Objektorienteret netværkskommuniation(ITONK1) Persistence

Ingeniørhøjskolen i Århus Slide 2 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 –Ready to explore the subject further –You will not: Be an expert on persistence, as this is a huge area in itself

Ingeniørhøjskolen i Århus Slide 3 Outline Plenum – experience with persistent datastorage Continuing exercise Principles of Persistence Datastore Technology for Persistence –Files –Relational Databases Examplified OR-mapping with EJB –Object Databases

Ingeniørhøjskolen i Århus Slide 4 Experience with Persistent datastorage To establish a picture of your knowledge –Which types of persistent datastorage do you know? –What experience do you have in using it with (oo) programming languages? –What problems did you face – and how did you solve them?

Ingeniørhøjskolen i Århus Slide 5 Exercise: comparing strategies for persistence This will be a continuing discussion Select one or more entity classes from your own required assignment 1 an discuss how to solve the persistent requirements of the assignment using: –File persistence –Relational Database Management System (toolkit or emb. JDBC) –Object database system (ODBMS, e.g. JDO) Using information obtained from the examples from ONK-2 and the following slides Feel free to use the classroom PC for searching for more information We will stop after each section (files, RDBMS, ODBMS), and allow for 5 min. group work We will end with a follow-up discussion

Principles of Persistence

Ingeniørhøjskolen i Århus Slide 7 What is Persistence – a definition 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 We remember the activation/deactivation discussion? –The state needs to be retained between object deactivation and object activation –Why?

Ingeniørhøjskolen i Århus Slide 8 How to achieve Persistence? Storing object state on persistent datastore before deactivation Upon activation, load object state from persistent datastore –E.g. RMI activation Persistent storage can be obtained by? –File system embedded systems on disk-storage, Flash-RAM and others –Relational Database All from embedded, to desktop and enterprise servers (most widely used is SQL servers) –Object-Database Has been emerging technology for years, but no widespread support yet. JDO for Java is a promising technology however –Others?

Datastore Technology for Persistence

Ingeniørhøjskolen i Århus Slide 10 Datastore Technology Persistence can be implemented using Files –CORBA Externalization –C#/Java type File Serialization –Structured Storage in COM Relational Databases –Object Relational Mapping “Homegrown”, CMP, JDO, Hibernate, Torque, LLBLGen Pro –JDBC/ODBC Oracle, IBM DB2, Microsoft SQL Server & Access, MySQL Object Databases – Versant, JDOhttp://

Ingeniørhøjskolen i Århus Slide 11 Problems with File-based Persistence Mapping to Files can be inefficient for large composite objects 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

Ingeniørhøjskolen i Århus Slide 12 Continuing exercise part 1 – file-based Select one or more entity classes from your own required assignment 1 an discuss how to solve the persistent requirements of the assignment using: –File persistence –Pro’s and Con’s –How to implement?

Ingeniørhøjskolen i Århus Slide 13 Relational Database Relational Database Management Systems (RDBMSs) Examples: –Ingres –Oracle –Sybase –DB2 –Microsoft SQL Server –Microsoft Access –MySQL –PostGree DB

Ingeniørhøjskolen i Århus Slide 14 Mapping to RDBMS’s Relational database schemas consist of sets of tables Types of mapping: –Define a table for each type –Complex mapping 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 RDBMS secondary keys for object references Resolve inheritance statically –This and other problems -> Objects and RDBMS does not map perfectly: –the OR impedance mismatch

Ingeniørhøjskolen i Århus Slide 15 Embedding Queries into Programs Embedded SQL –Macros to embed queries into programs –RDBMS provides processor to expand macros –API to traverse queries –Not standardized Open Database Connectivity (Microsoft) –Standardized API for RDBMS Access available on all Microsoft Platforms Java Database Connectivity (Sun) –Standardized RDBMS Access from Java

Ingeniørhøjskolen i Århus Slide 16 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, CCM, Hibernate, others –JDBC is at a low level –Hibernate project freeware

RDBMS mapping illustrated with EJB’s

Ingeniørhøjskolen i Århus Slide 18 Enterprise JavaBeans (EJB’s) 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, entity representations –Enterprise Java Beans (EJB) designed for inter-process purposes

Ingeniørhøjskolen i Århus Slide 19 EJB’s (cont.) Restricted to Java (only implementation language) –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)

Ingeniørhøjskolen i Århus Slide 20 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 Use Case model (Control classes)

Ingeniørhøjskolen i Århus Slide 21 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 CCM / 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

Ingeniørhøjskolen i Århus Slide 22 Bean Managed Persistence Have to handle all database interaction –except distributed transactions 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

Ingeniørhøjskolen i Århus Slide 23 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: insert ejbRemove: delete ejbLoad: select ejbStore: update ejbFindByPrimaryKey: select And more can be implemented: ejbFindBooksByAuthor: select

Ingeniørhøjskolen i Århus Slide 24 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 { … } } idtitleauthortopic 42123EJBSWEJB 43423EJB2SWEJB ………… BookBMP OR-mapping

Ingeniørhøjskolen i Århus Slide 25 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 { … } …

Ingeniørhøjskolen i Århus Slide 26 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 { … } }

Ingeniørhøjskolen i Århus Slide 27 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 { … } }

Ingeniørhøjskolen i Århus Slide 28 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 {... } }

Ingeniørhøjskolen i Århus Slide 29 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 {... } }

Ingeniørhøjskolen i Århus Slide 30 Continuing exercise part 2 – RDBMS based Select one or more entity classes from your own required assignment 1 an discuss how to solve the persistent requirements of the assignment using: –RDBMS Embedded SQL If you have experience using toolkits, be free to elaborate on this –Pro’s and Con’s –How to implement –Compared to using file externalization

Principles of Persistence continued ODBMS

Ingeniørhøjskolen i Århus Slide 32 ODBMS ODBMSs 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) –JDO has replaced the Java binding Support persistence of OO programming language objects

Ingeniørhøjskolen i Århus Slide 33 JDO: POJO’s

Ingeniørhøjskolen i Århus Slide 34 Continuing exercise part 3 – ODBMS based Select one or more entity classes from your own required assignment 1 an discuss how to solve the persistent requirements of the assignment using: –ODBMS Use the JDO example from ONK-2 –Pro’s and Con’s –How to implement? –Compared to using RDBMS and file externalization

Ingeniørhøjskolen i Århus Slide 35 Comparison File externalization / serialization is not transparent for implementors of server objects Persistence in RDBMS’s is –complicated by OR-impedance mismatch –simplified by wide availability of RDBMS’s –Well-known tested technology Persistence in ODBMS’s is –simplified by conceptual similarities of object models programming language bindings –No widespread support –(personal view) I would not risk high-performance system on this