1. 1 Chapter 26 Object-Oriented DBMSs – Concepts and Design Transparencies © Pearson Education Limited 1995, 2005

2. 2 Chapter 26 - Objectives u Framework for an OODM. u Basics of the FDM. u Basics of persistent programming languages. u Main points of OODBMS Manifesto. u Main strategies for developing an OODBMS. u Single-level v. two-level storage models. u Pointer swizzling. u How an OODBMS accesses records. u Persistent schemes. © Pearson Education Limited 1995, 2005

3. 3 Chapter 26 - Objectives u Advantages and disadvantages of orthogonal persistence. u Issues underlying OODBMSs. u Advantages and disadvantages of OODBMSs. © Pearson Education Limited 1995, 2005

4. 4 Object-Oriented Data Model No one agreed object data model. One definition: Object-Oriented Data Model (OODM) –Data model that captures semantics of objects supported in object-oriented programming. Object-Oriented Database (OODB) –Persistent and sharable collection of objects defined by an ODM. Object-Oriented DBMS (OODBMS) –Manager of an ODB. © Pearson Education Limited 1995, 2005

5. 5 Object-Oriented Data Model u Zdonik and Maier present a threshold model that an OODBMS must, at a minimum, satisfy: –It must provide database functionality. –It must support object identity. –It must provide encapsulation. –It must support objects with complex state. © Pearson Education Limited 1995, 2005

6. 6 Object-Oriented Data Model u Khoshafian and Abnous define OODBMS as: –OO = ADTs + Inheritance + Object identity –OODBMS = OO + Database capabilities. u Parsaye et al. gives: –High-level query language with query optimization. –Support for persistence, atomic transactions: concurrency and recovery control. –Support for complex object storage, indexes, and access methods. –OODBMS = OO system + (1), (2), and (3). © Pearson Education Limited 1995, 2005

7. 7 Commercial OODBMSs u GemStone from Gemstone Systems Inc., u Objectivity/DB from Objectivity Inc., u ObjectStore from Progress Software Corp., u Ontos from Ontos Inc., u FastObjects from Poet Software Corp., u Jasmine from Computer Associates/Fujitsu, u Versant from Versant Corp. © Pearson Education Limited 1995, 2005

8. 8 Origins of the Object-Oriented Data Model © Pearson Education Limited 1995, 2005

9. 9 Functional Data Model (FDM) u Interesting because it shares certain ideas with object approach including object identity, inheritance, overloading, and navigational access. u In FDM, any data retrieval task can viewed as process of evaluating and returning result of a function with zero, one, or more arguments. u Resulting data model is conceptually simple but very expressive. u In the FDM, the main modeling primitives are entities and functional relationships. © Pearson Education Limited 1995, 2005

10. 10 FDM - Entities u Decomposed into (abstract) entity types and printable entity types. u Entity types correspond to classes of ‘real world’ objects and declared as functions with 0 arguments that return type ENTITY. u For example: Staff() → ENTITY PropertyForRent() → ENTITY. © Pearson Education Limited 1995, 2005

11. 11 FDM – Printable Entity Types and Attributes u Printable entity types are analogous to base types in a programming language. u Include: INTEGER, CHARACTER, STRING, REAL, and DATE. u An attribute is a functional relationship, taking the entity type as an argument and returning a printable entity type. u For example: staffNo(Staff) → STRING sex(Staff) → CHAR salary(Staff) → REAL © Pearson Education Limited 1995, 2005

12. 12 FDM – Composite Attributes Name() → ENTITY Name(Staff) → NAME fName(Name) → STRING lName(Name) → STRING © Pearson Education Limited 1995, 2005

13. 13 FDM – Relationships u Functions with arguments also model relationships between entity types. u Thus, FDM makes no distinction between attributes and relationships. u Each relationship may have an inverse relationship defined. u For example: Manages(Staff) —» PropertyForRent ManagedBy(PropertyForRent) → Staff INVERSE OF Manages © Pearson Education Limited 1995, 2005

14. 14 FDM – Relationships u Can also model *:* relationships: –Views(Client) —» PropertyForRent –ViewedBy(PropertyForRent) —» Client INVERSE OF Views u and attributes on relationships: –viewDate(Client, PropertyForRent) → DATE © Pearson Education Limited 1995, 2005

15. 15 FDM – Inheritance and Path Expressions u Inheritance supported through entity types. u Principle of substitutability also supported. Staff()→ ENTITY Supervisor()→ ENTITY IS-A-STAFF(Supervisor) → Staff u Derived functions can be defined from composition of multiple functions (note overloading): fName(Staff) → fName(Name(Staff)) fName(Supervisor) → fName(IS-A-STAFF(Supervisor)) u Composition is a path expression (cf. dot notation): Supervisor.IS-A-STAFF.Name.fname © Pearson Education Limited 1995, 2005

16. 16 FDM – Declaration of FDM Schema © Pearson Education Limited 1995, 2005

17. 17 FDM – Diagrammatic Representation of Schema © Pearson Education Limited 1995, 2005

18. 18 FDM – Functional Query Languages u Path expressions also used within a functional query. u For example: RETRIEVE lName(Name(ViewedBy(Manages(Staff)))) WHERE staffNo(Staff) = ‘SG14’ u or in dot notation: RETRIEVE Staff.Manages.ViewedBy.Name.lName WHERE Staff.staffNo = ‘SG14’ © Pearson Education Limited 1995, 2005

19. 19 FDM – Advantages u Support for some object-oriented concepts. u Support for referential integrity. u Irreducibility. u Easy extensibility. u Suitability for schema integration. u Declarative query language. © Pearson Education Limited 1995, 2005

20. 20 Persistent Programming Languages (PPLs) Language that provides users with ability to (transparently) preserve data across successive executions of a program, and even allows such data to be used by many different programs. u In contrast, database programming language (e.g. SQL) differs by its incorporation of features beyond persistence, such as transaction management, concurrency control, and recovery. © Pearson Education Limited 1995, 2005

21. 21 Persistent Programming Languages (PPLs) u PPLs eliminate impedance mismatch by extending programming language with database capabilities. –In PPL, language’s type system provides data model, containing rich structuring mechanisms. u In some PPLs procedures are ‘first class’ objects and are treated like any other object in language. –Procedures are assignable, may be result of expressions, other procedures or blocks, and may be elements of constructor types. –Procedures can be used to implement ADTs. © Pearson Education Limited 1995, 2005

22. 22 Persistent Programming Languages (PPLs) u PPL also maintains same data representation in memory as in persistent store. –Overcomes difficulty and overhead of mapping between the two representations. u Addition of (transparent) persistence into a PPL is important enhancement to IDE, and integration of two paradigms provides more functionality and semantics. © Pearson Education Limited 1995, 2005

23. 23 OODBMS Manifesto u Complex objects must be supported. u Object identity must be supported. u Encapsulation must be supported. u Types or Classes must be supported. u Types or Classes must be able to inherit from their ancestors. u Dynamic binding must be supported. u The DML must be computationally complete. © Pearson Education Limited 1995, 2005

24. 24 OODBMS Manifesto u The set of data types must be extensible. u Data persistence must be provided. u The DBMS must be capable of managing very large databases. u The DBMS must support concurrent users. u DBMS must be able to recover from hardware/software failures. u DBMS must provide a simple way of querying data. © Pearson Education Limited 1995, 2005

25. 25 OODBMS Manifesto u The manifesto proposes the following optional features: –Multiple inheritance, type checking and type inferencing, distribution across a network, design transactions and versions. u No direct mention of support for security, integrity, views or even a declarative query language. © Pearson Education Limited 1995, 2005

26. 26 Alternative Strategies for Developing an OODBMS u Extend existing object-oriented programming language. –GemStone extended Smalltalk. u Provide extensible OODBMS library. –Approach taken by Ontos, Versant, and ObjectStore. u Embed OODB language constructs in a conventional host language. –Approach taken by O 2,which has extensions for C. © Pearson Education Limited 1995, 2005

27. 27 Alternative Strategies for Developing an OODBMS u Extend existing database language with object- oriented capabilities. –Approach being pursued by RDBMS and OODBMS vendors. –Ontos and Versant provide a version of OSQL. u Develop a novel database data model/language. © Pearson Education Limited 1995, 2005

28. 28 Single-Level v. Two-Level Storage Model u Traditional programming languages lack built-in support for many database features. u Increasing number of applications now require functionality from both database systems and programming languages. u Such applications need to store and retrieve large amounts of shared, structured data. © Pearson Education Limited 1995, 2005

29. 29 Single-Level v. Two-Level Storage Model u With a traditional DBMS, programmer has to: –Decide when to read and update objects. –Write code to translate between application’s object model and the data model of the DBMS. –Perform additional type-checking when object is read back from database, to guarantee object will conform to its original type. © Pearson Education Limited 1995, 2005

30. 30 Single-Level v. Two-Level Storage Model u Difficulties occur because conventional DBMSs have two-level storage model: storage model in memory, and database storage model on disk. u In contrast, OODBMS gives illusion of single- level storage model, with similar representation in both memory and in database stored on disk. –Requires clever management of representation of objects in memory and on disk (called “pointer swizzling”). © Pearson Education Limited 1995, 2005

31. 31 Two-Level Storage Model for RDBMS © Pearson Education Limited 1995, 2005

32. 32 Single-Level Storage Model for OODBMS © Pearson Education Limited 1995, 2005

33. 33 Pointer Swizzling Techniques The action of converting object identifiers (OIDs) to main memory pointers. u Aim is to optimize access to objects. u Should be able to locate any referenced objects on secondary storage using their OIDs. u Once objects have been read into cache, want to record that objects are now in memory to prevent them from being retrieved again. © Pearson Education Limited 1995, 2005

34. 34 Pointer Swizzling Techniques u Could hold lookup table that maps OIDs to memory pointers (e.g. using hashing). u Pointer swizzling attempts to provide a more efficient strategy by storing memory pointers in the place of referenced OIDs, and vice versa when the object is written back to disk. © Pearson Education Limited 1995, 2005

35. 35 No Swizzling u Easiest implementation is not to do any swizzling. u Objects faulted into memory, and handle passed to application containing object’s OID. u OID is used every time the object is accessed. u System must maintain some type of lookup table - Resident Object Table (ROT) - so that object’s virtual memory pointer can be located and then used to access object. u Inefficient if same objects are accessed repeatedly. u Acceptable if objects only accessed once. © Pearson Education Limited 1995, 2005

36. 36 Resident Object Table (ROT) © Pearson Education Limited 1995, 2005

37. 37 Object Referencing u Need to distinguish between resident and non- resident objects. u Most techniques variations of edge marking or node marking. u Edge marking marks every object pointer with a tag bit: –if bit set, reference is to memory pointer; –else, still pointing to OID and needs to be swizzled when object it refers to is faulted into. © Pearson Education Limited 1995, 2005

38. 38 Object Referencing u Node marking requires that all object references are immediately converted to virtual memory pointers when object is faulted into memory. u First approach is software-based technique but second can be implemented using software or hardware-based techniques. © Pearson Education Limited 1995, 2005

39. 39 Hardware-Based Schemes u Use virtual memory access protection violations to detect accesses of non-resident objects. u Use standard virtual memory hardware to trigger transfer of persistent data from disk to memory. u Once page has been faulted in, objects are accessed via normal virtual memory pointers and no further object residency checking is required. u Avoids overhead of residency checks incurred by software approaches. © Pearson Education Limited 1995, 2005

40. 40 Pointer Swizzling - Other Issues u Three other issues that affect swizzling techniques: –Copy versus In-Place Swizzling. –Eager versus Lazy Swizzling. –Direct versus Indirect Swizzling. © Pearson Education Limited 1995, 2005

41. 41 Copy versus In-Place Swizzling u When faulting objects in, data can either be copied into application’s local object cache or accessed in-place within object manager’s database cache. u Copy swizzling may be more efficient as, in the worst case, only modified objects have to be swizzled back to their OIDs. u In-place may have to unswizzle entire page of objects if one object on page is modified. © Pearson Education Limited 1995, 2005

42. 42 Eager versus Lazy Swizzling u Moss defines eager swizzling as swizzling all OIDs for persistent objects on all data pages used by application, before any object can be accessed. u More relaxed definition restricts swizzling to all persistent OIDs within object the application wishes to access. u Lazy swizzling only swizzles pointers as they are accessed or discovered. © Pearson Education Limited 1995, 2005

43. 43 Direct versus Indirect Swizzling u Only an issue when swizzled pointer can refer to object that is no longer in virtual memory. u With direct swizzling, virtual memory pointer of referenced object is placed directly in swizzled pointer. u With indirect swizzling, virtual memory pointer is placed in an intermediate object, which acts as a placeholder for the actual object. –Allows objects to be uncached without requiring swizzled pointers to be unswizzled. © Pearson Education Limited 1995, 2005

44. 44 Accessing an Object with a RDBMS © Pearson Education Limited 1995, 2005

45. 45 Accessing an Object with an OODBMS © Pearson Education Limited 1995, 2005

46. 46 Persistent Schemes u Consider three persistent schemes: –Checkpointing. –Serialization. –Explicit Paging. u Note, persistence can also be applied to (object) code and to the program execution state. © Pearson Education Limited 1995, 2005

47. 47 Checkpointing u Copy all or part of program’s address space to secondary storage. u If complete address space saved, program can restart from checkpoint. u In other cases, only program’s heap saved. u Two main drawbacks: –Can only be used by program that created it. –May contain large amount of data that is of no use in subsequent executions. © Pearson Education Limited 1995, 2005

48. 48 Serialization u Copy closure of a data structure to disk. u Write on a data value may involve traversal of graph of objects reachable from the value, and writing of flattened version of structure to disk. u Reading back flattened data structure produces new copy of original data structure. u Sometimes called serialization, pickling, or in a distributed computing context, marshaling. © Pearson Education Limited 1995, 2005

49. 49 Serialization u Two inherent problems: –Does not preserve object identity. –Not incremental, so saving small changes to a large data structure is not efficient. © Pearson Education Limited 1995, 2005

50. 50 Explicit Paging u Explicitly ‘page’ objects between application heap and persistent store. u Usually requires conversion of object pointers from disk-based scheme to memory-based scheme. u Two common methods for creating/updating persistent objects: –Reachability-based. –Allocation-based. © Pearson Education Limited 1995, 2005

51. 51 Explicit Paging - Reachability-Based Persistence u Object will persist if it is reachable from a persistent root object. u Programmer does not need to decide at object creation time whether object should be persistent. u Object can become persistent by adding it to the reachability tree. u Maps well onto language that contains garbage collection mechanism (e.g. Smalltalk or Java). © Pearson Education Limited 1995, 2005

52. 52 Explicit Paging - Allocation-Based Persistence u Object only made persistent if it is explicitly declared as such within the application program. u Can be achieved in several ways: –By class. –By explicit call. © Pearson Education Limited 1995, 2005

53. 53 Explicit Paging - Allocation-Based Persistence u By class –Class is statically declared to be persistent and all instances made persistent when they are created. –Class may be subclass of system-supplied persistent class. u By explicit call –Object may be specified as persistent when it is created or dynamically at runtime. © Pearson Education Limited 1995, 2005

54. 54 Orthogonal Persistence u Three fundamental principles: –Persistence independence. –Data type orthogonality. –Transitive persistence (originally referred to as ‘persistence identification’ but ODMG term ‘transitive persistence’ used here). © Pearson Education Limited 1995, 2005

55. 55 Persistence Independence u Persistence of object independent of how program manipulates that object. u Conversely, code fragment independent of persistence of data it manipulates. u Should be possible to call function with its parameters sometimes objects with long term persistence and sometimes only transient. u Programmer does not need to control movement of data between long-term and short-term storage. © Pearson Education Limited 1995, 2005

56. 56 Data Type Orthogonality u All data objects should be allowed full range of persistence irrespective of their type. u No special cases where object is not allowed to be long-lived or is not allowed to be transient. u In some PPLs, persistence is quality attributable to only subset of language data types. © Pearson Education Limited 1995, 2005

57. 57 Transitive Persistence u Choice of how to identify and provide persistent objects at language level is independent of the choice of data types in the language. u Technique that is now widely used for identification is reachability-based. © Pearson Education Limited 1995, 2005

58. 58 Orthogonal Persistence - Advantages u Improved programmer productivity from simpler semantics. u Improved maintenance. u Consistent protection mechanisms over whole environment. u Support for incremental evolution. u Automatic referential integrity. © Pearson Education Limited 1995, 2005

59. 59 Orthogonal Persistence - Disadvantages u Some runtime expense in a system where every pointer reference might be addressing persistent object. –System required to test if object must be loaded in from disk-resident database. u Although orthogonal persistence promotes transparency, system with support for sharing among concurrent processes cannot be fully transparent. © Pearson Education Limited 1995, 2005

60. 60 Versions Allows changes to properties of objects to be managed so that object references always point to correct object version. u Itasca identifies 3 types of versions: –Transient Versions. –Working Versions. –Released Versions. © Pearson Education Limited 1995, 2005

61. 61 Versions and Configurations © Pearson Education Limited 1995, 2005

62. 62 Versions and Configurations © Pearson Education Limited 1995, 2005

63. 63 Schema Evolution u Some applications require considerable flexibility in dynamically defining and modifying database schema. u Typical schema changes: (1) Changes to class definition: (a) Modifying Attributes. (b) Modifying Methods. © Pearson Education Limited 1995, 2005

64. 64 Schema Evolution (2) Changes to inheritance hierarchy: (a) Making a class S superclass of a class C. (b) Removing S from list of superclasses of C. (c) Modifying order of superclasses of C. (3) Changes to set of classes, such as creating and deleting classes and modifying class names. u Changes must not leave schema inconsistent. © Pearson Education Limited 1995, 2005

65. 65 Schema Consistency 1.Resolution of conflicts caused by multiple inheritance and redefinition of attributes and methods in a subclass. 1.1Rule of precedence of subclasses over superclasses. 1.2Rule of precedence between superclasses of a different origin. 1.3Rule of precedence between superclasses of the same origin. © Pearson Education Limited 1995, 2005

66. 66 Schema Consistency 2.Propagation of modifications to subclasses. 2.1Rule for propagation of modifications. 2.2Rule for propagation of modifications in the event of conflicts. 2.3Rule for modification of domains. © Pearson Education Limited 1995, 2005

67. 67 Schema Consistency 3.Aggregation and deletion of inheritance relationships between classes and creation and removal of classes. 3.1Rule for inserting superclasses. 3.2Rule for removing superclasses. 3.3Rule for inserting a class into a schema. 3.4Rule for removing a class from a schema. © Pearson Education Limited 1995, 2005

68. 68 Schema Consistency © Pearson Education Limited 1995, 2005

69. 69 Client-Server Architecture u Three basic architectures: –Object Server. –Page Server. –Database Server. © Pearson Education Limited 1995, 2005

70. 70 Object Server u Distribute processing between the two components. u Typically, client is responsible for transaction management and interfacing to programming language. u Server responsible for other DBMS functions. u Best for cooperative, object-to-object processing in an open, distributed environment. © Pearson Education Limited 1995, 2005

71. 71 Page and Database Server Page Server u Most database processing is performed by client. u Server responsible for secondary storage and providing pages at client’s request. Database Server u Most database processing performed by server. u Client simply passes requests to server, receives results and passes them to application. u Approach taken by many RDBMSs. © Pearson Education Limited 1995, 2005

72. 72 Client-Server Architecture © Pearson Education Limited 1995, 2005

73. 73 Architecture - Storing and Executing Methods u Two approaches: – Store methods in external files. – Store methods in database. u Benefits of latter approach: –Eliminates redundant code. –Simplifies modifications. © Pearson Education Limited 1995, 2005

74. 74 Architecture - Storing and Executing Methods –Methods are more secure. –Methods can be shared concurrently. –Improved integrity. u Obviously, more difficult to implement. © Pearson Education Limited 1995, 2005

75. 75 Architecture - Storing and Executing Methods © Pearson Education Limited 1995, 2005

76. 76 Benchmarking - Wisconsin benchmark u Developed to allow comparison of particular DBMS features. u Consists of set of tests as a single user covering: –updates/deletes involving key and non-key attributes; –projections involving different degrees of duplication in the attributes and selections with different selectivities on indexed, non-index, and clustered attributes; –joins with different selectivities; –aggregate functions. © Pearson Education Limited 1995, 2005

77. 77 Benchmarking - Wisconsin benchmark u Original benchmark had 3 relations: one called Onektup with 1000 tuples, and two others called Tenktup1/Tenktup2 with 10000 tuples. u Generally useful although does not cater for highly skewed attribute distributions and join queries used are relatively simplistic. u Consortium of manufacturers formed Transaction Processing Council (TPC) in 1988 to create series of transaction-based test suites to measure database/TP environments. © Pearson Education Limited 1995, 2005

78. 78 TPC Benchmarks u TPC-A and TPC-B for OLTP (now obsolete). u TPC-C replaced TPC-A/B and based on order entry application. u TPC-H for ad hoc, decision support environments. u TPC-R for business reporting within decision support environments. u TPC-W, a transactional Web benchmark for eCommerce. © Pearson Education Limited 1995, 2005

79. 79 Object Operations Version 1 (OO1) Benchmark u Intended as generic measure of OODBMS performance. Designed to reproduce operations common in advanced engineering applications, such as finding all parts connected to a random part, all parts connected to one of those parts, and so on, to a depth of seven levels. u About 1990, benchmark was run on GemStone, Ontos, ObjectStore, Objectivity/DB, and Versant, and INGRES and Sybase. Results showed an average 30-fold performance improvement for OODBMSs over RDBMSs. © Pearson Education Limited 1995, 2005

80. 80 OO7 Benchmark u More comprehensive set of tests and a more complex database based on parts hierarchy. u Designed for detailed comparisons of OODBMS products. u Simulates CAD/CAM environment and tests system performance in area of object-to-object navigation over cached data, disk-resident data, and both sparse and dense traversals. u Also tests indexed and nonindexed updates of objects, repeated updates, and the creation and deletion of objects. © Pearson Education Limited 1995, 2005

81. 81 Advantages of OODBMSs u Enriched Modeling Capabilities. u Extensibility. u Removal of Impedance Mismatch. u More Expressive Query Language. u Support for Schema Evolution. u Support for Long Duration Transactions. u Applicability to Advanced Database Applications. u Improved Performance. © Pearson Education Limited 1995, 2005

82. 82 Disadvantages of OODBMSs u Lack of Universal Data Model. u Lack of Experience. u Lack of Standards. u Query Optimization compromises Encapsulation. u Object Level Locking may impact Performance. u Complexity. u Lack of Support for Views. u Lack of Support for Security. © Pearson Education Limited 1995, 2005