Objektorienteret Middleware (OOMI) Persistence. Ingeniørhøjskolen i Århus Slide 2 af 53 Goal with todays lesson After these 2x35 minutes you will be:

Objektorienteret Middleware (OOMI)

Persistence

Ingeniørhøjskolen i ÅrhusSlide 2 af 53

Goal with todays lesson

• After these 2x35 minutes you will be:– Comfortable with the expression “persistence”, and how

it relates to Distributed Systems– Knowledgeable about different strategies for obtaining

persistence for Distributed Systems– You will not:

• Be an expert on persistence, as this is a huge area in itself


Outline

• Plenum – experince with persistent datastorage• Principles of Persistence• Datastore Technology for Persistence

– Files– Relational Databases– Object Databases

• Exemplified OR-mapping with EJB


Experience with Persistent datastorage

• To establish a picture of your knowledge – What kind of persistent datastorage do you know?– What experience do you have in using it with

programming languages?– What problems did you face – and how did you solve

them?

Principles of Persistence


What is Persistence?

• Persistence is the ability of an object to survive the lifetime of the process in which it resides.

• Persistence is relevant for stateful server objects.• What is State?

– State = object instance attributes – private & public– Not methods

• RMI & CORBA activation/deactivation– The state needs to be retained between object

deactivation and object activation– Why?


How to achieve Persistence?

• Storing object state on persistent datastore before de-activation

• Upon activation, load object state from persistent datastore– As we shall see in RMI & CORBA

• Persistent storage can be obtained by?– File system (Hard Drive, Disk-on-Chip, Flash Memory)

• embedded systems with a disk storage– Relational Database

• All from embedded, to desktop and enterprise servers (most widely used is SQL servers)– Object-Database

• no widespread support yet


Transparency of Persistence

• Persistence should be transparent to users and designers of client objects

Client Objects Server Objects Datastore Objects

Client Interface

Persistence Interface


Persistence Concepts

Datastore

Sessions

StorageObjects

StorageHomes

Storage HomeIncarnation

Storage ObjectIncarnation

RDBMSServer

RDBMSTable

Object= row in

RDBMS table

C++/Java object

C++/Java object or class

C++/Java object

Datastore Technology for Persistence


Datastore Technology

• Persistence can be implemented using • Files

– CORBA Externalization– Java Serialization– Structured Storage in COM

• Relational Databases– Object Relational Mappings– JDBC/ODBC

• Oracle, IBM DB2, Microsoft SQL Server & Access, MySQL

• Object Databases


How to obtain persistence?

• Roll your own persistence – SOAP, RMI

• For Java – use JDBC for RDBMS, serialization for filesystems• CORBA Externalization for filesystems• COM Serialization or Structured Storage• Possible to construct your own framework (e.g. ROAD)

• Use a standard service– CORBA Persistence service (PSS)– COM Persistence service– Enterprise Java Beans– May still be necessary to obtain persistence manually


Externalization in CORBA

• Technique to – write composite objects into a byte stream– load composite objects from a byte stream

• Byte stream can then be written to/read from the file system

• Supported by several CORBA products• Also used to store Java objects


Java Serialization

• Transforms Java objects into stream• Follows all referenced Java objects• Stream can be written onto a file to achieve

persistence• Objects must implement Serializable• Attributes that need not be made persistent can

be declared as transient in the class definition


Example of using Serialization for RMI

• http://java.sun.com/j2se/1.3/docs/guide/rmi/activation/activation.4.html– Class:

• public class MyPersistentClass extends Activatable

– This Activatable class will– Restore its State upon activation (if the file

persistentObjectStore.ser exist)– Save its State after a new transaction is registered– This could also have been done utilizing an RDBMS

http://java.sun.com/j2se/1.3/docs/guide/rmi/activation/activation.4.html



private Vector transactions; private File holder;

public MyPersistentClass(ActivationID id, MarshalledObject data) throws RemoteException, ClassNotFoundException, java.io.IOException {

// Register the object with the activation system // then export it on an anonymous port super(id, 0);

// Extract the File object from the MarshalledObject that was // passed to the constructor holder = (File)data.get(); if (holder.exists()) { // Use the MarshalledObject to restore my state

this.restoreState(); } else {

transactions = new Vector(1,1); transactions.addElement("Initializing transaction vector");

} }

CONSTRUCTOR of MyPersistentClassFind complete example and tutorial at:


Every time a passivated object is called,the constructor is called as the rmid daemon

Instantiates it to activate – and here itis checked whether the File exist – or not.

If it does exist, then its State is restored

Every time a passivated object is called,the constructor is called as the rmid daemon

Instantiates it to activate – and here itis checked whether the File exist – or not.

If it does exist, then its State is restored




public Vector calltheServer(Vector v) throws RemoteException { int limit = v.size(); for (int i = 0; i < limit; i++) { transactions.addElement(v.elementAt(i)); } // Save this object's data out to file // this.saveState(); return transactions; }

private void restoreState() throws IOException, ClassNotFoundException { File f = holder; FileInputStream fis = new FileInputStream(f); ObjectInputStream ois = new ObjectInputStream(fis); transactions = (Vector)ois.readObject(); ois.close(); } private void saveState() { try {

File f = holder; FileOutputStream fos = new FileOutputStream(f); ObjectOutputStream oos = new ObjectOutputStream(fos); oos.writeObject(getTransactions()); oos.close();

} catch (Exception e) { throw new RuntimeException("Error saving vector of data");

} }

Using simpel Java SerilizationTo obtain perstistence

Using simpel Java SerilizationTo obtain perstistence

Find complete example and tutorial at: http://java.sun.com/j2se/1.3/docs/

guide/rmi/activation/activation.4.html




Persistence Interfaces in COM

IPersist

+ GetClassID()

<<Interface>>

IPersistStorage

+ IsDirty()+ InitNew()+ Load()+ Save()+ SaveCompleted()+ HandsOffStorage()

<<Interface>>

IPersistStream

+ IsDirty()+ Load()+ Save()+ GetSizeMax()

<<Interface>>

IPersistFile

+ IsDirty()+ Load()+ Save()+ SaveCompleted()+ GetCurFile()

<<Interface>>

IUnknown<<Interface>>


Problems with File-based Persistence

• Mapping to Files can be inefficient for large composite objects

• Finder methods diffuclt to develop – Naming & File search difficult

• File systems only have crude support for concurrency control

• File systems lack support for fault-tolerance (transactions)

• Application specific code needed for every persistent class


Relational Database

• Relational Database Management Systems (RDBMSs) Examples:– Oracle– DB2– Microsoft SQL Server– Sybase– Microsoft Access– MySQL (freeware)– PostGree DB (freeware)


Mapping to RDBMSs

• Relational database schemas consist of sets of tables

• Define a table for each type• In each table create

– primary key for object identifier– a column for each attribute of the object

• mapping of middleware atomic types to primitive types supported by RDMBS

• secondary keys for object references

• Resolve inheritance statically– This and other problems -> the OR impedance

mismatch


Embedding Queries into Programs

• Pure Embedded SQL– Macros to embed queries into programs– RDBMS provides processor to expand macros– API to traverse queries– Not standardized

• Open Database Connectivity - ODCB (Microsoft)– Standardized API for RDBMS Access available on all

Microsoft Platforms

• Java Database Connectivity – JDCB (Sun)– Standardized RDBMS Access from Java


Issues with mapping

• Does this mean that we should figure out for ourselfes how to obtain the OR-mapping?– No– Frameworks available– CORBA PSS, COM persistence, EJB for Java– JDBC/ODBC is at a low level – but very feasible– Torque/Turbine project freeware– JDO maps to XMI– In the following EJB will be presented

RDBMS mapping illustrated with EJB’s


Enterprise JavaBeans (EJBs)• Standard server-side component model for Java Enterprise

Applications– security– resource pooling– persistence– concurrency– transactional integrity

• Has nothing to do with “JavaBeans”– JavaBeans designed for intra-process purposes

• GUIs

• non-visual widgets

– Enterprise Java Beans (EJB) designed for inter-process purposes


EJBs (cont.)

• Java– platform independence– “write once, run anywhere”

• EJB components– platform/implementation independence– write once, run in any Application Server complying with the

EJB spec• J2EE reference implementation• Oracle’s Application Server (OAS)• IBM’s Websphere• BEA’s Weblogic Server and Weblogic Enterprise• Sybase’s EAServer• Open Source – JBoss (see links)


EJB Architecture

• Interfaces and Classes– beans

• entity bean (maps perfectly to the “Entity stereotype”• session beans (no primary key – maps to “Control”)

– Stateless session bean

– Stateful session bean

• GUI? JSP/Servlet or Java Application

– primary key (implemented in a class)– home interface (local and remote)– EJB object interface (local and remote)

• Container / Application Server


Bean Types• Entity beans

• models persistent state - this state is maintained through all method and server invocations

• nouns of the domain • real world objects (e.g. Owner, Account, Transaction)

• Session beans• models non-persistent state - this state will be lost between method

invocations (stateless session) or server invocations (Stateful session)• manage tasks performed on behalf of a single client (e.g. Teller,

Monthly Statement)– contains the business processes in which to use entity beans

• manages actions that may cross entity beans or go outside the concern of an entity bean

– e.g. Teller may authenticate the user and transfer funds between accounts– e.g. Statement may include transactions from multiple accounts


Bean Usage

• Entity beans– model state maintained across all client interactions– represent a row of data in a database

• Session beans– model business process being performed by a single

client involving one or more entity beans– it extends the actions of the client into the server

• simplifies the actions programmed by the client• limits the number of distributed calls required between the client

and the entity beans• limits the number of stubs that have to be loaded by the client

– are not persisted to a database

Maps to domain model (Entity classes)Maps to domain model (Entity classes)

Maps to Use Case model (Control classes)Maps to Use Case model (Control classes)


Persistence Concepts in EJB

Datastore

Sessions

StorageObjects

StorageHomes

Storage HomeIncarnation

Storage ObjectIncarnation


State Management

• State synchronization methods– ejbLoad– ejbStore

• Resource Management methods– ejbActivate– ejbPassivate


Entity Bean Types

• Bean can have total control over loading and storing from database– Bean Managed Persistence (BMP)

• Container can take over this responsibility– Container Managed Persistence (CMP)

• Still need to define an OR mapping in admin tool• This is the same in CORBA PSS

– Specialized Implementations• Legacy applications such as CICS

• When to choose what?– Well – start out with CMP if possible, and then migrate

code as performance issues pops up during testing


Bean Manged Perstistence

• Have to handle all database interaction– except transactions (more of this next time)

• ejbLoad() and ejbStore() called when bean instance state must be synchronized with database

• ejbActivate() and ejbPassivate() called when bean is moved between the ready state and pooled state


Implement a BMP Entity Bean

package java.examples.ejb.entity.bean;

import javax.ejb.EntityBean;

import javax.ejb.EntityContext;

…

public class BookBMP extends BookEJB {

private DataSource dataSource_;

private EntityContext ctx_;

…

Additional setup of database connections needed – some are done in the configuration tool

Important features: Entity beans always implement the following event handles:•ejbCreate: inseret•ejbRemove: delete•ejbLoad: select•ejbStore: update•ejbFindByPrimaryKey: select•And more can be implemented:•ejbFindBooksByAuthor: select


Implement DB Insertion

public String ejbCreate(String id, String title, String author, String topic) {

super.ejbCreate(id, title, author, topic);

Connection conn = null;

PreparedStatement pstatement = null;

try {

conn = dataSource_.getConnection();

pstatement=conn.prepareStatement("insert into Book (id, title, author, topic)"+

” values (?, ?, ?, ?)");

pstatement.setString(1,id_); pstatement.setString(2,title_);

pstatement.setString(3,author_); pstatement.setString(4,topic_);

pstatement.execute(); return id_;

}

catch(SQLException ex) { throw new EJBException(ex); }

finally { … }

}

id title author topic

42123 EJB SW EJB

43423 EJB2 SW EJB

… … … …

BookBMP

OR mapping


Implement DB Load

public void ejbLoad() { Connection conn = null; PreparedStatement pstatement = null; ResultSet rs = null; try { conn = dataSource_.getConnection(); pstatement = conn.prepareStatement( "select id, title, author, topic from Book " + "where id = ?"); pstatement.setString(1, (String)ctx_.getPrimaryKey()); rs = pstatement.executeQuery(); if (rs.next()) {

id_ = rs.getString("id"); title_ = rs.getString("title"); author_ = rs.getString("author"); topic_ = rs.getString("topic"); super.ejbLoad(); } else { throw new EJBException("unable to locate row"); } } catch(SQLException ex) { throw new EJBException(getText(ex)); } finally { … } ...


Implement DB Store

public void ejbStore() {



try {

super.ejbStore();


pstatement = conn.prepareStatement(

"update Book set title=?, author=?, topic=? " + "where id = ?");

pstatement.setString(1,title_); pstatement.setString(2,author_);

pstatement.setString(3,topic_); pstatement.setString(4,id_);

pstatement.executeUpdate();

}

catch(SQLException ex) { throw new EJBException(getText(ex)); }

finally { … }

}


Implement DB Remove

public void ejbRemove() {



try {

super.ejbRemove();


pstatement = conn.prepareStatement("delete from Book " + "where id = ?");

pstatement.setString(1, (String)ctx_.getPrimaryKey());

pstatement.executeUpdate();

}


finally { … }

}


Implement Finders

public String ejbFindByPrimaryKey(String pk) throws FinderException {



ResultSet rs = null;

try {


pstatement = conn.prepareStatement("select id from Book " + "where id = ?");

pstatement.setString(1, pk);

rs = pstatement.executeQuery();

if (rs.next()) {

return rs.getString("id");

}

else { throw new ObjectNotFoundException(pk + " no found"); }

}


finally {... }

}


Implement Finders (cont.)

public Collection ejbFindAll() throws FinderException {



ResultSet rs = null;

try {

Vector pKeys = new Vector();


pstatement = conn.prepareStatement("select id from Book ");

rs = pstatement.executeQuery();

while (rs.next()) {

pKeys.add(rs.getString("id"));

}

return pKeys;

}

catch(SQLException ex) {throw new EJBException(getText(ex)); }

finally {... }

}

RDBMS JDBC DataBean / JavaBean example from DEITEL


This is another way of using JDBC access to a RDBMS.

It consists of two elements, the JavaBean (code to left is a GuestBean), and a DataBean (GuestDataBean).

1 // Fig. 31.20: GuestBean.java2 // JavaBean to store data for a guest in the guest book.3 package com.deitel.advjhtp1.jsp.beans;4 5 public class GuestBean {6 private String firstName, lastName, email;7 8 // set the guest's first name9 public void setFirstName( String name )10 {11 firstName = name; 12 }13 14 // get the guest's first name15 public String getFirstName()16 {17 return firstName; 18 }19 20 // set the guest's last name21 public void setLastName( String name )22 {23 lastName = name; 24 }25 26 // get the guest's last name27 public String getLastName()28 {29 return lastName; 30 }31 32 // set the guest's email address33 public void setEmail( String address )34 {35 email = address;


GuestBean.java

36 }37 38 // get the guest's email address39 public String getEmail()40 {41 return email; 42 }43 }


GuestDataBean.java

Set up database

1 // Fig. 31.21: GuestDataBean.java2 // Class GuestDataBean makes a database connection and supports 3 // inserting and retrieving data from the database.4 package com.deitel.advjhtp1.jsp.beans;5 6 // Java core packages7 import java.io.*;8 import java.sql.*;9 import java.util.*;10 11 public class GuestDataBean {12 private Connection connection;13 private PreparedStatement addRecord, getRecords;14 15 // construct TitlesBean object 16 public GuestDataBean() throws Exception17 {18 // load the Cloudscape driver19 Class.forName( "COM.cloudscape.core.RmiJdbcDriver" );20 21 // connect to the database22 connection = DriverManager.getConnection(23 "jdbc:rmi:jdbc:cloudscape:guestbook" );24 25 getRecords =26 connection.prepareStatement(27 "SELECT firstName, lastName, email FROM guests"28 );29 30 addRecord =31 connection.prepareStatement(32 "INSERT INTO guests ( " +33 "firstName, lastName, email ) " +34 "VALUES ( ?, ?, ? )"35 );

Defines the database driver, URL, and SQL statements.


GuestDataBean.java

Database access methods

36 }37 38 // return an ArrayList of GuestBeans39 public List getGuestList() throws SQLException40 {41 List guestList = new ArrayList();42 43 // obtain list of titles44 ResultSet results = getRecords.executeQuery();45 46 // get row data47 while ( results.next() ) {48 GuestBean guest = new GuestBean();49 50 guest.setFirstName( results.getString( 1 ) );51 guest.setLastName( results.getString( 2 ) );52 guest.setEmail( results.getString( 3 ) );53 54 guestList.add( guest ); 55 } 56 57 return guestList;58 }59 60 // insert a guest in guestbook database61 public void addGuest( GuestBean guest ) throws SQLException62 {63 addRecord.setString( 1, guest.getFirstName() );64 addRecord.setString( 2, guest.getLastName() );65 addRecord.setString( 3, guest.getEmail() );66 67 addRecord.executeUpdate();68 }69 70 // close statements and terminate database connection

Obtain the guest list from the database. Return each guest in a bean.


GuestDataBean.java

Close database

71 protected void finalize()72 {73 // attempt to close database connection74 try {75 getRecords.close();76 addRecord.close();77 connection.close();78 }79 80 // process SQLException on close operation81 catch ( SQLException sqlException ) {82 sqlException.printStackTrace();83 }84 }85 }


Example of usage of the JavaBean/Databean in JSP

GuestBookLogin.jsp

Add new guest to book

71 <input type = "text" name = "email" />72 </td>73 </tr>74 75 <tr>76 <td colspan = "2">77 <input type = "submit" 78 value = "Submit" />79 </td>80 </tr>81 </table>82 </form>83 84 <% // continue scriptlet85 86 } // end if87 else {88 guestData.addGuest( guest );89 90 %> <%-- end scriptlet to insert jsp:forward action --%>91 92 <%-- forward to display guest book contents --%>93 <jsp:forward page = "guestBookView.jsp" />94 95 <% // continue scriptlet96 97 } // end else98 99 %> <%-- end scriptlet --%>100 </body>101 102 </html>

Calls the guestData Bean to write this new guest into the address book. The forward the client to the guestBookView JSP.


RDBMS JDBC DataBean / JavaBean

• This may be better suited for WebServices / RMI• You may also employ SQL directly in your Servant

objects• Also see:

http://db.apache.org/torque/torque-301/tutorial/index.html on how to use Torque

http://db.apache.org/torque/torque-301/tutorial/index.html

http://db.apache.org/torque/torque-301/tutorial/index.html


Using the Torque framework

Excerpt from tutorial – defining the Schema

…

<table name="author" description="Author Table">

<column name="author_id" required="true" primaryKey="true" type="INTEGER" description="Author Id"/>

<column name="first_name" required="true" type="VARCHAR" size="128" description="First Name"/>

<column name="last_name" required="true" type="VARCHAR" size="128" description="Last Name"/>

</table>

</database>

Excerpt from tutorial – sample code

Publisher addison = new Publisher(); addison.setName("Addison Wesley Professional"); addison.save(); Author bloch = new Author(); bloch.setFirstName("Joshua"); bloch.setLastName("Bloch"); bloch.save();

Principles of Persistence continuedODBMS


ODBMS

• ODBMS have been standardized by the Object Database Management Group– Schema definition language (ODL) – subset of CORBA

IDL– Programming language bindings to

• C++ • Java• And many others

– Object Query Language (OQL)

• Support persistence of OO programming language objects


Mapping to ODBMSs

• ODL is a superset of OMG/IDL• Programming language bindings of ODBMS are

also supported by CORBA• CORBA portable object adapter supports ODBMS• ODBMS objects can be

– clients of CORBA objects– servers for CORBA objects


Comparison

• Externalization/Serilization – only well suited for smaller size projects

• Persistence in RDBMS is – complicated by OR Impedence Mismatch– simplified by wide availability of RDBMS– very performant (if OR imp.mis. is handled)

• Persistence in ODBMS is– simplified by conceptual similarities of

• object models• programming language bindings

– but fairly unproven yet

Documents

Objektorienteret Middleware (OOMI) Persistence. Ingeniørhøjskolen i Århus Slide 2 af 53 Goal with todays lesson After these 2x35 minutes you will be: