An Exploration of Object Oriented Database Management Systems

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By Dare Obasanjo

Introduction: Why Object Oriented Database Management Systems?

In today's world, Client-Server applications that rely on a database on the server as a data store while servicing requests from multiple clients are quite commonplace. Most of these applications use a Relational Database Management System (RDBMS) as their data store while using an object oriented programming language for development. This causes a certain inefficency as objects must be mapped to tuples in the database and vice versa instead of the data being stored in a way that is consistent with the programming model. The "impedance mismatch" caused by having to map objects to tables and vice versa has long been accepted as a necessary performance penalty. This paper is aimed at seeking out an alternative that avoids this penalty.

The purpose of this paper is to provide answers to the following questions



Overview of Object Oriented Database Management Systems

An OODBMS is the result of combining object oriented programming principles with database management principles. Object oriented programming concepts such as encapsulation, polymorphism and inheritance are enforced as well as database management concepts such as the ACID properties (Atomicity, Consistency, Isolation and Durability) which lead to system integrity, support for an ad hoc query language and secondary storage management systems which allow for managing very large amounts of data. The Object Oriented Database Manifesto [Atk 89] specifically lists the following features as mandatory for a system to support before it can be called an OODBMS; Complex objects, Object identity, Encapsulation , Types and Classes , Class or Type Hierarchies, Overriding,overloading and late binding, Computational completeness , Extensibility, Persistence , Secondary storage management, Concurrency, Recovery and an Ad Hoc Query Facility.

From the aforementioned description, an OODBMS should be able to store objects that are nearly indistinguishable from the kind of objects supported by the target programming language with as little limitation as possible. Persistent objects should belong to a class and can have one or more atomic types or other objects as attributes. The normal rules of inheritance should apply with all their benefits including polymorphism, overridding inherited methods and dynamic binding. Each object has an object identifier (OID) which used as a way of uniquely identifying a particuler object. OIDs are permanent, system generated and not based on any of the member data within the object. OIDs make storing references to other objects in the database simpler but may cause referential intergrity problems if an object is deleted while other objects still have references to its OID. An OODBMS is thus a full scale object oriented development environment as well as a database management system. Features that are common in the RDBMS world such as transactions, the ability to handle large amounts of data, indexes, deadlock detection, backup and restoration features and data recovery mechanisms also exist in the OODBMS world.

A primary feature of an OODBMS is that accessing objects in the database is done in a transparent manner such that interaction with persistent objects is no different from interacting with in-memory objects. This is very different from using an RDBMSs in that there is no need to interact via a query sub-language like SQL nor is there a reason to use a Call Level Interface such as ODBC, ADO or JDBC. Database operations typically involve obtaining a database root from the the OODBMS which is usually a data structure like a graph, vector, hash table, or set and traversing it to obtain objects to create, update or delete from the database. When a client requests an object from the database, the object is transferred from the database into the application's cache where it can be used either as a transient value that is disconnected from its representation in the database (updates to the cached object do not affect the object in the database) or it can be used as a mirror of the version in the database in that updates to the object are reflected in the database and changes to object in the database require that the object is refetched from the OODBMS.

Comparisons of OODBMSs to RDBMSs

There are concepts in the relational database model that are similar to those in the object database model. A relation or table in a relational database can be considered to be analogous to a class in an object database. A tuple is similar to an instance of a class but is different in that it has attributes but no behaviors. A column in a tuple is similar to a class attribute except that a column can hold only primitive data types while a class attribute can hold data of any type. Finally classes have methods which are computationally complete (meaning that general purpose control and computational structures are provided [McF 99]) while relational databases typically do not have computationally complete programming capabilities although some stored procedure languages come close.

Below is a list of advantages and disadvantages of using an OODBMS over an RDBMS with an object oriented programming language.

Advantages

  1. Composite Objects and Relationships: Objects in an OODBMS can store an arbitrary number of atomic types as well as other objects. It is thus possible to have a large class which holds many medium sized classes which themselves hold many smaller classes, ad infinitum. In a relational database this has to be done either by having one huge table with lots of null fields or via a number of smaller, normalized tables which are linked via foreign keys. Having lots of smaller tables is still a problem since a join has to be performed every time one wants to query data based on the "Has-a" relationship between the entities. Also an object is a better model of the real world entity than the relational tuples with regards to complex objects. The fact that an OODBMS is better suited to handling complex,interrelated data than an RDBMS means that an OODBMS can outperform an RDBMS by ten to a thousand times depending on the complexity of the data being handled.

  2. Class Hierarchy: Data in the real world is usually has hierarchical characteristics. The ever popular Employee example used in most RDBMS texts is easier to describe in an OODBMS than in an RDBMS. An Employee can be a Manager or not, this is usually done in an RDBMS by having a type identifier field or creating another table which uses foreign keys to indicate the relationship between Managers and Employees. In an OODBMS, the Employee class is simply a parent class of the Manager class.

  3. Circumventing the Need for a Query Language: A query language is not necessary for accessing data from an OODBMS unlike an RDBMS since interaction with the database is done by transparently accessing objects. It is still possible to use queries in an OODBMS however.

  4. No Impedence Mismatch: In a typical application that uses an object oriented programming language and an RDBMS, a signifcant amount of time is usually spent mapping tables to objects and back. There are also various problems that can occur when the atomic types in the database do not map cleanly to the atomic types in the programming language and vice versa. This "impedance mismatch" is completely avoided when using an OODBMS.

  5. No Primary Keys: The user of an RDBMS has to worry about uniquely identifying tuples by their values and making sure that no two tuples have the same primary key values to avoid error conditions. In an OODBMS, the unique identification of objects is done behind the scenes via OIDs and is completely invisible to the user. Thus there is no limitation on the values that can be stored in an object.

  6. One Data Model: A data model typically should model entities and their relationships, constraints and operations that change the states of the data in the system. With an RDBMS it is not possible to model the dynamic operations or rules that change the state of the data in the system because this is beyond the scope of the database. Thus applications that use RDBMS systems usually have an Entity Relationship diagram to model the static parts of the system and a seperate model for the operations and behaviors of entities in the application. With an OODBMS there is no disconnect between the database model and the application model because the entities are just other objects in the system. An entire application can thus be comprehensively modelled in one UML diagram.

Disadvantages

  1. Schema Changes: In an RDBMS modifying the database schema either by creating, updating or deleting tables is typically independent of the actual application. In an OODBMS based application modifying the schema by creating, updating or modifying a persistent class typically means that changes have to be made to the other classes in the application that interact with instances of that class. This typically means that all schema changes in an OODBMS will involve a system wide recompile. Also updating all the instance objects within the database can take an extended period of time depending on the size of the database.

  2. Language Dependence: An OODBMS is typically tied to a specific language via a specific API. This means that data in an OODBMS is typically only accessible from a specific language using a specific API, which is typically not the case with an RDBMS.

  3. Lack of Ad-Hoc Queries: In an RDBMS, the relational nature of the data allows one to construct ad-hoc queries where new tables are created from joining existing tables then querying them. Since it is currently not possible to duplicate the semantics of joining two tables by "joining" two classes then there is a loss of flexibility with an OODBMS. Thus the queries that can be performed on the data in an OODBMS is highly dependent on the design of the system.

Who is currently using an OODBMS to handle mission critical data?

The following information was gleaned from the ODBMS Facts website.

Interacting With An OODBMS

Below are Java code samples for accessing a relational database and accessing an object database. Compare the size of the code in both examples. The examples are for an instant messaging application.

  1. Validating a user.
    
      

    Java code accessing an ObjectStore™ database

         import COM.odi.*; 
         import COM.odi.util.query.*; 
         import COM.odi.util.*; 
         import java.util.*;
    
       
    try {
    
             //start database session 
         Session session = Session.create(null, null); 
         session.join(); 
    
             //open database and start transaction 
             Database db = Database.open("IMdatabase", ObjectStore.UPDATE); 
             Transaction tr = Transaction.begin(ObjectStore.READONLY); 
         
             //get hashtable of user objects from DB
         OSHashMap users = (OSHashMap) db.getRoot("IMusers"); 
    
             //get password and username from user 
         String username = getUserNameFromUser(); 
         String passwd   = getPasswordFromUser(); 
    
             
             //get user object from database and see if it exists and whether password is correct
         UserObject user = (UserObject) users.get(username); 
    
         if(user == null)
               System.out.println("Non-existent user");
         else
           if(user.getPassword().equals(passwd))
             System.out.println("Successful login");
               else
                 System.out.println("Invalid Password");
    
             //end transaction, close database and retain terminate session 
             tr.commit();
             db.close();
         session.termnate(); 
         }
         //exception handling would go here ...
    
    
    

    Java JDBC code accessing an IBM's DB2 Database™

         import java.sql.*;
         import sun.jdbc.odbc.JdbcOdbcDriver; 
         import java.util.*;
    
       
    try {
    
             //Launch instance of database driver. 
         Class.forName("COM.ibm.db2.jdbc.app.DB2Driver").newInstance(); 
    
             //create database connection 
             Connection conn = DriverManager.getConnection("jdbc:db2:IMdatabase"); 
    
             //get password and username from user  
         String username = getUserNameFromUser(); 
         String passwd   = getPasswordFromUser(); 
    
         //perform SQL query 
         Statement sqlQry = conn.createStatement(); 
         ResultSet rset = sqlQry.executeQuery("SELECT password from user_table WHERE username='" + username +"'"); 
         
            
         if(rset.next()){
           if(rset.getString(1).equals(passwd))
             System.out.println("Successful login");
               else
                 System.out.println("Invalid Password");
              }else{
              System.out.println("Non-existent user");
              }
    
             //close database connection
         sqlQry.close(); 
         conn.close(); 
    
         }
         //exception handling would go here ...
    
    There isn't much difference in the above examples although it does seem a lot clearer to perform operations on a UserObject instead of a ResultSet when validating the user.

  2. Getting the user's contact list.
    
      

    Java code accessing an ObjectStore™ database

         import COM.odi.*; 
         import COM.odi.util.query.*; 
         import COM.odi.util.*; 
         import java.util.*;
    
       
    try {
    
          /* start session and open DB, same as in section 1a */ 
    
          //get hashmap of users from the DB 
          OSHashMap users = (OSHashMap) db.getRoot("IMusers"); 
    
         //get user object from database
         UserObject c4l = (UserObject) users.get("Carnage4Life");  
         UserObject[] contactList = c4l.getContactList(); 
    
         System.out.println("This are the people on Carnage4Life's contact list");
    
         for(int i=0; i < contactList.length; i++)
            System.out.println(contactList[i].toString());  //toString() prints fullname, username, online status and webpage URL
    
         /* close session and close DB, same as in  section 1a */ 
    
        }//exception handling code
    
    
    

    Java JDBC code accessing an IBM's DB2 Database™

         import java.sql.*;
         import sun.jdbc.odbc.JdbcOdbcDriver; 
         import java.util.*;
    
       
    try {
    
          /*  open DB connection, same as in section 1b */
    
             //perform SQL query 
         Statement sqlQry = conn.createStatement(); 
         ResultSet rset = sqlQry.executeQuery("SELECT fname, lname, user_name, online_status, webpage FROM contact_list, user_table" +
                                              "WHERE contact_list.owner_name='Carnage4Life' and contact_list.buddy_name=user_table.user_name");
    
        System.out.println("This are the people on Carnage4Life's contact list");                    
            
        while(rset.next())
             System.out.println("Full Name:" + rset.getString(1) + " " + rset.getString(2) + " User Name:" + rset.getString(3) + " OnlineStatus:"
                            + rset.getString(4) + " HomePage URL:" + rset.getString(5));
    
          /*  close DB connection, same as in section 1b */ 
    
      }//exception handling code
    
    The benefits of using an OODBMS over an RDBMS in Java slowly become obvious. Consider also that if the data from the select needs to be returned to another method then all the data from the result set has to be mapped to another object (UserObject).

  3. Get all the users that are online.
    
      

    Java code accessing an ObjectStore™ database

         import COM.odi.*; 
         import COM.odi.util.query.*; 
         import COM.odi.util.*; 
         import java.util.*;
    
        try{
              /* same as above */ 
          
          //use a OODBMS query to locate all the users whose status is 'online' 
          Query q = new Query = (UserObject.class, "onlineStatus.equals(\"online\"");
          Collection users = db.getRoot("IMusers"); 
              Set onlineUsers = q.select(users); 
              
              Iterator iter = onlineUsers.iterator();
    
             // iterate over the results
             while ( iter.hasNext() )
             {
                 UserObject user = (UserObject) iter.next();
    
                 // send each person some announcement
             sendAnnouncement(user); 
    
             }
    
             /* same as above */ 
    
         }//exception handling goes here 
    
    

    Java JDBC code accessing an IBM's DB2 Database™

         import java.sql.*;
         import sun.jdbc.odbc.JdbcOdbcDriver; 
         import java.util.*;
    
    try{
              /* same as above */ 
          
           //perform SQL query 
         Statement sqlQry = conn.createStatement(); 
         ResultSet rset = sqlQry.executeQuery("SELECT fname, lname, user_name, online_status, webpage FROM user_table WHERE online_status='online'");
    
         while(rset.next()){
    
         UserObject user = new UserObject(rset.getString(1),rset.getString(2),rset.getString(3),rset.getString(4),rset.getString(5));
         sendAnnouncement(user);     
    
         }
    
    
             /* same as above */ 
         }//exception handling goes here
    
    

List of Object Oriented Database Management Systems

Proprietary

Description of the MultiEdit Application

MultiEdit allows multiple users, potentially on different machines to edit a file simultaneously. Each user has his or her own view of the file, and each view includes its own cursor. Users may enter text into (the same or different points of) the file simultaneously. It is an essential requirement of the application that the contents of the file must always be kept consistent with the actions of users.

Static UML diagram of system

Javadocs For The Project Are Available

This project uses a Client/Server architecture to handle the difficulty of maintaining consistency between multiple clients trying to edit the same document at once. Each document is an object of class ShareableDocument stored in an Object Oriented Database which is remotely accessible via a DocumentManager which sits on the server and handles client requests. Whenever a user needs to access a document it is loaded from the database by the DocumentManager and sent to them over the network. From then on whenever an edit is performed by the user the actual key stroke and the position of the cursor is sent to the server which updates an in memory copy of the object before broadcasting the event to all users who are currently accessing the document including the user that originally performed the edit.

The main drawback of the above method is that the user who is typing the document will most likely experience a lag between when a character is typed and when it shows up on the GUI which is dependent on the speed of the network. Also if there is a network outage or similar error then the user cannot edit the document.

Saves are simply requests to the server to persist its in memory copy of the document which is more efficient than sending the whole document to the server. ShareableDocuments are not saved unless explicitly specified by a user or when a user closes a document. The application also allows the user to lock entire ShareableDocuments which prevents others from modifying the documents but they can still see the edits being made by the owner of the lock in real-time

Description of Major Classes in System
User
This is a representation of a user of the system which is stored in the ObjectStore™ database.
PersistableRemoteObject
This is an interface that describes operations that can be performed on a remote object that is shared by multiple users.
Owner
An interface that describes any entity that owns (i.e. created) a PersistableRemoteObject.
ShareableDocument
This is a representation of a document that can be editted by multiple users at once or locked by a single user. It also is stored in the ObjectStore™ database and implements the PersistableRemoteObject interface.
RemoteObjectFactory
This is an interface that describes the minimum amount of services that a server that manages PersistableRemoteObjects must provide.
DocumentManager
This is an interface describing the methods that are remotely accessible on an object that manages ShareableDocuments. It is a subclass of the RemoteObjectFactory interface.
DocumentManagerImpl
This is the actual class that handles managing access to ShareableDocuments by multiple users. It implements the DocumentManager interface and is a subclass of Java.rmi.UnicastRemoteObject which allows it to be accessed remotely.
DocumentContext
This maintains information on a ShareableDocument that is currently being accessed by one or more users. A table of these is kept in the DocumentManagerImpl class to keep track of all the ShareableDocuments that are being shared at once.
ClientView
This is the GUI that the user interacts with.
ClientControl
This does event handling for the ClientView.
ClientModel
This is the underlying model for the ClientView that contains data structures, maintains state and communicates with the server. It implements the ClientListener interface.
ClientListener
This is an interface that describes the methods that are remotely accessible on the ClientModel. The method that allows the client receive keystroke events from server is in this interface.
ClientState
This is an object that represents the current state of the ClientView.
LoginDialog
GUI used for logging on to the server.
AddUserDialog
GUI used for adding a new user to the system.
DocumentSelectionDialog
GUI used to select which document to edit.

MultiEdit State Diagram

UML State diagram of system

Discarded Design Decisions and Potential Future Development of MultiEdit

The original design called for a Paragraph object that would be a part of a ShareableDocument and would be the atomic unit capable of being locked by a user. Unfortunately the amount of semantic analysis required during runtime to constantly reparse the document to decide what textual divisions indicated a paragraph degraded performance to an unsatisfactory extent. Also a good solution is a non-trivial task. Certain considerations were made towards locking lines if they were being accessed by other users but this was outside the original specifications. Also originally each user updated their GUI before sending the key stroke to the server, this meant that if some other client had gotten to the server just before it and updated its GUI, then their representations of the document would beout of sync. The solution was to make sure that GUIs were not updated until the keystrokes had reached the server and were broadcast to all interested parties. Finally the decision to keep a copy of the document on the server instead of simply using the server to relay updates is so that when a new user requests a document that is already being accessed by two or more users, there is an effective way to give send the document without worrying about consistency issues.

Conclusion

The gains from using an OODBMS while developing an application using an OO programming language are many. The savings in development time by not having to worry about seperate data models as well as the fact that there is less code to write due to the lack of impedance mismatch is very attractive. In my opinion, there is little reason to pick an RDBMS over an OODBMS system for newapplication development unless there are legacy issues that have to be dealt with.

This paper is the final part of my indepedent study supervised by Dr. Sham Navathe and Wai Gen Yee.

Bibliography

  1. OODBMS Facts. Barry & Associates. 29 April 2001<http://www.odbmsfacts.com/>

  2. Atkinson, Malcolm et al. The Object-Oriented Database Manifesto. In Proceedings of the First International Conference on Deductive and Object-Oriented Databases, pages 223-40, Kyoto, Japan, December 1989 < http://www.cs.cmu.edu/People/clamen/OODBMS/Manifesto/htManifesto/Manifesto.html >

  3. McFarland, Gregory, Andres Rudmik, and David Lange. Object-Oriented Database Management Systems Revisited. 31 January 1999 .<http://www.dacs.dtic.mil/techs/oodbms2/>
?2001 Dare Obasanjo

Note: This page is modified from An Exploration of ODMS.