Transcript 1 UCN Technology: Computer Science: Autumn 2012 ADO.NET 2.0

ADO.NET 2.0
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Architecture
DataReader
DataSet
Connection factory
Config file
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Data access
Swap?
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Data Providers
• OLE DB:
A COM object, that maps the OLE DB api to the DBMS' api
Is Microsoft propritary.
• ODBC:
Works similar to a printer driver undependent of DBMS and OS
• .NET Data provider:
A set of classes that implements a set of interfaces and
abstract classes
• Some are provided by Microsoft and are installed with the .Net
framework.
• Others are written by the DBMS provider
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Microsoft Data Providers
MS Access
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Data Provider
OLE DB
Microsoft SQL Server
Microsoft SQL Server Mobile
ODBC
Oracle
Namespace
System.Data.OleDb
System.Data.SqlClient
System.Data.SqlServerCe
System.Data.Odbc
System.Data.OracleClient
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Assembly
System.Data.dll
System.Data.dll
System.Data.SqlServerCe.dll
System.Data.dll
System.Data.OracleClient.dll
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Other providers
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Data Provider
Firebird Interbase
IBM DB2 Universal Database
MySQL
PostgreSQL
Sybase
Oracle (proprietary)
Website
http://www.mono-project.com/Firebird_Interbase
http://www-306.ibm.com/software/data/db2
http://dev.mysql.com/downloads/connector/net/1.0.html
http://www.mono-project.com/PostgreSQL
http://www.mono-project.com/Sybase
http://www.oracle.com/technology/tech/windows/odpnet/index.html
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Classes and Interfaces
• Object
Base Class
Implemented Interfaces
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DbConnection
DbCommand
DbDataReader
DbDataAdapter
DbParameter
DbTransaction
IDbConnection
IDbCommand
IDataReader
IDataAdapter
IDataParameter
IDbTransaction
Connection
Command
DataReader
DataAdapter
Parameter
Transaction
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- Connection
- SQL command
- Forward reader
- Used with Dataset
- Parameters for Command
- Transaction
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Two ways of DB access
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Connected:
– Open connection.
– Read/Write access (select, insert, update and delete) using a
Command object.
– When reading (select) a DataReader object is returned. A
DataReader is an iterator (cursor) into the result table.
– Close Connection.
Disconnected:
– Fill a DataSet object (a copy of a part of the database) using a
DataAdapter.
– DataAdapter wraps SQL-statement(s).
– A DataSet object contains DataTable objects.
– DataTable objects contain collections of rows and columns.
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Connection vs. Connectionless
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Connection:
– Open Connection
– Execute DB operations
– Close Connection
– Working on actual (live)
data
– Other applications can not
access data.
SQL
DB
Domæneklasser
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Dataklasser
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Connection vs. Connectionless
SQL
DB
Domæneklasser
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Dataklasser
Connectionless:
– Create a copy of a part of
the database
– Execute DB operations on
the copy
– Other applications may
change date
– The copy may be come
inconsistent.
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Data are changed in the local copy:
– at update it is checked if the data in the
database have been modified by others
– in that case the update is rejected
(ConcurrencyException).
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Overview of using databases
• 4 steps:
1. Open connection to database
2. Execute SQL for updating DB or fetching records
3. Handle data
4. Close connection
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Step 1: Open Connection
• Connection are opened according to connection string info
– here is a connection to a MS Access database opened
– The database is located here: @"c:\AnyPlace\MyDb.mdb"
– @ means that escape characters are discarded
<%@ import namespace="System.Data"%>
<%@ import namespace="System.Data.OleDb"%>
string sConnection;
sConnection = "Provider=Microsoft.Jet.OLEDB.4.0;"
+ "Data Source="+@"c:\AnyPlace\MyDb.mdb");
connection
OleDbConnection dbConn;
dbConn = new OleDbConnection(sConnection);
dbConn.Open();
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Open Connection
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Connections are opened using a connection string
– May be found using Visual Studio:
// Create and open a connection.
SqlConnection cn = new SqlConnection();
cn.ConnectionString = "Data Source=PCM06073\\SQLEXPRESS;”
+”Initial Catalog=vw;Integrated Security=True;";
cn.Open();
ShowConnectionStatus(cn);
Data Source
or
Server?
// or should it be?
cn.ConnectionString = “Server=PCM06073\\SQLEXPRESS;”
+”Initial Catalog=vw;Integrated Security=True;";
cn.Open();
Server is preferable.
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Connection Strings
• Connection strings are product specific (DB specific) anf
often very well-documented.
• Help may be found at:
– www.connectionstrings.com
– www.able-consulting.com/ADO_conn.htm
• In many cases VS can help.
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Step 2: Get / fetch data
• Data are fetched by creating a Command
object and use it to execute a SQL statement.
• Data can be stored in either a DataReader
object or a DataSet object.
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Differences between DataReader and
DataSet/DataAdapter
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DataReader can only be used for reading data.
It can only be traversed once (forward).
DBCommand can update the database by ExecuteNonQuery. This
update is executed immediately.
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DataAdapter is the connection between DataSet and database.
Data are fetched to the DataSet, might be modified and sent back to
the database.
Updates are executed on a local copy. Concurrency problems must
be handled.
Possible to traverse forward and backward.
A DataSet can contain multiple tables.
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Step 2: Get records
Method 1: DataSet
• Get records by SQL Select query and DataAdapter
– Data can be read and changed
string sql;
sql = " Select lastname, firstname"
+ " From employee"
+ " Order By lastname Asc, firstname Asc;";
OleDbCommand dbCmd;
dbCmd = new OleDbCommand(dbConn, sql);
adapter.SelectCommand = dbCmd;
dataset =new DataSet("MyTable");
adapter.Fill(dataset);
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dataset
record
record
record
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Step 2: Get records
Method 2: DataReader
• Get records via SQL Select query
– read-only access to the database
string sql;
sql = " Select lastname, firstname"
+ " From employee"
+ " Order By lastname Asc, firstname Asc;";
OleDbCommand dbCmd;
dbCmd = new OleDbCommand(sql, dbConn);
OleDbDataReader dbReader;
dbReader = dbCmd.ExecuteReader();
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data reader
record
record
record
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What is achieved?
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We have created a connection to a
database.
The connection is placed in the
connection object.
We have done a search by using a
SQL-statement.
The search was executed by using a
command object.
The result of the search was stored in
a DataSet or as here a DataReader
object.
Now it is possible to get the data from
this object for viewing, passing on to
client or handle in other ways.
Example: ..\vwReader
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Factory
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Problem:
Unlike ODBC, knowledge of the implementation might be
spread to many places in the application.
That's because of the set of dedicated classes
The solution is to use a factory (from factory pattern)
In ADO.NET 2.0, it is implementet in the framework.
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Use factory to create a Connection and a Command object
Example: \ReaderWithConfig
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Use factory to create Connection and Command objects
• Here is no references in the source code to concrete classes
• provider and connectionString may be placed in the config file
string provider = "System.Data.OleDb";
string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source=firma.mdb";
DbProviderFactory factory= DbProviderFactories.GetFactory(provider);
DbConnection conn=factory.CreateConnection();
conn.ConnectionString=connectionString;
conn.Open();
Console.WriteLine(conn.State);
DbCommand command = factory.CreateCommand();
command.CommandText = "SELECT * FROM employee";
command.Connection = conn;
DbDataReader dataReader = command.ExecuteReader();
while (dataReader.Read()){
Console.WriteLine(dataReader[1]);
}
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Connection strings
• Connection strings depends on the provider and are not
well documented
• Where to get help?
– www.connectionstrings.com
– www.carlprothman.net/Default.aspx?tabid=81
– Providers documentation
• Visual Studio can generate some of them
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.Config fil
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Configuration files are used several places in .Net
i.e in ASP.NET for setting site properties and security
In Remoting for setting remote object properties
In ADO.NET to set provider and connection string
You might also use own application dependent properties
In a .Net application the config file should be called
<full assemblyname.config>,
i.e test.exe.config or test.dll.config
• If you call it app.config in Visual Studio, it automatically
copied and renamed when the application is built.
• In ASP.NET it must be named web.config. You might define
one in different sub folders, but the security part
(authentification e.g.) must be in the root folder of the site.
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.config fil and ADO.NET
• Example of SQLExpress connection string:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
Internal name
</configSections>
Database
<connectionStrings>
Machine name
<add name="TestConnection"
connectionString="Data Source=pcm06463\sqlexpress;Initial Catalog=dbtest; User ID=test;Password="
providerName="System.Data.SqlClient" />
</connectionStrings>
</configuration>
Nessesary for sqlexpress
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Get provider and connection string
• From ConfigurationManager:
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string provider = ConfigurationManager.ConnectionStrings["TestConnection"].ProviderName;
string connStr = ConfigurationManager.ConnectionStrings["TestConnection"].ConnectionString;
.....
Example: \ReaderWithConfig
Example: \ReaderWithConfig2
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Parameters in SQL statements
• Makes it possible to use the same SQL-statement in multible
situations.
• But harder to debugge
string sqlString = "INSERT INTO datotest Values (@dato)";
SqlParameter sqlParam = new SqlParameter("@dato", System.Data.SqlDbType.DateTime);
sqlParam.Value = DateTime.Now;
SqlCommand comm = _conn.CreateCommand();
comm.CommandText = sqlString;
comm.Parameters.Add(sqlParam);
int result = comm.ExecuteNonQuery();
ExecuteNonQuery is
used for insert, delete
and update
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Transaction - Definition
• A transaction is an operation on data in the database.
• A transaction may be composed of several database
operations, but is viewed as a logical unit of work
• A transaction must be done completely or not done at all
• A transaction must have the ACID properties:
– A: Either it is done in total or it is not done at all (Atomicity)
– C: The database moves from one consistent state to an other
consistent state (Consistency)
– I: If more operations are accessing the same data, they are
not to disturb each other – they must execute as if they
executed alone (Isolation)
– D: When a transaction completes, its changes to the
database are permanent (Durability)
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UCN T&B - CBH (FEN) Databases/Transactions
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Transactions – example:
T1 and T2 are executing concurrently
T1: Transfers N DKKs from
account X to account Y:
T2: Deposits M DKK on
account Y:
read_item(X);
X:= X-N;
write_item(X);
read_item(Y);
read_item(Y);
Y:= Y+M;
write_item(Y);
Y:= Y+N;
write_item(Y);
Any possible
problems?
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Transactions – Problems
• We want several transactions to execute concurrently
(Why?)
• Three types of problems:
– lost update
– uncommitted dependency (temporary update)
– inconsistent analysis (incorrect summary)
• Crash during execution of a transaction must be handled
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Lost Update
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Uncommitted Dependency
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Inconsistent Analysis
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Transactions
• In C#: Bank Transactions
Example: demos\Transactions\BankTransaction1
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Multiple Databases
• Moving from (badly designed) database to another (better
designed) database:
Example: demos\flytDataApp
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