.NET Mobile Application Development Data in Distributed Systems Accessing Data with.NET

Introduction  In this session we will consider >The use of databases in mobile and distributed applications >Connected, client-server database access ­.NET connected data access features >Disconnected database architecture ­ADO.NET and disconnected data access features >Distributed data access from mobile and disconnected clients ­SQL Server CE

Database Terminology  Relational database >data and relations between them organised in tables. A table is collection of rows or records. Each row in a table contains the same fields.tablesrecordsfields  Query >formal request to a database for information or updates to informationdatabase  Stored procedure >set of SQL commands that can be stored in the server >gives improved performance as less information needs to be sent between the server and the client  Structured Query Language (SQL) >industry-standard language for creating, updating and, querying relational databases

Databases in Distributed Systems  Distributed applications often rely on databases for >persistent storage >synchronizing state and ensuring consistency between multiple clients  Databases >normally reside on a server >accessed by clients via the network  Clients interact with databases using explicit SQL queries >active connection to database required for each query

Distributed Data Management  Three key aspects of managing data in distributed applications >Caching ­keeping local copies of required data eliminates need for continuous network connections >Replication ­propagating changes in cached data back to main data store ­periodically refreshing cached data >Conflict Resolution ­multiple clients operate simultaneously ­updates of a data item by several clients => conflict ­conflict resolution mechanisms needed

Database Access Methods  Traditional applications follow a layered model  Business logic/client layers interact with the database in a connected client/server manner >connection to database opened at start of session and maintained until session completes >SQL queries sent individually or in batches to database when required by application ­application cannot operate if network connection not present ­not ideally suited to mobile / intermittently connected clients Computer A Application front end Computer C Database Computer B Business Logic

Disconnected Database Access  In a disconnected data architecture client >opens temporary connection to database; required data cached locally and connection closed >operates using the cached data >reconnects to database when connection available and merges any changes from the locally cached data  Guaranteed network connection not required for application to function  Ideally suited to >mobile device applications >intermittently connected devices >clients with high network latencies (e.g. Internet)

.NET Data Access Features .NET supports >Connected and disconnected data architectures >Microsoft SQL Server data sources interaction from desktop and mobile platforms >Interaction with other database servers through ODBC and OLE from desktop platforms only .NET data access classes contained in >System.Data - Common data access types >System.Data.SqlClient - Types used for interacting with MS SQL Server >System.Data.Odbc - Types used for ODBC database interaction >System.Data.OleDb - Types used for OLE database interaction

.NET Connected Data Objects  Connection object (e.g. SqlConnection) >Represents connection to data source; used by other objects to interact with data source  Command object (e.g. OdbcCommand) >Represents queries to database or calls to stored procedures  DataReader object (e.g. OleDbDataReader) >Allows iteration through rows returned from a query  Transaction object (e.g. SqlTransaction) >Provides transactional support  Parameter object (e.g. SqlParameter) >Represents parameters to queries or stored procedures  DataAdapter object (e.g. SqlDataAdapter) >Bridge between database and the disconnected data objects of ADO.NET >Used in passing data between ADO.NET objects and data source

Connected Data Access Example using System.Data.SqlClient;... string connStr = "Initial Catalog=Northwind;Data Source=localhost;Integrated Security=SSPI;"; string insertQuery = "INSERT INTO Customers (CustomerID, CompanyName) Values('NWIND', 'Northwind Traders')"; SqlConnection conDB = new SqlConnection(connStr); SqlCommand cmd = new SqlCommand(insertQuery); cmd.Connection = conDB; conDB.Open(); cmd.ExecuteNonQuery(); conDB.Close();

ADO.NET Disconnected Data Objects  DataTable object >Stores data as collection of rows and columns  DataColumn object >Stores information about structure of a DataTable column  Constraint object >Enables constraints to be placed on data stored in a DataTable  DataRow object >Contains single record from a DataTable  DataSet object >Most important ADO.NET object >Container for multiple DataTables and DataRelations  DataRelation object >Defines relationships between DataTables in a DataSet  DataView object >Provides sorting and filtering of data in a DataSet >Enables binding of data to WinForms controls

Using ADO.NET Objects  ADO.NET objects can >be created and manipulated independently of a data source >be constructed to mirror the relational structure of a data source  Using a DataAdapter object, a DataSet can >be filled with data from a data source >merge changed values back in to a data source >become an offline cache of the data required by an application ­Allows application to operate even when network connection is not present

Disconnected Data Access Example using System.Data; using System.Data.SqlClient;... string connStr = "Initial Catalog=Northwind;Data Source=localhost; Integrated Security=SSPI;"; SqlConnection conDB = new SqlConnection(connStr); SqlDataAdapter adapter = new SqlDataAdapter(); adapter.SelectCommand = new SqlCommand("SELECT * FROM Suppliers", conDB); conDB.Open(); DataSet ds = new DataSet("Customers"); adapter.Fill(ds); conDB.Close(); string criteria = "Surname = Smith"; DataRow[] matchRows = ds.Tables["Customers"].Select(criteria); foreach (DataRow row in matchRows) { string forename = row["Forename"]; string forename = row["Surname"]; Console.WriteLine("Customer "+forename+" "+surname); }

Databases and Mobile Devices  All the preceding techniques can be used with mobile devices and Compact Framework  Mobile devices support three other techniques >SQL Server CE ­SQL Server database engine for WinCE devices >SQL Server CE Remote Data Access (RDA) ­enables two-way transfer of data between SQL CE database and networked SQL Server database >SQL Server Merge Replication ­more powerful mechanism for two-way data transfer between SQL CE database and networked SQL Server database with automatic conflict resolution

SQL Server CE  Lightweight version of MS SQL Server for Windows CE devices  Supports >creation, querying, updating and deleting of databases on device >useful for local caching of data from network databases to enable offline operation  Supports databases up to 2GB but size limited by available memory  Accessible from Compact Framework using classes in System.Data.SQlServerCe namespace >SqlCeEngine - used to creates new databases >SqlCeConnection >SqlCeCommand >SqlCeDataAdapter >SqlCeDataReader  Use is similar to standard SQL client

SqlCE Remote Data Access  Based around SqlCeRemoteDataAccess class  Two primary methods >Pull() ­retrieves data from networked SQL server and copies it to table in local SQL CE database ­pulled records not locked on networked database >Push() ­writes modifications back to networked SQL server based on data in local SQL CE database table ­overwrites changes made by other users to networked database (optimistic concurrency control) ­may result in lost updates  Networked SQL Server requires IIS Web server and specific configuration for RDA

SqlCe Merge Replication  Extension of SQL Server Merge Replication to mobile devices  Three databases used >Publisher - makes data available for replication >Distributor - manages replication and updates >Subscriber - receives/uses replicated data (on CE device)  Configurable resolvers used to automatically deal with multiple updates without loss of data  Merge replication vs Remote Data Access >requires much more configuration than RDA >much easier to use – automatically deals with conflicts

 In this session we have discussed >Databases in distributed applications >Connected database access with.NET >ADO.NET and the disconnected database architecture >Distributed data access from mobile and disconnected clients  The accompanying practical exercises demonstrate >Use of ADO.NET classes in manipulating data >Using ADO.NET to retrieve data from a networked database using Web services Summary

Reading and Resources Reading  Thomas Connolly & Carolyn Begg, Database Systems: A Practical Approach to Design, Implementation, and Management, Addison- Wesley, 2002  David Sceppa, Microsoft ADO.NET Core Reference, Microsoft Press, 2002  Andy Wigley & Stephen Wheelwright, Microsoft Compact Framework Core Reference, Microsoft Press, 2003 Chapters 13 – 17, pp 429 – 586 Resources  Microsoft Developer Network, SQL Server Documentation,