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L08 Data Source Layer
 

L08 Data Source Layer

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    L08 Data Source Layer L08 Data Source Layer Presentation Transcript

    • Lecture 08 Data Source Layer
    • Lectures L01 Enterprise Application Architecture Introduction, 1 L02 Software Design L03 Design Patterns Introduction L04 Base Patterns 18 L05 Frameworks L06 Process Design L07 Organizing the Domain Layer 2, 9
    • Lectures L08 Mapping to Relational Databases 3, 10 L09 Behavioral Design 3, 11 L10 Web Presentation 4, 14 L11 Putting it all together 8 L12 Concurrent Programming 5, 16 L13 Session State and Distribution Strategies 6, 7, 15, 16 L14 Summary and Conclusions
    • Lecture 08 Data Source Layer
    • Reading  Fowler 3 Mapping to Relational Database  Fowler 10 Data Source Architectural Patterns – Table Data Gateway (144) – Row Data Gateway (152) – Active Record (160) – Data Mapper (165)  Fowler 15 Distribution Patterns – Data Transfer Object (401)  Fowler 18 Base Pattern – Record set (508)
    • Agenda  Design Objectives  Data Source Patterns – Data Transfer Object (401) – Row Data Gateway (152) – Table Data Gateway (144) – Active Record (160) – Data Mapper (165) – Record set (508)
    • Design Objectives
    • Relational Databases  We are dealing with relational databases – Wide-spread and well understood – SQL based – These are important in enterprise software  Alternatives – NoSQL databases – O/R Mappers
    • Connecting to Data Sources  Programs need to interface the Data Source – Usually this means relational databases  Database vendors usually supply drivers for database  Rational databases use SQL language – Fairly standard Program Database classes Driver Database
    • Objectives  Hide SQL from the Domain Layer  Access to database needs to ensure – Speed and data integrity – Concurrent access of many clients  Database independence – It can be an objective to keep the system independent of particular database technology  Data Source Layer needs to be maintainable – Database will change
    • Impedance Mismatch
    • The Legacy Problem  In most cases the data model exists – The schema already exists – We cannot assume that we create the schema  Data tends to stick where it lends – Cannot assume that our application controls the schema – The schema will likely outlive the application
    • The Usability Problem  The Database API determines the usability of the data access – Should be easy to use  The programming model is important – Does matter how efficient and good a persistence framework is, if it is complex and cumbersome to use  Tools may help, but should not be used to conceal excessive complexity – If tools are required to generate data access the programming model is likely to be complex
    • Using Databases  Programmers tend to want to solve all problems in their domain – Should we solve all problems in our object domain? – Should we write everything in Java or C#?  Databases are good at what they do – But it’s necessary to let them do it in a natural way
    • Database Code  Database programming can be very repetitive – Opportunities for reusability – JDBC is too low-level  Code is Bad! – Don’t write code unless you have to – Try to write code for the business layer  Persistence Frameworks are difficult to build – Use the frameworks that exist
    • Which of these statements is false A) Data tends to stick where it lends B) Database programming tends to be low-level C) Objects tend to map nicely to the database D) Database programming tends to be repetitive QUIZ ✔
    • Data Source Patterns
    • Domain Layer Patterns Recap  Transaction Script – Organizes business logic by procedures where each procedure handles a single request from the presentation  Domain Model – An object model of the domain that incorporates both behaviour and data  Table Module – A single instance that handles the business logic for all rows in a database table or view
    • Good Design  Separate database code from other code – Provide database classes to access the database – All SQL code in the same place – Factories for each database – Use of Connection Pools  Error handling – SQLException is isolated in the Data Source Layer – Wrap in domain specific exceptions – use of runtime exceptions
    • Design Example  Domain Model uses gateways
    • Useful Patterns  Data Transfer Object (401) – An object that carries data between processes in order to deduce the number of method calls  Record Set (508) – An in-memory representation of tabular data
    • Data Transfer Object (401) An object that carries data between processes in order to deduce the number of method calls  Object that is used to transfer data between layers – Data Source returns data objects to web layer
    • Data Transfer Object (401)  How it Works – Similar to Value Object but is constructed to carry data between layers – Data source layer creates DTO for transfer – DTOs holds data – get/set method – Can be mutable or immutable – Could have methods to transform data – for example serialize the data or convert to XML – Simple Domain Objects can be used as DTO • Creates dependencies
    • Data Transfer Object (401)  Assembling DTO from domain objects – Assembler reduces dependencies  When To Use It – Whenever you need to transfer multiple items of data between two processes in a single method call
    • Record Set (508)  An in-memory representation of tabular data  How It Works – Contains the result of a database query – Common in ADO.NET and JDBC – One record is current, clients can traverse the set – Usually provided by the database code  When to Use It – When returning data from a query
    • Data Source Patterns  Table Data Gateway (144) – Acts as a Gateway to a database table  Row Data Gateway (152) – Acts as a Gateway to a single record in a data source  Active Record (160) – Wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data  Data Mapper (165) – A layer of Mappers that moves data between objects and database while keeping them independent
    • Pattern Overview
    • Data Source Layer  Domain layer has influence on the patterns  Transaction Script – Table Data Gateway for single access to a table – Row Data Gateway for single access to a row of a table  Domain Model – Active Record or Row Data Gateway – Data Mapper  Table Module – Table Data Gateway with Record Set
    • Data Source Patterns  Table Data Gateway (144) – Acts as a Gateway to a database table  Row Data Gateway (152) – Acts as a Gateway to a single record in a data source  Active Record (160) – Wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data  Data Mapper (165) – A layer of Mappers that moves data between objects and database while keeping them independent
    • Table Data Gateway (114) An object that acts as a Gateway (446) to a database table. One instance handles all the rows in the table.  Also called Data Access Objects – DAO  How It Works – Simple interface to a table with several find methods and methods for maintaining data – CRUD methods (Create, Read, Update, Delete) – Acts as a gateway to a table – One gateway for each table – Finders return Collection of DTOs or Record Set
    • Table Data Gateway (114)  When to Use It – Works for Table Module (125) since it is based on Record Set (509) – Useful for web application where Domain Model (116) is used
    • Row Data Gateway (152) An object that acts as a Gateway (446) to a single record in a data source. There is only one instance per row.  How It Works – Object that is exactly one single record – Each table column is a field in the object – Do not have logic – Finder object – Can be generated
    • Row Data Gateway (152)  When to Use It – Works well for simple domain layer for example Transaction Script
    • Active Record (160) An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data  How It Works – Each object can read and store itself – Contain domain logic  When to Use It – When Domain Logic is not too complex – When using Transaction Script +insert() +update() +delete() +getExemption() +isFlaggedForAudit(in CompanyID) +getTaxableEarnings() -lastname -firstname -NumerOfDependents Person
    • Data Mapper (165) A layer of Mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself  Sparates the in-memory objects from the databse
    • Data Mapper  How It works – Simple Data Mappers map in-memory object to table on a field-to-field basis – Others need to map more complicated object hierarchies to multiple tables – Mapper uses Identity Map to see if object is already loaded  For insert and updates – The mapper must know what objects have changed, which are new, and which must be destroyed – Unit of Work pattern
    • Data Mapper  When to Use It – Database and object model must be independent – Data Mappers are useful with Domain Model – For simple Domain Model an Active Record could be used, but as it becomes more complicated some mapping is needed  O/R mapping solutions can provide the mappers – For example Hibernate
    • Data Mapper  Simple example – Loading from the database
    • Data Mapper  Simple example – Updating data – Client asks the mapper to save a domain object – The mapper pulls the data out of the domain object and saves to the database
    • Data Source class maps nicely to the rows in a table and contains some useful methods A) Row Data Gateway B) Table Data Gateway C) Active Record D) Data Mapper QUIZ ✔
    • Spring JDBC
    • Spring JDBC  Spring JDBC packages – org.springframework.jdbc  datasource – Classes for connecting to the database  core – Base classes for accessing the database – JdbcTemplate  object – Classes that support updating, inserting and deleting data from the database  support – Utility classes
    • Loading the DataSource  Code  Configuration – data.xml Resource res = new FileSystemResource("data.xml"); XmlBeanFactory factory = new XmlBeanFactory(res); DataSource ds = (DataSource)factory.getBean("dataSource"); <beans> <bean id="dataSource“ class= "org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="driverClassName"> <value>net.sourceforge.jtds.jdbc.Driver</value> </property> <property name="url”> <value>jdbc:jtds:sqlserver://honn.ru.is:1433</value> </property> <property name="username"><value>andri</value></property> <property name="password"><value>abc123</value></property> </bean> </beans>
    • JdbcTemplate  Main class of core package – Simplifies queries  Template Method pattern – JdbcTemplate handles the processing and calls our code – Dependency Injection
    • JdbcTemplate Example ParameterizedRowMapper<String> rm = new ParameterizedRowMapper<String>() { public String mapRow(ResultSet rs, int rowNum) throws SQLException { return rs.getString("title"); } }; JdbcTemplate tpl = new JdbcTemplate(getDataSource()); Collection<String> col = tpl.query("select * from contents", rm); for(String s : col) { System.out.println(s); } Kenya's elephants send text messages to rangers (AP) Flexible OLEDs could be part of lighting's future (AP)
    • Collecting Data  Spring Interface RowMapper – An interface used by JdbcTemplate for mapping returned result sets – Class that implements this interface can be used to collect data public interface ParameterizedRowMapper<T> extends RowMapper<T> { Object mapRow(ResultSet rs, int rowNum) throws SQLException; }
    • ContentRowMapper public class ContentRowMapper implements ParameterizedRowMapper<Content> { public Content mapRow(ResultSet rs, int rowNum) throws SQLException { Content content = new Content (rs.getInt (1), // id rs.getString (2), // title rs.getString (3), // link rs.getString (4), // description rs.getDate (5), // pubdate rs.getString(6)); // author return content; } }
    • Using ContentRowMapper  JdbcTemplate method query takes RowMapper interface as parameter ContentRowMapper crm = new ContentRowMapper(); JdbcTemplate tpl = new JdbcTemplate(ds); List l = tpl.query("select * from contents", crm); Iterator i = l.iterator(); Content cont; while (i.hasNext()) { cont = (Content) i.next(); System.out.println(cont); }
    • Insert  SimpleJdbcInsert – Class for inserts public int add(Content content) { SimpleJdbcInsert insertContent = new SimpleJdbcInsert(getDataSource()) .withTableName("contents") .usingGeneratedKeyColumns("id"); Map<String, Object> parameters = new HashMap<String, Object>(5); parameters.put("title", content.getTitle()); parameters.put("link", content.getLink()); parameters.put("description", content.getDescription()); parameters.put("pubdate", content.getPubDate()); parameters.put("author", content.getAuthor()); return insertContent.executeAndReturnKey(parameters).intValue(); }
    • RU Data Framework Content Example
    • Content Example  Table contents – Contains content information CREATE TABLE contents ( id int Identity (1, 1) primary key NOT NULL, title varchar(128), link varchar(512) unique, description text, pubDate datetime, author varchar(128), )
    • Content Example  Gateway class for the contents table – ContentDataGateway interface contains the CRUD operations – Class ContentData implements the gateway and provides the JDBC code – Content is simple JavaBean – acts as Data Transfer Object
    • RU Data Framework  Classes and interfaces for accessing the database – Implementation of the Data Gateway  For Table Data Gateway – Each table has an Gateway interface – Implementation in Data classes – Factory pattern returns the implementation for each Date Gateway
    • RuDataAccessFactory  Factory for creating Gateway interfaces – Reads information about the DataSource – Spring Bean Definition file: data.xml – Uses Spring Bean Factory – RuDataAccessFactory reads information on each gateway interface and which classes to use as implementation – Code using the gateway interface calls getDataAccess in the factory class factory = RuDataAccessFactory.getInstance("data.xml"); contentDataGateway = (ContentDataGateway) factory.getDataAccess("contentDataAccess");
    • Data Definition  File data.xml <beans> <!-- Data Source --> <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource" > <property name="driv erClassName"> <value>net.sourceforge.jtds.jdbc.Driver</value></property> <property name="url"> <value>jdbc:jtds:sqlserver://honn.ru.is:1433</value></property> <property name="username"><value>andri</value></property> <property name="password"><value>abc123</value></property> </bean> <!– Content Gateway Interface --> <bean id="contentGateway" class="is.ru.honn.tube.data.content.ContentData"/> </bean> </beans>
    • DataSource  DataSource is a connection to a database – Driver Class Name (net.sourceforge.jtds.jdbc.Driver) – URL (jdbc:jtds:sqlserver://honn.ru.is:1433) – username (andri) – password (abc123)  RU framework uses Spring – Load the DataSource information – DriverManagerDataSource extends AbstractDataSource which implements DataSource
    • DataSource in Ru Framework  RU Framework uses Spring to get DataSource
    • Using RU Framework
    • RuDataAccess  RuDataAccess is base interface for Data gateway interfaces – All gateway interfaces extend RuDataAccess – Has methods to set and get DataSource
    • RuData  RuData is a class implementing RuDataAccess – Handles DataSource – Data classes extend this class
    • ContentDataGateway  Contains all the method that are needed to manage contents – Gateway to the contents table – Pattern Table Data Gateway public interface ContentDataGateway extends RuDataAccess { public int add(Content content); public List<Content> getContents(); }
    • ContentData public class ContentData extends RuData implements ContentDataGateway { public int add(Content content) { SimpleJdbcInsert insertContent = new SimpleJdbcInsert(getDataSource()) .withTableName("contents") .usingGeneratedKeyColumns("id"); Map<String, Object> parameters = new HashMap<String, Object>(5); parameters.put("title", content.getTitle()); parameters.put("link", content.getLink()); parameters.put("description", content.getDescription()); parameters.put("pubdate", content.getPubDate()); parameters.put("author", content.getAuthor()); return insertContent.executeAndReturnKey(parameters).intValue(); }
    • ContentData public List getContents() { JdbcTemplate queryContent = new JdbcTemplate(getDataSource()); List<Content> contents = queryContent.query ("select * from contents", new ContentRowMapper()); return contents; }
    • Usage public class ContentServiceData implements ContentService { private ContentDataGateway contentDataGateway = null; public ContentServiceData() { RuDataAccessFactory factory = null; try { factory = RuDataAccessFactory.getInstance("data.xml"); } catch (RuException e) { ... } contentDataGateway = (ContentDataGateway) factory.getDataAccess("contentDataGateway"); } public void addContent(Content content) { contentDataGateway.add(content); } public List<Content> getContents() { return contentDataGateway.getContents(); } }
    • Contents  Use ContentServiceData instead of a Service Stub – Change one line in the app.xml file <bean id="contentService" class="is.ru.honn.tube.service.ContentServiceData"> </bean>
    • Contents  Use ContentServiceData instead of a Service Stub – Change one line in the app.xml file
    • Summary  Design Objectives – Object-relational impedance mismatch – The Usability Problem – The Legacy Problem  Patterns – Table Data Gateway (144) – Row Data Gateway (152) – Active Record (160) – Data Mapper (165) – Data Transfer Object (401) and Record set (508)