Procedura: dividere in blocchi i dati serializzati
Durante l'invio di grandi set di dati in messaggi del servizio Web si verificano i problemi seguenti:
Un working set (memoria) di grandi dimensioni, a causa del buffer creato dal modulo di gestione della serializzazione.
Consumo eccessivo di larghezza di banda, a causa dell'aumento del 33% dopo la codifica Base64.
Per risolvere questi problemi, implementare l'interfaccia IXmlSerializable per controllare la serializzazione e la deserializzazione. In particolare, implementare i metodi WriteXml e ReadXml per dividere in blocchi i dati.
Per implementare la divisione in blocchi sul lato server
Nel server è necessario che il metodo Web disattivi il buffer ASP.NET e restituisca un tipo che implementa IXmlSerializable.
Il tipo che implementa IXmlSerializable divide in blocchi i dati nel metodo WriteXml.
Per implementare l'elaborazione sul lato client
Modificare il metodo Web nel proxy client in modo che venga restituito il tipo che implementa IXmlSerializable. Per eseguire tale operazione automaticamente, è possibile utilizzare SchemaImporterExtension, ma questa soluzione non è illustrata nel presente esempio.
Implementare il metodo ReadXml per leggere il flusso dei dati divisi in blocchi e scrivere i byte su disco. Questa implementazione genera inoltre eventi di stato utilizzabili da un controllo grafico, ad esempio una barra di stato.
Esempio
Nell'esempio di codice seguente viene illustrato il metodo Web per la disattivazione del buffer ASP.NET nel client. Viene inoltre illustrata l'implementazione sul lato client dell'interfaccia IXmlSerializable che divide in blocchi i dati nel metodo WriteXml.
<WebMethod(), System.Web.Services.Protocols.SoapDocumentMethodAttribute(ParameterStyle := SoapParameterStyle.Bare)> _
Public Function DownloadSong(ByVal Authorization As DownloadAuthorization, ByVal filePath As String) As SongStream
' Turn off response buffering.
System.Web.HttpContext.Current.Response.Buffer = False
' Return a song.
Dim song As New SongStream(filePath)
Return song
End Function
...
<XmlSchemaProvider("MySchema")> _
Public Class SongStream
Implements IXmlSerializable
Private Const ns As String = "http://demos.Contoso.com/webservices"
Private filePath As String
Public Sub New()
End Sub
Public Sub New(ByVal filePath As String)
Me.filePath = filePath
End Sub
' This is the method named by the XmlSchemaProviderAttribute applied to the type.
Public Shared Function MySchema(ByVal xs As XmlSchemaSet) As XmlQualifiedName
' This method is called by the framework to get the schema for this type.
' We return an existing schema from disk.
Dim schemaSerializer As New XmlSerializer(GetType(XmlSchema))
Dim xsdPath As String = Nothing
' NOTE: replace SongStream.xsd with your own schema file.
xsdPath = System.Web.HttpContext.Current.Server.MapPath("SongStream.xsd")
Dim s As XmlSchema = CType(schemaSerializer.Deserialize(New XmlTextReader(xsdPath)), XmlSchema)
xs.XmlResolver = New XmlUrlResolver()
xs.Add(s)
Return New XmlQualifiedName("songStream", ns)
End Function
Sub WriteXml(ByVal writer As System.Xml.XmlWriter) Implements IXmlSerializable.WriteXml
' This is the chunking code.
' ASP.NET buffering must be turned off for this to work.
Dim bufferSize As Integer = 4096
Dim songBytes(bufferSize) As Char
Dim inFile As FileStream = File.Open(Me.filePath, FileMode.Open, FileAccess.Read)
Dim length As Long = inFile.Length
' Write the file name.
writer.WriteElementString("fileName", ns, Path.GetFileNameWithoutExtension(Me.filePath))
' Write the size.
writer.WriteElementString("size", ns, length.ToString())
' Write the song bytes.
writer.WriteStartElement("song", ns)
Dim sr As New StreamReader(inFile, True)
Dim readLen As Integer = sr.Read(songBytes, 0, bufferSize)
While readLen > 0
writer.WriteStartElement("chunk", ns)
writer.WriteChars(songBytes, 0, readLen)
writer.WriteEndElement()
writer.Flush()
readLen = sr.Read(songBytes, 0, bufferSize)
End While
writer.WriteEndElement()
inFile.Close()
End Sub
Function GetSchema() As System.Xml.Schema.XmlSchema Implements IXmlSerializable.GetSchema
Throw New System.NotImplementedException()
End Function
Sub ReadXml(ByVal reader As System.Xml.XmlReader) Implements IXmlSerializable.ReadXml
Throw New System.NotImplementedException()
End Sub
End Class
...
Public Class SongFile
Implements IXmlSerializable
Public Shared Event OnProgress As ProgressMade
Public Sub New()
End Sub
Private Const ns As String = "http://demos.teched2004.com/webservices"
Public Shared MusicPath As String
Private filePath As String
Private size As Double
Sub ReadXml(ByVal reader As System.Xml.XmlReader) Implements IXmlSerializable.ReadXml
reader.ReadStartElement("DownloadSongResult", ns)
ReadFileName(reader)
ReadSongSize(reader)
ReadAndSaveSong(reader)
reader.ReadEndElement()
End Sub
Sub ReadFileName(ByVal reader As XmlReader)
Dim fileName As String = reader.ReadElementString("fileName", ns)
Me.filePath = Path.Combine(MusicPath, Path.ChangeExtension(fileName, ".mp3"))
End Sub
Sub ReadSongSize(ByVal reader As XmlReader)
Me.size = Convert.ToDouble(reader.ReadElementString("size", ns))
End Sub
Sub ReadAndSaveSong(ByVal reader As XmlReader)
Dim outFile As FileStream = File.Open(Me.filePath, FileMode.Create, FileAccess.Write)
Dim songBase64 As String
Dim songBytes() As Byte
reader.ReadStartElement("song", ns)
Dim totalRead As Double = 0
While True
If reader.IsStartElement("chunk", ns) Then
songBase64 = reader.ReadElementString()
totalRead += songBase64.Length
songBytes = Convert.FromBase64String(songBase64)
outFile.Write(songBytes, 0, songBytes.Length)
outFile.Flush()
RaiseEvent OnProgress((100 *(totalRead / size)))
Else
Exit While
End If
End While
outFile.Close()
reader.ReadEndElement()
End Sub
<PermissionSet(SecurityAction.Demand, Name :="FullTrust")> _
Public Sub Play()
System.Diagnostics.Process.Start(Me.filePath)
End Sub
Function GetSchema() As System.Xml.Schema.XmlSchema Implements IXmlSerializable.GetSchema
Throw New System.NotImplementedException()
End Function
Public Sub WriteXml(ByVal writer As XmlWriter) Implements IXmlSerializable.WriteXml
Throw New System.NotImplementedException()
End Sub
End Class
[WebMethod]
[System.Web.Services.Protocols.SoapDocumentMethodAttribute
(ParameterStyle= SoapParameterStyle.Bare)]
public SongStream DownloadSong(DownloadAuthorization Authorization, string filePath)
{
// Turn off response buffering.
System.Web.HttpContext.Current.Response.Buffer = false;
// Return a song.
SongStream song = new SongStream(filePath);
return song;
}
...
[XmlSchemaProvider("MySchema")]
public class SongStream : IXmlSerializable
{
private const string ns = "http://demos.Contoso.com/webservices";
private string filePath;
public SongStream(){ }
public SongStream(string filePath)
{
this.filePath = filePath;
}
// This is the method named by the XmlSchemaProviderAttribute applied to the type.
public static XmlQualifiedName MySchema(XmlSchemaSet xs)
{
// This method is called by the framework to get the schema for this type.
// We return an existing schema from disk.
XmlSerializer schemaSerializer = new XmlSerializer(typeof(XmlSchema));
string xsdPath = null;
// NOTE: replace the string with your own path.
xsdPath = System.Web.HttpContext.Current.Server.MapPath("SongStream.xsd");
XmlSchema s = (XmlSchema)schemaSerializer.Deserialize(
new XmlTextReader(xsdPath), null);
xs.XmlResolver = new XmlUrlResolver();
xs.Add(s);
return new XmlQualifiedName("songStream", ns);
}
void IXmlSerializable.WriteXml(System.Xml.XmlWriter writer)
{
// This is the chunking code.
// ASP.NET buffering must be turned off for this to work.
int bufferSize = 4096;
char[] songBytes = new char[bufferSize];
FileStream inFile = File.Open(this.filePath, FileMode.Open, FileAccess.Read);
long length = inFile.Length;
// Write the file name.
writer.WriteElementString("fileName", ns, Path.GetFileNameWithoutExtension(this.filePath));
// Write the size.
writer.WriteElementString("size", ns, length.ToString());
// Write the song bytes.
writer.WriteStartElement("song", ns);
StreamReader sr = new StreamReader(inFile, true);
int readLen = sr.Read(songBytes, 0, bufferSize);
while (readLen > 0)
{
writer.WriteStartElement("chunk", ns);
writer.WriteChars(songBytes, 0, readLen);
writer.WriteEndElement();
writer.Flush();
readLen = sr.Read(songBytes, 0, bufferSize);
}
writer.WriteEndElement();
inFile.Close();
}
System.Xml.Schema.XmlSchema IXmlSerializable.GetSchema()
{
throw new System.NotImplementedException();
}
void IXmlSerializable.ReadXml(System.Xml.XmlReader reader)
{
throw new System.NotImplementedException();
}
}
...
public class SongFile : IXmlSerializable
{
public static event ProgressMade OnProgress;
public SongFile()
{ }
private const string ns = "http://demos.teched2004.com/webservices";
public static string MusicPath;
private string filePath;
private double size;
void IXmlSerializable.ReadXml(System.Xml.XmlReader reader)
{
reader.ReadStartElement("DownloadSongResult", ns);
ReadFileName(reader);
ReadSongSize(reader);
ReadAndSaveSong(reader);
reader.ReadEndElement();
}
void ReadFileName(XmlReader reader)
{
string fileName = reader.ReadElementString("fileName", ns);
this.filePath =
Path.Combine(MusicPath, Path.ChangeExtension(fileName, ".mp3"));
}
void ReadSongSize(XmlReader reader)
{
this.size = Convert.ToDouble(reader.ReadElementString("size", ns));
}
void ReadAndSaveSong(XmlReader reader)
{
FileStream outFile = File.Open(
this.filePath, FileMode.Create, FileAccess.Write);
string songBase64;
byte[] songBytes;
reader.ReadStartElement("song", ns);
double totalRead=0;
while(true)
{
if (reader.IsStartElement("chunk", ns))
{
songBase64 = reader.ReadElementString();
totalRead += songBase64.Length;
songBytes = Convert.FromBase64String(songBase64);
outFile.Write(songBytes, 0, songBytes.Length);
outFile.Flush();
if (OnProgress != null)
{
OnProgress(100 * (totalRead / size));
}
}
else
{
break;
}
}
outFile.Close();
reader.ReadEndElement();
}
[PermissionSet(SecurityAction.Demand, Name="FullTrust")]
public void Play()
{
System.Diagnostics.Process.Start(this.filePath);
}
System.Xml.Schema.XmlSchema IXmlSerializable.GetSchema()
{
throw new System.NotImplementedException();
}
public void WriteXml(XmlWriter writer)
{
throw new System.NotImplementedException();
}
}
Compilazione del codice
- Nel codice vengono utilizzati gli spazi dei nomi seguenti: System, System.Runtime.Serialization, System.Web.Services, System.Web.Services.Protocols, System.Xml, System.Xml.Serialization e System.Xml.Schema.