturer...

getManufacturerAddress() : Address

1

{if ( realSubject == null ) realSubject = PersistenceFacade.get(oid, Manufacturer.class);return realSubject;}

PersistentObject

oid

...

1

{return manufacturer.getAddress()}

actually references an instance of ManufacturerProxy

1

23

// EAGER MATERIALIZATION OF MANUFACTURERclass ProductSpecificationRDBMapper extends AbstractPersistenceMapper{ protected Object getObjectFromStorage( OID oid ){ ResultSet rs =

RDBOperations.getlnstance().getProductSpecificationData( oid ); ProductSpecification ps = new ProductSpecification(); ps.setPrice( rs.getDouble( "PRICE" ) );

// here's the essence of it String manufacturerForeignKey = rs.getString( "MANU_OID" ); OID manuOID = new OID( manufacturerForeignKey );

ps.setManufacturer((Manufacturer) PersistenceFacade.getInstance(). get(manuOID,

Manufacturer.class) );

// or LAZY MATERIALIZATION OF MANUFACTURERps.setManufacturer( new ManufacturerProxy( manuOID ) );

the Representing Object Relationships as Tables

one-to-one associationsPlace an OID foreign key in one or both

tables representing the objects in relationship.

Or, create an associative table that records the OIDs of each object in relationship.

one-to-many associations, such as a collectionmany-to-many associationsCreate an associative table that records

the OIDs of each object in relationship.

Unresolved Issues

• dematerializing objects Briefly, the mappers must define putObjectToStorage.

methods. Dematerializing composition hierarchies requires collaboration

between multiple mappers and the maintenance of associative tables (if an RDB is used).

• materialization and dematerialization of collections• queries for groups of objects• thorough transaction handling• error handling when a database operation fails• multiuser access and locking strategies• security—controlling access to the database