压缩文本是一个可以减少文本内容尺寸达80%的过程。这意味着存储压缩的文本将会比存储没有压缩的文本少80％的空间。也意味着在网络上传输内容需要更少的时间，对于使用文本通信的客户端服务器应用程序来说，将会表现出更高的效率，例如XML Web services。
本文的主要目的就是寻找在客户端和服务器之间使交换的数据尺寸最小化的方法。一些有经验的开发者会使用高级的技术来优化通过网络特别是互联网传送的数据，这样的做法在许多分布式系统中都存在瓶颈。解决这个问题的一个方法是获取更多的带宽，但这是不现实的。另一个方法是通过压缩的方法使得被传输的数据达到最小。
当内容是文本的时候，通过压缩，它的尺寸可以减少80%。这就意味着在客户端和服务器之间带宽的需求也可以减少类似的百分比。为了压缩和解压缩，服务端和客户端则占用了CPU的额外资源。但升级服务器的CPU一般都会比增加带宽便宜，所以压缩是提高传输效率的最有效的方法。
XML/SOAP在网络中
让我们仔细看看SOAP在请求或响应XML Web service的时候，是什么在网络上传输。我们创建一个XML Web service，它包含一个 add 方法。这个方法有两个输入参数并返回这两个数的和：
<WebMethod()> Public Function add(ByVal a As Integer, ByVal b As _Integer) As Integer
add = a + b
End Function
当 XML Web service 消费端调用这个方法的时候，它确实发送了一个SOAP请求到服务器：
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<soap:Body><add xmlns="1020http://tempuri.org/"><a>10</a><b>20</b></add>
</soap:Body></soap:Envelope>
服务端使用一个SOAP响应来回应这个SOAP请求：
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<soap:Body><addResponse xmlns="http://tempuri.org/">
<addResult>30</addResult></addResponse>
</soap:Body></soap:Envelope>
这是调用XML Web service的方法后，在网络上传输的实际信息。在更复杂的XML Web service中，SOAP响应可能是一个很大的数据集。例如，当Northwind中的表orders中的内容被序列化为XML后，数据可能达到454KB。如果我们创建一个应用通过XML Web service来获取这个数据集，那么SOAP响应将会包含所有的数据。
为了提高效率，在传输之前，我们可以压缩这些文本内容。我们怎样才能做到呢？当然是使用SOAP扩展！
SOAP 扩展
SOAP扩展是ASP.NET的Web方法调用的一个拦截机制，它能够在SOAP请求或响应被传输之前操纵它们。开发者可以写一段代码在这些消息序列化之前和之后执行。（SOAP扩展提供底层的API来实现各种各样的应用。）
使用SOAP扩展，当客户端从XML Web service调用一个方法的时候，我们能够减小SOAP信息在网络上传输的尺寸。许多时候，SOAP请求要比SOAP响应小很多（例如，一个大的数据集），因此在我们的例子中，仅对SOAP响应进行压缩。就像你在图1中所看到的，在服务端，当SOAP响应被序列化后，它会被压缩，然后传输到网络上。在客户端，SOAP信息反序列化之前，为了使反序列化成功，SOAP信息会被解压缩。
图 1. SOAP信息在序列化后被压缩（服务端），在反序列化前被解压缩（客户端）
我们也可以压缩SOAP请求，但在这个例子中，这样做的效率增加是不明显的。
为了压缩我们的Web service的SOAP响应，我们需要做两件事情：
· 在服务端序列化SOAP响应信息后压缩它。
· 在客户端反序列化SOAP信息前解压缩它。
这个工作将由SOAP扩展来完成。在下面的段落中，你可以看到所有的客户端和服务端的代码。
首先，这里是一个返回大数据集的XML Web service：
Imports System.Web.Services
<WebService(Namespace := "http://tempuri.org/")> _
Public Class Service1
Inherits System.Web.Services.WebService
<WebMethod()> Public Function getorders() As DataSet
Dim OleDbConnection1 = New System.Data.OleDb.OleDbConnection()
OleDbConnection1.ConnectionString = "Provider=SQLOLEDB.1; _
Integrated Security=SSPI;Initial Catalog=Northwind; _
Data Source=.;Workstation ID=T-MNIKIT;"
Dim OleDbCommand1 = New System.Data.OleDb.OleDbCommand()
OleDbCommand1.Connection = OleDbConnection1
OleDbConnection1.Open()
Dim OleDbDataAdapter1 = New System.Data.OleDb.OleDbDataAdapter()
OleDbDataAdapter1.SelectCommand = OleDbCommand1
OleDbCommand1.CommandText = "Select * from orders"
Dim objsampleset As New DataSet()
OleDbDataAdapter1.Fill(objsampleset, "Orders")
OleDbConnection1.Close()
Return objsampleset
End Function
End Class
在客户端，我们构建了一个Windows应用程序来调用上面的XML Web service，获取那个数据集并显示在DataGrid中：
Public Class Form1
Inherits System.Windows.Forms.Form
'This function invokes the XML Web service, which returns the dataset
'without using compression.
Private Sub Button1_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Button1.Click
Dim ws As New wstest.Service1()
Dim test1 As New ClsTimer()
'Start time counting…
test1.StartTiming()
'Fill datagrid with the dataset
DataGrid1.DataSource = ws.getorders()
test1.StopTiming()
'Stop time counting…
TextBox5.Text = "Total time: " & test1.TotalTime.ToString & "msec"
End Sub
'This function invokes the XML Web service, which returns the dataset
'using compression.
Private Sub Button2_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Button2.Click
Dim ws As New wstest2.Service1()
Dim test1 As New ClsTimer()
'Start time counting…
test1.StartTiming()
'Fill datagrid with dataset
DataGrid1.DataSource = ws.getorders()
test1.StopTiming()
'Stop time counting…
TextBox4.Text = "Total time: " & test1.TotalTime.ToString & "msec"
End Sub
End Class
客户端调用了两个不同的XML Web services, 仅其中的一个使用了SOAP压缩。下面的Timer类是用来计算调用时间的：
Public Class ClsTimer
' Simple high resolution timer class
'
' Methods:
' StartTiming reset timer and start timing
' StopTiming stop timer
'
'Properties
' TotalTime Time in milliseconds
'Windows API function declarations
Private Declare Function timeGetTime Lib "winmm" () As Long
'Local variable declarations
Private lngStartTime As Integer
Private lngTotalTime As Integer
Private lngCurTime As Integer
Public ReadOnly Property TotalTime() As String
Get
TotalTime = lngTotalTime
End Get
End Property
Public Sub StartTiming()
lngTotalTime = 0
lngStartTime = timeGetTime()
End Sub
Public Sub StopTiming()
lngCurTime = timeGetTime()
lngTotalTime = (lngCurTime - lngStartTime)
End Sub
End Class
服务端的SOAP扩展
在服务端，为了减小SOAP响应的尺寸，它被压缩。下面这段告诉你怎么做：
第一步
使用Microsoft Visual Studio .NET, 我们创建一个新的Visual Basic .NET 类库项目（使用"ServerSoapExtension"作为项目名称），添加下面的类：
Imports System
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.IO
Imports zipper
Public Class myextension
Inherits SoapExtension
Private networkStream As Stream
Private newStream As Stream
Public Overloads Overrides Function GetInitializer(ByVal _
methodInfo As LogicalMethodInfo, _
ByVal attribute As SoapExtensionAttribute) As Object
Return System.DBNull.Value
End Function
Public Overloads Overrides Function GetInitializer(ByVal _
WebServiceType As Type) As Object
Return System.DBNull.Value
End Function
Public Overrides Sub Initialize(ByVal initializer As Object)
End Sub
Public Overrides Sub ProcessMessage(ByVal message As SoapMessage)
Select Case message.Stage
Case SoapMessageStage.BeforeSerialize
Case SoapMessageStage.AfterSerialize
AfterSerialize(message)
Case SoapMessageStage.BeforeDeserialize
BeforeDeserialize(message)
Case SoapMessageStage.AfterDeserialize
Case Else
Throw New Exception("invalid stage")
End Select
End Sub
' Save the stream representing the SOAP request or SOAP response into a
' local memory buffer.
Public Overrides Function ChainStream(ByVal stream As Stream) As Stream
networkStream = stream
newStream = New MemoryStream()
Return newStream
End Function
' Write the compressed SOAP message out to a file at
'the server's file system..
Public Sub AfterSerialize(ByVal message As SoapMessage)
newStream.Position = 0
Dim fs As New FileStream("c:\temp\server_soap.txt", _
FileMode.Append, FileAccess.Write)
Dim w As New StreamWriter(fs)
w.WriteLine("-----Response at " + DateTime.Now.ToString())
w.Flush()
'Compress stream and save it to a file
Comp(newStream, fs)
w.Close()
newStream.Position = 0
'Compress stream and send it to the wire
Comp(newStream, networkStream)
End Sub
' Write the SOAP request message out to a file at the server's file system.
Public Sub BeforeDeserialize(ByVal message As SoapMessage)
Copy(networkStream, newStream)
Dim fs As New FileStream("c:\temp\server_soap.txt", _
FileMode.Create, FileAccess.Write)
Dim w As New StreamWriter(fs)
w.WriteLine("----- Request at " + DateTime.Now.ToString())
w.Flush()
newStream.Position = 0
Copy(newStream, fs)
w.Close()
newStream.Position = 0
End Sub
Sub Copy(ByVal fromStream As Stream, ByVal toStream As Stream)
Dim reader As New StreamReader(fromStream)
Dim writer As New StreamWriter(toStream)
writer.WriteLine(reader.ReadToEnd())
writer.Flush()
End Sub
Sub Comp(ByVal fromStream As Stream, ByVal toStream As Stream)
Dim reader As New StreamReader(fromStream)
Dim writer As New StreamWriter(toStream)
Dim test1 As String
Dim test2 As String
test1 = reader.ReadToEnd
'String compression using NZIPLIB
test2 = zipper.Class1.Compress(test1)
writer.WriteLine(test2)
writer.Flush()
End Sub
End Class
' Create a SoapExtensionAttribute for the SOAP extension that can be
' applied to an XML Web service method.
<AttributeUsage(AttributeTargets.Method)> _
Public Class myextensionattribute
Inherits SoapExtensionAttribute
Public Overrides ReadOnly Property ExtensionType() As Type
Get
Return GetType(myextension)
End Get
End Property
Public Overrides Property Priority() As Integer
Get
Return 1
End Get
Set(ByVal Value As Integer)
End Set
End Property
End Class
第二步
我们增加ServerSoapExtension.dll程序集作为引用，并且在web.config声明SOAP扩展：
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<system.web>
<webServices>
<soapExtensionTypes>
<add type="ServerSoapExtension.myextension,
ServerSoapExtension" priority="1" group="0"/>
</soapExtensionTypes>
</webServices>
...
</system.web>
</configuration>
就象你在代码中看到的那样，我们使用了一个临时目录("c:\temp")来捕获SOAP请求和压缩过的SOAP响应到文本文件("c:\temp\server_soap.txt")中 。
客户端的SOAP扩展
在客户端，从服务器来的SOAP响应被解压缩，这样就可以获取原始的响应内容。下面就一步一步告诉你怎么做：
第一步
使用Visual Studio .NET, 我们创建一个新的Visual Basic .NET类库项目（使用 "ClientSoapExtension"作为项目名称），并且添加下面的类：
Imports System
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.IO
Imports zipper
Public Class myextension
Inherits SoapExtension
Private networkStream As Stream
Private newStream As Stream
Public Overloads Overrides Function GetInitializer(ByVal _
methodInfo As LogicalMethodInfo, _
ByVal attribute As SoapExtensionAttribute) As Object
Return System.DBNull.Value
End Function
Public Overloads Overrides Function GetInitializer(ByVal _
WebServiceType As Type) As Object
Return System.DBNull.Value
End Function
Public Overrides Sub Initialize(ByVal initializer As Object)
End Sub
Public Overrides Sub ProcessMessage(ByVal message As SoapMessage)
Select Case message.Stage
Case SoapMessageStage.BeforeSerialize
Case SoapMessageStage.AfterSerialize
AfterSerialize(message)
Case SoapMessageStage.BeforeDeserialize
BeforeDeserialize(message)
Case SoapMessageStage.AfterDeserialize
Case Else
Throw New Exception("invalid stage")
End Select
End Sub
' Save the stream representing the SOAP request or SOAP response
' into a local memory buffer.
Public Overrides Function ChainStream(ByVal stream As Stream) _
As Stream
networkStream = stream
newStream = New MemoryStream()
Return newStream
End Function
' Write the SOAP request message out to a file at
' the client's file system.
Public Sub AfterSerialize(ByVal message As SoapMessage)
newStream.Position = 0
Dim fs As New FileStream("c:\temp\client_soap.txt", _
FileMode.Create, FileAccess.Write)
Dim w As New StreamWriter(fs)
w.WriteLine("----- Request at " + DateTime.Now.ToString())
w.Flush()
Copy(newStream, fs)
w.Close()
newStream.Position = 0
Copy(newStream, networkStream)
End Sub
' Write the uncompressed SOAP message out to a file
' at the client's file system..
Public Sub BeforeDeserialize(ByVal message As SoapMessage)
'Decompress the stream from the wire
DeComp(networkStream, newStream)
Dim fs As New FileStream("c:\temp\client_soap.txt", _
FileMode.Append, FileAccess.Write)
Dim w As New StreamWriter(fs)
w.WriteLine("-----Response at " + DateTime.Now.ToString())
w.Flush()
newStream.Position = 0
'Store the uncompressed stream to a file
Copy(newStream, fs)
w.Close()
newStream.Position = 0
End Sub
Sub Copy(ByVal fromStream As Stream, ByVal toStream As Stream)
Dim reader As New StreamReader(fromStream)
Dim writer As New StreamWriter(toStream)
writer.WriteLine(reader.ReadToEnd())
writer.Flush()
End Sub
Sub DeComp(ByVal fromStream As Stream, ByVal toStream As Stream)
Dim reader As New StreamReader(fromStream)
Dim writer As New StreamWriter(toStream)
Dim test1 As String
Dim test2 As String
test1 = reader.ReadToEnd
'String decompression using NZIPLIB
test2 = zipper.Class1.DeCompress(test1)
writer.WriteLine(test2)
writer.Flush()
End Sub
End Class
' Create a SoapExtensionAttribute for the SOAP extension that can be
' applied to an XML Web service method.
<AttributeUsage(AttributeTargets.Method)> _
Public Class myextensionattribute
Inherits SoapExtensionAttribute
Public Overrides ReadOnly Property ExtensionType() As Type
Get
Return GetType(myextension)
End Get
End Property
Public Overrides Property Priority() As Integer
Get
Return 1
End Get
Set(ByVal Value As Integer)
End Set
End Property
End Class
就象你在代码中看到的那样，我们使用了一个临时目录("c:\temp") 来捕获SOAP请求和解压缩的SOAP响应到文本文件("c:\temp\client_soap.txt")中。
第二步
我们添加ClientSoapExtension.dll程序集作为引用，并且在我们的应用程序的XML Web service引用中声明SOAP扩展：
'-------------------------------------------------------------------------
' <autogenerated>
' This code was generated by a tool.
' Runtime Version: 1.0.3705.209
'
' Changes to this file may cause incorrect behavior and will be lost if
' the code is regenerated.
' </autogenerated>
'-------------------------------------------------------------------------
Option Strict Off
Option Explicit On
Imports System
Imports System.ComponentModel
Imports System.Diagnostics
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.Xml.Serialization
'
'This source code was auto-generated by Microsoft.VSDesigner,
'Version 1.0.3705.209.
'
Namespace wstest2
'<remarks/>
<System.Diagnostics.DebuggerStepThroughAttribute(), _
System.ComponentModel.DesignerCategoryAttribute("code"), _
System.Web.Services.WebServiceBindingAttribute(Name:="Service1Soap", _
[Namespace]:="http://tempuri.org/")> _
Public Class Service1
Inherits System.Web.Services.Protocols.SoapHttpClientProtocol
'<remarks/>
Public Sub New()
MyBase.New
Me.Url = "http://localhost/CompressionWS/Service1.asmx"
End Sub
'<remarks/>
<System.Web.Services.Protocols.SoapDocumentMethodAttribute _
("http://tempuri.org/getproducts",RequestNamespace:= _
"http://tempuri.org/", ResponseNamespace:="http://tempuri.org/", _
Use:=System.Web.Services.Description.SoapBindingUse.Literal,_
ParameterStyle:=System.Web.Services.Protocols.SoapParameterStyle _
.Wrapped), ClientSoapExtension.myextensionattribute()> _
Public Function getproducts() As System.Data.DataSet
Dim results() As Object = Me.Invoke("getproducts", _
New Object(-1) {})
Return CType(results(0), System.Data.DataSet)
End Function
'<remarks/>
Public Function Begingetproducts(ByVal callback As _
System.AsyncCallback, ByVal asyncState As Object) As System.IAsyncResult _
Return Me.BeginInvoke("getproducts", New Object(-1) {}, _
callback, asyncState)
End Function
'<remarks/>
Public Function Endgetproducts(ByVal asyncResult As _
System.IAsyncResult) As System.Data.DataSet
Dim results() As Object = Me.EndInvoke(asyncResult)
Return CType(results(0), System.Data.DataSet)
End Function
End Class
End Namespace
这里是zipper类的源代码，它是使用免费软件NZIPLIB库实现的：
using System;
using NZlib.GZip;
using NZlib.Compression;
using NZlib.Streams;
using System.IO;
using System.Net;
using System.Runtime.Serialization;
using System.Xml;
namespace zipper
{
public class Class1
{
public static string Compress(string uncompressedString)
{
byte[] bytData = System.Text.Encoding.UTF8.GetBytes(uncompressedString);
MemoryStream ms = new MemoryStream();
Stream s = new DeflaterOutputStream(ms);
s.Write(bytData, 0, bytData.Length);
s.Close();
byte[] compressedData = (byte[])ms.ToArray();
return System.Convert.ToBase64String(compressedData, 0, _
compressedData.Length);
}
public static string DeCompress(string compressedString)
{
string uncompressedString="";
int totalLength = 0;
byte[] bytInput = System.Convert.FromBase64String(compressedString);;
byte[] writeData = new byte[4096];
Stream s2 = new InflaterInputStream(new MemoryStream(bytInput));
while (true)
{
int size = s2.Read(writeData, 0, writeData.Length);
if (size > 0)
{
totalLength += size;
uncompressedString+=System.Text.Encoding.UTF8.GetString(writeData, _
0, size);
}
else
{
break;
}
}
s2.Close();
return uncompressedString;
}
}
}
分析
软件&硬件
· 客户方: Intel Pentium III 500 MHz, 512 MB RAM, Windows XP.
· 服务器方: Intel Pentium III 500 MHz, 512 MB RAM, Windows 2000 Server, Microsoft SQL Server 2000.
在客户端，一个Windows应用程序调用一个XML Web service。这个XML Web service 返回一个数据集并且填充客户端应用程序中的DataGrid。
图2. 这是一个我们通过使用和不使用SOAP压缩调用相同的XML Web Service的样例程序。这个XML Web service返回一个大的数据集。
CPU 使用记录
就象在图3中显示的那样，没有使用压缩的CPU使用时间是 29903 milliseconds.
图 3. 没有使用压缩的CPU使用记录
在我们的例子中，使用压缩的CPU使用时间显示在图4中, 是15182 milliseconds.
图 4. 使用压缩的CPU 使用记录
正如你看到的，我们在客户方获取这个数据集的时候，使用压缩与不使用压缩少用了近50%的CPU时间，仅仅在CPU加载时有一点影响。当客户端和服务端交换大的数据时，SOAP压缩能显著地增加XML Web Services效率。 在Web中，有许多改善效率的解决方案。我们的代码是获取最大成果但是最便宜的解决方案。SOAP扩展是通过压缩交换数据来改善XML Web Services性能的，这仅仅对CPU加载时间造成了一点点影响。并且值得提醒的是，大家可以使用更强大的和需要更小资源的压缩算法来获取更大的效果
