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Lecture 15

Lecture 15. Object oriented databases. Object Definition Language (ODL). module DreamHome Class Branch (extent branchOffices key branchNo) { attribute string branchNo; …. relationship Manager ManagedBy inverse Manager::Manages; void takeOnPropertyForRent(in string propertyNo)

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Lecture 15

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  1. Lecture 15 Object oriented databases

  2. Object Definition Language (ODL) module DreamHome Class Branch (extent branchOffices key branchNo) { attribute string branchNo; …. relationship Manager ManagedBy inverse Manager::Manages; void takeOnPropertyForRent(in string propertyNo) raises(propertyAlreadyForRent); }

  3. Object Definition Language (ODL) class Person { attribute struct Pname {string fName, string lName} name; } Class Staff extends Person (extent staff key staffNo) { attribute staffNo; attribute date DOB; …. short getAge(); }

  4. Object Definition Language (ODL) class Manager extends Staff (extent managers) { relationship Branch Manages inverse Branch::ManagedBy; }

  5. Object Query Language (OQL) • Provides declarative access to object database using SQL-like syntax. • Does not provide explicit update operators - leaves this to operations defined on object types. • Can be used as a standalone language and as a language embedded in another language, for which an ODMG binding is defined (Smalltalk, C++, and Java). • OQL can also invoke operations programmed in these languages.

  6. Object Query Language (OQL) • Can be used for associative and navigational access: • Associative query returns collection of objects. How these objects are located is responsibility of ODMS. • Navigational query accesses individual objects and object relationships used to navigate from one object to another. Responsibility of application program to specify procedure for accessing the objects.

  7. Object Query Language (OQL) • An OQL query is a function that delivers an object whose type may be inferred from operator contributing to query expression. • Query definition expressions is of form: DEFINE Q as e • Defines query with name Q given query expression e.

  8. Object Query Language (OQL) • Expression can take several forms: • Elementary - Construction • Atomic type - Object • Collection - Indexed collections • Binary set - Conversion • Query consists of a (possibly empty) set of query definition expressions followed by an expression. • Result is object with or without identity.

  9. Example 28.2 OQL: Extents & Traversal Paths Get set of all staff (with identity) staff Get set of all branch managers (with identity) branchOffices.ManagedBy

  10. Example 28.2 OQL: Extents & Traversal Paths Find all branches in London SELECT b.branchNo FROM b IN branchOffices WHERE b.address.city = “London”; This returns a literal of type bag<string>.

  11. Example 28.2 OQL: Extents & Traversal Paths Assume londonBranches is named object (from last query). Find all staff who work at that branch. londonBranches.Has This returns set<SalesStaff>.

  12. Example 28.2 OQL: Extents & Traversal Paths Because of ambiguity over return result, cannot access sales staff salaries using: londonBranches.Has.salary Result may be set<float> or bag<float>. Instead use: SELECT [DISTINCT] s.salary FROM s IN londonBranches.Has;

  13. Example 28.3 - OQL: Use of DEFINE Get set of all staff who work in London (without identity). DEFINE Londoners AS SELECT s FROM s IN salesStaff WHERE s.WorksAt.address.city = “London”; SELECT s.name.lName FROM s IN Londoners; This returns a literal of type set<string>.

  14. Example 28.3 - OQL: Use of DEFINE Can generalize this: DEFINE CityWorker(cityname) AS SELECT s FROM s IN salesStaff WHERE s.WorksAt.address.city = cityname; CityWorker(“London”); CityWorker(“Glasgow”);

  15. Example 28.4 OQL: Use of structures Get structured set (without identity) containing name, sex, and age of all staff who live in London. SELECT struct (lName:s.name.lName, sex:s.sex, age:s.age) FROM s IN Staff WHERE s.WorksAt.address.city = “London” This returns a literal of type set<struct>.

  16. Example 28.4 OQL: Use of structures Get structured set (with identity) with name, sex, and age of all deputy managers over 60: class Deputy {attribute string lName; attribute sexType sex; attribute integer age;}; Typedef bag<Deputy>Deputies; Deputies (SELECT Deputy (lName:s.name.lName, sex:s.sex, age:x.age) FROM s IN salesStaff WHERE position = “Deputy” AND s.getAge > 60) This returns a mutable object of type deputies.

  17. Example 28.4 OQL: Use of structures Get structured set (without identity) containing branch number and set of all Assistants at branches in London. SELECT struct (branchNo:x.branchNo, assistants: (SELECT y FROM y IN x.WorksAt WHERE y.position = “Assistant”)) FROM x IN (SELECT b FROM b IN branchOffices WHERE b.address.city = “London”) This returns a literal of type set<struct>.

  18. Example 28.5 OQL: Use of aggregates How many staff work in Glasgow. COUNT (s IN CityWorker(“Glasgow”); • OQL aggregate can be applied within SELECT or to result of SELECT. • Following equivalent: SELECT COUNT(s) FROM s IN salesStaff WHERE s.WorksAt.branchNo = “B003”; COUNT(SELECT s FROM s IN salesStaff WHERE s.WorksAt.branchNo = “B003”);

  19. Example 28.6 OQL: GROUP BY Determine number of sales staff at each branch. SELECT struct(branchNumber, numberOfStaff:COUNT(partition)) FROM s IN salesStaff GROUP BY branchNumber: s.WorksAt.branchNo; Result is of type: set<struct(branchNumber: string, partition: bag<struct(s:SalesStaff)>)> Note use of keyword partition to refer to each partition.

  20. OQL - Creating Objects A type name constructor is used to create an object with identity. Manager(staffNo: “SL21”, fName: “John”, lName: “White”, address: “19 Taylor St, London”, position: “Manager”, sex: “M”, DOB: date“1945-10-01”, salary: 30000)

  21. Language Bindings • Specify how ODL/OML constructs are mapped to programming language constructs. • Basic design principle is that programmer should think there is only one language being used. • C++ class library provided containing classes and functions that implement ODL constructs. Also, OML is used to specify how database objects are retrieved and manipulated within application program.

  22. Language Bindings - Creating a Working Application

  23. Language Bindings • Features that implement interface are prefixed d_ (e.g. d_Float, d_String, d_List, d_Set, and d_Bag). • Also class d_Iterator a class d_Extent. • Template class d_Ref(T) defined for each class T in database schema that can refer to both persistent and transient objects of class T.

  24. Language Bindings • Relationships handled by including either a reference (for 1:1) or a collection (for 1:*). For example, to represent 1:* Has relationship in Branch class: d_Rel_Set<SalesStaff, _WorksAt> Has; const char _WorksAt[] = “WorksAt”; • and to represent same relationship in SalesStaff class: d_Rel_Ref<Branch, _Has> WorksAt; const char _Has[] = “Has”;

  25. Language Bindings - OML • new operator overloaded so that it can create persistent or transient objects. • To create a persistent object, a database name and a name for the object must be provided. For example, to create a transient object: d_Ref<SalesStaff> tempSalesStaff = newSalesStaff; • and to create a persistent object: d_Database *myDB; d_Ref<SalesStaff> s1 = new(myDb, “John White”) SalesStaff;

  26. Language Bindings - OQL • OQL queries can be executed from within C++ in one of following ways: • using query member function of the d_Collection class; • using d_OQL_Query interface. d_Bag<d_Ref<SalesStaff>> wellPaidStaff; SaleStaff->query(wellPaidStaff, “salary > 30000”); d_OQL_Query q(“SELECT s.WorksAt FROM s IN SalesStaff WHERE salary > $1”); d_Bag<d_Ref<Branch>> branches; q << 30000; d_oql_execute(q, branches);

  27. Mapping Conceptual Design to Logical OO Design • Step 1 Mapping classes • Map each class or subclass to an ODL class, including all appropriate attributes and methods. • Map composite attributes to a tuple constructor using a struct declaration. • Map any multivalued attributes as follows: • if values are ordered, map to a list constructor; • if values contain duplicates, map to a bag constructor; • otherwise, map to a set constructor. • Create an extent for each class that will be iterated over. Specify EXTENDS for each ODL class that represents a subclass to inherit attributes and methods of superclass.

  28. Mapping Conceptual Design to Logical OO Design • Step 2 Mapping binary relationships • Add a relationship property (or reference attribute) into each class that participates in relationship. • If supported, use inverse relationships where possible to ensure system automatically maintains RI; otherwise program this functionality into class methods. • If 1:1, each relationship property will be single-valued. • If 1:*, relationship property will be single-valued on one side and collection type (list or set) on the other. • If *:*, each side will be a collection type. • Create tuple constructor (struct) for relationships attributes of form <relationship reference, relationship attributes>.

  29. Mapping Conceptual Design to Logical OO Design • Step 3 Mapping n-ary relationships • For each relationship with degree greater than 2, create separate class to represent relationship and include relationship property (based on a 1:* relationship) to each participating class. • Step 4 Mapping categories • For each category (union type), create class to represent category and define a 1:1 relationship between category class and each of its superclasses. • Alternatively, a union type can be used if OODBMS supports it.

  30. ObjectStore - Architecture • Based on multi-client/multi-server architecture, with each server responsible for controlling access to an object store and for managing concurrency control (locking-based), data recovery, and transaction log, among others. • A client can contact ObjectStore server on its host or any other ObjectStore server on any other host in network.

  31. ObjectStore - Architecture • For each host machine running one or more clients there is an associated cache manager process to facilitate concurrent access to data by handling callback messages from server to clients. • Also, each client has its own client cache, which acts as a holding area for data mapped (or waiting to be mapped) into physical memory.

  32. ObjectStore Architecture

  33. ObjectStore Server • Responsible for: • storage and retrieval of persistent data; • handling concurrent access by multiple client applications; • database recovery.

  34. Client Application • Objectstore client library is linked into each client application, allowing it to: • map persistent objects to virtual addresses; • allocate and deallocate storage for persistent objects; • maintain a cache of recently used pages and the lock status of those pages; • handle page faults on addresses that refer to persistent objects.

  35. Client Cache • Exists to improve access to persistent objects. • When client application needs to access a persistent object, then a page fault is generated when: • object is not in physical memory and not in client cache; • object is in client cache but has not yet been accessed; • object is in client cache but has been previously accessed with different read/write permissions.

  36. Virtual Memory Mapping Architecture (VMMA) • C++ object is stored in database in its native format with all pointers intact (unswizzled). • Basic idea of VMMA is same as for virtual memory management in operating systems. • References to objects are realized by virtual memory addresses. If object has to be dereferenced and the page object resides on is in memory, there is no extra overhead in dereferencing this object (dereferencing is as fast as for any C/C++ program).

  37. Virtual Memory Mapping Architecture (VMMA) • If required page not in memory, page fault occurs and page is brought into same virtual memory address it originally occupied. • Thus, pointers to this object in other transferred objects are valid virtual memory pointers referring to their original target. • Unmapped range of virtual memory reserved for persistent objects, thereby ensuring that this range will be used for no other purpose than database pages.

  38. Building an ObjectStore Application

  39. ObjectStore Databases • ObjectStore supports two types of databases: • file database, a native operating system file that contains an ObjectStore database; • rawfs (raw file system) database, a private file system managed by the ObjectStore server, independent of file system managed by operating system. • Database divided into clusters and segments. A cluster is basic unit of storage allocation. When a persistent object is created storage is allocated from a cluster. Clusters are divided into segments.

  40. Data Definition in ObjectStore • Can handle persistence for objects created in C/C++ and Java through separate class libraries, and objects created in one language can be accessed in other. • For C++, ObjectStore uses C++ as a schema language so that everything in database must be defined by C++ class. • Persistence is orthogonal to type and persistent object support achieved through overloading new operator. branchNo = new(DHomeDB, os_typespec::get_char(), 4) char[4];

  41. Data Definition in ObjectStore • Also a version of C++ deleteoperator to delete persistent objects and free persistent memory. • Once persistent memory has been allocated, pointers to this memory can be used in same way as pointers to virtual memory.

  42. Creating Relationships in ObjectStore • Relationship between Branch and SalesStaff handled by declaring two data members that are inverses of each other. • RI automatically maintained. • Macros provided for defining relationships: os_relationship_1_1; os_relationship_m_m; os_relationship_1_m; os_relationship_m_1. os_relationship_1_m(SalesStaff, WorksAt, Branch, Has, Branch*) WorksAt; os_relationship_m_1(Branch, Has, SalesStaff, WorksAt, os_Set<SalesStaff*>) Has;

  43. Data Manipulation in ObjectStore • Following operations must be performed before persistent memory can be accessed: • a database must be created or opened; • a transaction must be started; • a database root must be retrieved or created.

  44. Roots and Entry Point Objects • Database root provides way to give an object a persistent name, thereby allowing object to serve as entry point into the database. • From there, any object related to it can be retrieved using navigation (i.e., following data member pointers) or by a query (i.e., selecting all elements of a given collection that satisfy a specified predicate).

  45. Roots and Entry Point Objects

  46. Access Based on a Named Root aBranch = (Branch*)(db1->find_root(“Branch3_Root”) ->get_value(WorksAtType); cout << “Retrieval of branch B003 root: ” << aBranch->branchNo << “\n”;

  47. Iteration of Collections using Cursors os_Cursor<SalesStaff*> c(aBranch->Has); cout << “Staff associated with B003: \n” for (p = c.first(); c.more(); p = c.next()) cout << p->staffNo << “\n”;

  48. Lookup of Single Object Based on Value of one or more Data Members salesStaffExtent = (os_Set<SalesStaff*>*) (db1->find_root(“salesStaffExtent_Root”) ->get_value(salesStaffExtentType); aSalesPerson = salesStaffExtent ->query_pick(“SalesStaff*”,“!strcmp(staffNo,\“SG37\”)”, db1); cout << “Retrieval of specific member of sales staff: ” << aSalesPerson.staffNo << “\n”;

  49. Lookup of Single Object Based on Value of one or more Data Members os_Set<SalesStaff*> &highlyPaidStaff = salesStaffExtent->query(“SalesStaff*”, “salary > 30000”, db1); cout << “Retrieval of highly paid staff: \n”; os_Cursor<SalesStaff*> c(highlyPaidStaff); for (p = c.first(); c.more(); p = c.next()) cout << p->staffNo << “\n”;

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