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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 Slide 2 af 53 Ingeniørhøjskolen i Århus Outline • Plenum – experince with persistent datastorage • Principles of Persistence • Datastore Technology for Persistence – Files – Relational Databases – Object Databases • Exemplified OR-mapping with EJB Slide 3 af 53 Ingeniørhøjskolen i Århus 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? Slide 4 af 53 Ingeniørhøjskolen i Århus 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? Slide 6 af 53 Ingeniørhøjskolen i Århus 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 Slide 7 af 53 Ingeniørhøjskolen i Århus 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 Slide 9 af 53 Ingeniørhøjskolen i Århus 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 Slide 11 af 53 Ingeniørhøjskolen i Århus 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 Slide 12 af 53 Ingeniørhøjskolen i Århus 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 Slide 13 af 53 Ingeniørhøjskolen i Århus 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 Slide 14 af 53 Ingeniørhøjskolen i Århus 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 Slide 15 af 53 Ingeniørhøjskolen i Århus 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"); } } Slide 16 af 53 Ingeniørhøjskolen i Århus 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"); } } Slide 17 af 53 Using simpel Java Serilization To obtain perstistence Ingeniørhøjskolen i Århus Persistence Interfaces in COM <<Interface>> IUnknown <<Interface>> IPersist + GetClassID() <<Interface>> IPersistStorage + IsDirty() + InitNew() + Load() + Save() + SaveCompleted() + HandsOffStorage() <<Interface>> IPersistStream + IsDirty() + Load() + Save() + GetSizeMax() Slide 18 af 53 <<Interface>> IPersistFile + IsDirty() + Load() + Save() + SaveCompleted() + GetCurFile() Ingeniørhøjskolen i Århus 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 Slide 19 af 53 Ingeniørhøjskolen i Århus Relational Database • Relational Database Management Systems (RDBMSs) Examples: – – – – – – – Oracle DB2 Microsoft SQL Server Sybase Microsoft Access MySQL (freeware) PostGree DB (freeware) Slide 20 af 53 Ingeniørhøjskolen i Århus 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 Slide 21 af 53 Ingeniørhøjskolen i Århus 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 Slide 22 af 53 Ingeniørhøjskolen i Århus 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 Slide 23 af 53 Ingeniørhøjskolen i Århus 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 Ingeniørhøjskolen i Århus Slide 25 af 53 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) Slide 26 af 53 Ingeniørhøjskolen i Århus 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 Slide 27 af 53 Ingeniørhøjskolen i Århus 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 Slide 28 af 53 Ingeniørhøjskolen i Århus 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 Slide 29 af 53 Ingeniørhøjskolen i Århus Persistence Concepts in EJB Storage Objects Storage Home Incarnation Datastore Sessions Storage Homes Storage Object Incarnation Slide 30 af 53 Ingeniørhøjskolen i Århus State Management • State synchronization methods – ejbLoad – ejbStore • Resource Management methods – ejbActivate – ejbPassivate Slide 31 af 53 Ingeniørhøjskolen i Århus 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 Slide 32 af 53 Ingeniørhøjskolen i Århus 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 Slide 33 af 53 Ingeniørhøjskolen i Århus 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 Slide 34 af 53 Ingeniørhøjskolen i Århus 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 { … } } Slide 35 af 53 BookBMP OR mapping id title author topic 42123 EJB SW EJB 43423 EJB2 SW EJB … … … … Ingeniørhøjskolen i Århus 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 { … } ... Slide 36 af 53 Ingeniørhøjskolen i Århus 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 { … } } Slide 37 af 53 Ingeniørhøjskolen i Århus 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 { … } } Slide 38 af 53 Ingeniørhøjskolen i Århus 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 {... } } Slide 39 af 53 Ingeniørhøjskolen i Århus 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 {... } } Slide 40 af 53 Ingeniørhøjskolen i Århus RDBMS JDBC DataBean / JavaBean example from DEITEL 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 // 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; Slide 42 af 25 Ingeniørhøjskolen i Århus 36 37 38 39 40 41 42 43 } // get the guest's email address public String getEmail() { return email; } GuestBean.ja va } Slide 43 af 25 Ingeniørhøjskolen i Århus 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 // 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 ( ?, ?, ? )" ); Slide 44 af 25 Ingeniørhøjskolen i Århus 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 } // 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 Slide 45 af 25 Ingeniørhøjskolen i Århus 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 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 } } Slide 46 af 25 Ingeniørhøjskolen i Århus 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 <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> Slide 47 af 25 Ingeniørhøjskolen i Århus 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 Slide 48 af 53 Ingeniørhøjskolen i Århus 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(); Slide 49 af 53 Ingeniørhøjskolen i Århus 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 Slide 51 af 53 Ingeniørhøjskolen i Århus 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 Slide 52 af 53 Ingeniørhøjskolen i Århus 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 Slide 53 af 53 Ingeniørhøjskolen i Århus