Transcript Internetteknologi (ITNET1)

Objektorienteret Middleware (OOMI)
Persistence
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
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Outline
• Plenum – experince with persistent datastorage
• Principles of Persistence
• Datastore Technology for Persistence
– Files
– Relational Databases
– Object Databases
• Exemplified OR-mapping with EJB
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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?
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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?
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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
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Transparency of Persistence
• Persistence should be transparent to users and
designers of client objects
Client Objects
Server Objects Datastore Objects
Client
InterfaceSlide 8 af 53
Persistence
Interface Ingeniørhøjskolen i Århus
Persistence Concepts
C++/Java object or class
Object
= row in
RDBMS table
Storage
Objects
Storage Home
Incarnation
Datastore
C++/Java object
Sessions
Storage
Homes
RDBMS
Server
Storage Object
Incarnation
RDBMS
Table
C++/Java object
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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
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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
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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
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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
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Example of using Serialization for RMI
• http://java.sun.com/j2se/1.3/docs/guide/rmi/activat
ion/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
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private Vector transactions;
private File holder;
public MyPersistentClass(ActivationID id, MarshalledObject data)
throws RemoteException, ClassNotFoundException, java.io.IOException {
CONSTRUCTOR of MyPersistentClass
Find complete example and tutorial at:
http://java.sun.com/j2se/1.3/docs/
guide/rmi/activation/activation.4.html
// 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();
Every time a passivated object is called,
the constructor is called as the rmid daemon
Instantiates it to activate – and here it
is checked whether the File exist – or not.
If it does exist, then its State is restored
if (holder.exists()) {
// Use the MarshalledObject to restore my state
this.restoreState();
} else {
transactions = new Vector(1,1);
transactions.addElement("Initializing transaction vector");
}
}
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public Vector calltheServer(Vector v) throws RemoteException {
Find complete example and tutorial at:
http://java.sun.com/j2se/1.3/docs/
guide/rmi/activation/activation.4.html
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");
}
}
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Using simpel Java Serilization
To obtain perstistence
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Persistence Interfaces in COM
<<Interface>>
IUnknown
<<Interface>>
IPersist
+ GetClassID()
<<Interface>>
IPersistStorage
+ IsDirty()
+ InitNew()
+ Load()
+ Save()
+ SaveCompleted()
+ HandsOffStorage()
<<Interface>>
IPersistStream
+ IsDirty()
+ Load()
+ Save()
+ GetSizeMax()
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<<Interface>>
IPersistFile
+ IsDirty()
+ Load()
+ Save()
+ SaveCompleted()
+ GetCurFile()
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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
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Relational Database
• Relational Database Management Systems
(RDBMSs) Examples:
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Oracle
DB2
Microsoft SQL Server
Sybase
Microsoft Access
MySQL (freeware)
PostGree DB (freeware)
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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
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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
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Issues with mapping
• Does this mean that we should figure out for
ourselfes how to obtain the OR-mapping?
–
–
–
–
–
–
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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
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RDBMS mapping illustrated with EJB’s
Enterprise JavaBeans (EJBs)
• Standard server-side component model for Java
Enterprise Applications
–
–
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–
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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
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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)
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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
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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
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Bean Usage
• Entity beans
Maps to domain model (Entity classes)
– model state maintained across all client interactions
– represent a row of data in a database
• Session beans
Maps to Use Case model (Control classes)
– 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
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Persistence Concepts in EJB
Storage
Objects
Storage Home
Incarnation
Datastore
Sessions
Storage
Homes
Storage Object
Incarnation
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State Management
• State synchronization methods
– ejbLoad
– ejbStore
• Resource Management methods
– ejbActivate
– ejbPassivate
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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
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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
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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
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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 { … }
}
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BookBMP
OR mapping
id
title
author
topic
42123
EJB
SW
EJB
43423
EJB2
SW
EJB
…
…
…
…
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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 { … }
...
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Implement DB Store
public void ejbStore() {
Connection conn = null;
PreparedStatement pstatement = null;
try {
super.ejbStore();
conn = dataSource_.getConnection();
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 { … }
}
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Implement DB Remove
public void ejbRemove() {
Connection conn = null;
PreparedStatement pstatement = null;
try {
super.ejbRemove();
conn = dataSource_.getConnection();
pstatement = conn.prepareStatement("delete from Book " + "where id = ?");
pstatement.setString(1, (String)ctx_.getPrimaryKey());
pstatement.executeUpdate();
}
catch(SQLException ex) { throw new EJBException(getText(ex)); }
finally { … }
}
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Implement Finders
public String ejbFindByPrimaryKey(String pk) throws FinderException {
Connection conn = null;
PreparedStatement pstatement = null;
ResultSet rs = null;
try {
conn = dataSource_.getConnection();
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"); }
}
catch(SQLException ex) { throw new EJBException(getText(ex)); }
finally {... }
}
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Implement Finders (cont.)
public Collection ejbFindAll() throws FinderException {
Connection conn = null;
PreparedStatement pstatement = null;
ResultSet rs = null;
try {
Vector pKeys = new Vector();
conn = dataSource_.getConnection();
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 {... }
}
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RDBMS JDBC DataBean / JavaBean
example from DEITEL
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// Fig. 31.20: GuestBean.java
// JavaBean to store data for a guest in the guest book.
package com.deitel.advjhtp1.jsp.beans;
public class GuestBean {
private String firstName, lastName, email;
// set the guest's first name
public void setFirstName( String name )
{
firstName = name;
}
// get the guest's first name
public String getFirstName()
{
return firstName;
}
// set the guest's last name
public void setLastName( String name )
{
lastName = name;
}
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).
// get the guest's last name
public String getLastName()
{
return lastName;
}
// set the guest's email address
public void setEmail( String address )
{
email = address;
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}
// get the guest's email address
public String getEmail()
{
return email;
}
GuestBean.ja
va
}
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// Fig. 31.21: GuestDataBean.java
// Class GuestDataBean makes a database connection and supports
// inserting and retrieving data from the database.
package com.deitel.advjhtp1.jsp.beans;
// Java core packages
import java.io.*;
import java.sql.*;
import java.util.*;
Defines the database driver, URL,
public class GuestDataBean {
and SQL statements.
private Connection connection;
private PreparedStatement addRecord, getRecords;
GuestDataBea
n.java
Set up database
// construct TitlesBean object
public GuestDataBean() throws Exception
{
// load the Cloudscape driver
Class.forName( "COM.cloudscape.core.RmiJdbcDriver" );
// connect to the database
connection = DriverManager.getConnection(
"jdbc:rmi:jdbc:cloudscape:guestbook" );
getRecords =
connection.prepareStatement(
"SELECT firstName, lastName, email FROM guests"
);
addRecord =
connection.prepareStatement(
"INSERT INTO guests ( " +
"firstName, lastName, email ) " +
"VALUES ( ?, ?, ? )"
);
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}
// return an ArrayList
ofthe
GuestBeans
Obtain
guest list from the
public List getGuestList() throws SQLException
database. Return each guest in a
{
List guestList =bean.
new ArrayList();
// obtain list of titles
ResultSet results = getRecords.executeQuery();
// get row data
while ( results.next() ) {
GuestBean guest = new GuestBean();
GuestDataBea
n.java
Database access
methods
guest.setFirstName( results.getString( 1 ) );
guest.setLastName( results.getString( 2 ) );
guest.setEmail( results.getString( 3 ) );
guestList.add( guest );
}
return guestList;
}
// insert a guest in guestbook database
public void addGuest( GuestBean guest ) throws SQLException
{
addRecord.setString( 1, guest.getFirstName() );
addRecord.setString( 2, guest.getLastName() );
addRecord.setString( 3, guest.getEmail() );
addRecord.executeUpdate();
}
// close statements and terminate database connection
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protected void finalize()
{
// attempt to close database connection
try {
getRecords.close();
addRecord.close();
connection.close();
}
// process SQLException on close operation
catch ( SQLException sqlException ) {
sqlException.printStackTrace();
}
GuestDataBea
n.java
Close
database
}
}
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<input type = "text" name = "email" />
</td>
</tr>
<tr>
<td colspan = "2">
<input type = "submit"
value = "Submit" />
</td>
</tr>
Calls the guestData Bean to write this
</table>
</form>
new guest into the address book. The
forward the client to the guestBookView
JSP.
<% // continue scriptlet
} // end if
else {
guestData.addGuest( guest );
%> <%-- end scriptlet to insert jsp:forward action --%>
<%-- forward to display guest book contents --%>
<jsp:forward page = "guestBookView.jsp" />
Example of
usage of the
JavaBean/
Databean in
JSP
GuestBookLog
in.jsp
Add new guest to
book
<% // continue scriptlet
}
// end else
%> <%-- end scriptlet --%>
</body>
</html>
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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/torque301/tutorial/index.html on how to use Torque
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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();
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Principles of Persistence continued
ODBMS
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
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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
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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
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