SqlClient-Streamingunterstützung

Artikel
06/01/2023

Die Streamingunterstützung zwischen SQL Server und einer Anwendung (neu in .Net Framework 4.5) unterstützt unstrukturierte Daten auf dem Server (Dokumente, Bilder und Mediendateien). Eine SQL Server-Datenbank kann BLOBs (Binary Large Objects) speichern, beim Abrufen von BLOBs kann jedoch viel Arbeitsspeicher beansprucht werden.

Durch die Unterstützung des Streamings an und von SQL Server wird das Schreiben von Anwendungen vereinfacht, die Daten streamen. Daten müssen nicht vollständig in den Arbeitsspeicher geladen werden, was zu weniger Ausnahmefehlern aufgrund Arbeitsspeicherüberlaufs führt.

Die Streamingunterstützung ermöglicht auch Anwendungen der mittleren Ebene eine bessere Skalierung, insbesondere in Szenarien, in denen Geschäftsobjekte eine Verbindung mit SQL Azure herstellen, um große BLOBs zu senden, abzurufen und zu bearbeiten.

Warnung

Asynchrone Aufrufe werden nicht unterstützt, wenn eine Anwendung auch das Context Connection-Schlüsselwort für Verbindungszeichenfolgen verwendet.

Die zur Unterstützung des Streamings hinzugefügten Member werden verwendet, um Daten aus Abfragen abzurufen und um Parameter an Abfragen und gespeicherte Prozeduren zu übergeben. Das Streamingfeature behandelt grundlegende OLTP- und Datenmigrationsszenarien und ist für lokale und externe Datenmigrationsumgebungen geeignet.

Streamingunterstützung von SQL Server

Mit der Unterstützung des Streamings von SQL Server werden neue Funktionen in den Klassen DbDataReader und SqlDataReader eingeführt, um Stream-, XmlReader- und TextReader-Objekte abzurufen und darauf zu reagieren. Diese Klassen werden verwendet, um Daten aus Abfragen abzurufen. Daher betrifft das unterstützte Streaming von SQL Server OLTP-Szenarien und ist für lokale und externe Umgebungen geeignet.

Die folgenden Member wurden SqlDataReader hinzugefügt, um die Streamingunterstützung von SQL Server zu ermöglichen:

Die folgenden Member wurden DbDataReader hinzugefügt, um die Streamingunterstützung von SQL Server zu ermöglichen:

Streamingunterstützung zu SQL Server

Mit der Unterstützung des Streamings an SQL Server werden neue Funktionen in der SqlParameter-Klasse eingeführt, um XmlReader-, Stream- und TextReader-Objekte zu akzeptieren und darauf zu reagieren. SqlParameter wird verwendet, um Parameter an Abfragen und gespeicherte Prozeduren zu übergeben.

Durch das Verwerfen eines SqlCommand-Objekts oder das Aufrufen von Cancel muss jeglicher Streamingvorgang abgebrochen werden. Wenn eine Anwendung CancellationToken sendet, wird der Abbruch nicht sichergestellt.

Die folgenden SqlDbType-Typen akzeptieren den Value-Wert Stream:

Binär (Binary)
VarBinary

Die folgenden SqlDbType-Typen akzeptieren den Value-Wert TextReader:

Char
NChar
NVarChar
Xml

Der XmlSqlDbType-Typ akzeptiert ein Value von XmlReader.

SqlValue kann Werte des Typs XmlReader, TextReader und Stream akzeptieren.

Das XmlReader-, TextReader- und Stream-Objekt wird bis zu dem von Size definierten Wert übertragen.

Beispiel: Streaming von SQL Server

Verwenden Sie das folgende Transact-SQL, um die Beispieldatenbank zu erstellen:

CREATE DATABASE [Demo]
GO
USE [Demo]
GO
CREATE TABLE [Streams] (
[id] INT PRIMARY KEY IDENTITY(1, 1),
[textdata] NVARCHAR(MAX),
[bindata] VARBINARY(MAX),
[xmldata] XML)
GO
INSERT INTO [Streams] (textdata, bindata, xmldata) VALUES (N'This is a test', 0x48656C6C6F, N'<test>value</test>')
INSERT INTO [Streams] (textdata, bindata, xmldata) VALUES (N'Hello, World!', 0x54657374696E67, N'<test>value2</test>')
INSERT INTO [Streams] (textdata, bindata, xmldata) VALUES (N'Another row', 0x666F6F626172, N'<fff>bbb</fff><fff>bbc</fff>')
GO

Das Beispiel erläutert die folgenden Aufgaben:

Vermeiden, dass ein Benutzeroberflächenthread blockiert wird, indem eine asynchrone Methode zum Abrufen großer Dateien bereitgestellt wird.
Übertragen einer großen Textdatei aus SQL Server in .NET Framework 4.5.
Übertragen einer großen XML-Datei aus SQL Server in .NET Framework 4.5.
Abrufen von Daten aus SQL Server.
Übertragen großer Dateien (BLOB)s aus einer SQL Server-Datenbank in eine andere, ohne dass der Arbeitsspeicher knapp wird.

using System;
using System.Data;
using System.Data.SqlClient;
using System.IO;
using System.Threading.Tasks;
using System.Xml;

namespace StreamingFromServer {
   class Program {
      private const string connectionString = @"...";

      static void Main(string[] args) {
         CopyBinaryValueToFile().Wait();
         PrintTextValues().Wait();
         PrintXmlValues().Wait();
         PrintXmlValuesViaNVarChar().Wait();

         Console.WriteLine("Done");
      }

      // Retrieve a large BLOB from SQL Server in .NET Framework 4.5 using the asynchronous capability.
      private static async Task CopyBinaryValueToFile() {
         string filePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), "binarydata.bin");

         using (SqlConnection connection = new SqlConnection(connectionString)) {
            await connection.OpenAsync();
            using (SqlCommand command = new SqlCommand("SELECT [bindata] FROM [Streams] WHERE [id]=@id", connection)) {
               command.Parameters.AddWithValue("id", 1);

               // The reader needs to be executed with the SequentialAccess behavior to enable network streaming.
               // Otherwise ReadAsync will buffer the entire BLOB into memory which can cause scalability issues or even OutOfMemoryExceptions.
               using (SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess)) {
                  if (await reader.ReadAsync()) {
                     if (!(await reader.IsDBNullAsync(0))) {
                        using (FileStream file = new FileStream(filePath, FileMode.Create, FileAccess.Write)) {
                           using (Stream data = reader.GetStream(0)) {

                              // Asynchronously copy the stream from the server to the file we just created.
                              await data.CopyToAsync(file);
                           }
                        }
                     }
                  }
               }
            }
         }
      }

      // Transfer a large Text File from SQL Server in .NET Framework 4.5.
      private static async Task PrintTextValues() {
         using (SqlConnection connection = new SqlConnection(connectionString)) {
            await connection.OpenAsync();
            using (SqlCommand command = new SqlCommand("SELECT [id], [textdata] FROM [Streams]", connection)) {

               // The reader needs to be executed with the SequentialAccess behavior to enable network streaming.
               // Otherwise ReadAsync will buffer the entire text document into memory which can cause scalability issues or even OutOfMemoryExceptions.
               using (SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess)) {
                  while (await reader.ReadAsync()) {
                     Console.Write("{0}: ", reader.GetInt32(0));

                     if (await reader.IsDBNullAsync(1)) {
                        Console.Write("(NULL)");
                     }
                     else {
                        char[] buffer = new char[4096];
                        int charsRead = 0;
                        using (TextReader data = reader.GetTextReader(1)) {
                           do {
                              // Grab each chunk of text and write it to the console.
                              // If you are writing to a TextWriter, you should use WriteAsync or WriteLineAsync.
                              charsRead = await data.ReadAsync(buffer, 0, buffer.Length);
                              Console.Write(buffer, 0, charsRead);
                           } while (charsRead > 0);
                        }
                     }

                     Console.WriteLine();
                  }
               }
            }
         }
      }

      // Transfer a large Xml Document from SQL Server in .NET Framework 4.5.
      private static async Task PrintXmlValues() {
         using (SqlConnection connection = new SqlConnection(connectionString)) {
            await connection.OpenAsync();
            using (SqlCommand command = new SqlCommand("SELECT [id], [xmldata] FROM [Streams]", connection)) {

               // The reader needs to be executed with the SequentialAccess behavior to enable network streaming.
               // Otherwise ReadAsync will buffer the entire Xml Document into memory which can cause scalability issues or even OutOfMemoryExceptions.
               using (SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess)) {
                  while (await reader.ReadAsync()) {
                     Console.WriteLine("{0}: ", reader.GetInt32(0));

                     if (await reader.IsDBNullAsync(1)) {
                        Console.WriteLine("\t(NULL)");
                     }
                     else {
                        using (XmlReader xmlReader = reader.GetXmlReader(1)) {
                           int depth = 1;
                           // NOTE: The XmlReader returned by GetXmlReader does NOT support async operations.
                           // See the example below (PrintXmlValuesViaNVarChar) for how to get an XmlReader with asynchronous capabilities.
                           while (xmlReader.Read()) {
                              switch (xmlReader.NodeType) {
                                 case XmlNodeType.Element:
                                    Console.WriteLine("{0}<{1}>", new string('\t', depth), xmlReader.Name);
                                    depth++;
                                    break;
                                 case XmlNodeType.Text:
                                    Console.WriteLine("{0}{1}", new string('\t', depth), xmlReader.Value);
                                    break;
                                 case XmlNodeType.EndElement:
                                    depth--;
                                    Console.WriteLine("{0}</{1}>", new string('\t', depth), xmlReader.Name);
                                    break;
                              }
                           }
                        }
                     }
                  }
               }
            }
         }
      }

      // Transfer a large Xml Document from SQL Server in .NET Framework 4.5.
      // This goes via NVarChar and TextReader to enable asynchronous reading.
      private static async Task PrintXmlValuesViaNVarChar() {
         XmlReaderSettings xmlSettings = new XmlReaderSettings() {
            // Async must be explicitly enabled in the XmlReaderSettings otherwise the XmlReader will throw exceptions when async methods are called.
            Async = true,
            // Since we will immediately wrap the TextReader we are creating in an XmlReader, we will permit the XmlReader to take care of closing\disposing it.
            CloseInput = true,
            // If the Xml you are reading is not a valid document (as per <https://video2.skills-academy.com/previous-versions/dotnet/netframework-4.0/6bts1x50(v=vs.100)>) you will need to set the conformance level to Fragment.
            ConformanceLevel = ConformanceLevel.Fragment
         };

         using (SqlConnection connection = new SqlConnection(connectionString)) {
            await connection.OpenAsync();

            // Cast the XML into NVarChar to enable GetTextReader - trying to use GetTextReader on an XML type will throw an exception.
            using (SqlCommand command = new SqlCommand("SELECT [id], CAST([xmldata] AS NVARCHAR(MAX)) FROM [Streams]", connection)) {

               // The reader needs to be executed with the SequentialAccess behavior to enable network streaming.
               // Otherwise ReadAsync will buffer the entire Xml Document into memory which can cause scalability issues or even OutOfMemoryExceptions.
               using (SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess)) {
                  while (await reader.ReadAsync()) {
                     Console.WriteLine("{0}:", reader.GetInt32(0));

                     if (await reader.IsDBNullAsync(1)) {
                        Console.WriteLine("\t(NULL)");
                     }
                     else {
                        // Grab the row as a TextReader, then create an XmlReader on top of it.
                        // The code doesn't keep a reference to the TextReader since the XmlReader is created with the "CloseInput" setting (so it will close the TextReader when needed).
                        using (XmlReader xmlReader = XmlReader.Create(reader.GetTextReader(1), xmlSettings)) {
                           int depth = 1;
                           // The XmlReader above now supports asynchronous operations, so we can use ReadAsync here.
                           while (await xmlReader.ReadAsync()) {
                              switch (xmlReader.NodeType) {
                                 case XmlNodeType.Element:
                                    Console.WriteLine("{0}<{1}>", new string('\t', depth), xmlReader.Name);
                                    depth++;
                                    break;
                                 case XmlNodeType.Text:
                                    // Depending on what your data looks like, you should either use Value or GetValueAsync.
                                    // Value has less overhead (since it doesn't create a Task), but it may also block if additional data is required.
                                    Console.WriteLine("{0}{1}", new string('\t', depth), await xmlReader.GetValueAsync());
                                    break;
                                 case XmlNodeType.EndElement:
                                    depth--;
                                    Console.WriteLine("{0}</{1}>", new string('\t', depth), xmlReader.Name);
                                    break;
                              }
                           }
                        }
                     }
                  }
               }
            }
         }
      }
   }
}

Beispiel: Streaming zu SQL Server

Verwenden Sie das folgende Transact-SQL, um die Beispieldatenbank zu erstellen:

CREATE DATABASE [Demo2]
GO
USE [Demo2]
GO
CREATE TABLE [BinaryStreams] (
[id] INT PRIMARY KEY IDENTITY(1, 1),
[bindata] VARBINARY(MAX))
GO
CREATE TABLE [TextStreams] (
[id] INT PRIMARY KEY IDENTITY(1, 1),
[textdata] NVARCHAR(MAX))
GO
CREATE TABLE [BinaryStreamsCopy] (
[id] INT PRIMARY KEY IDENTITY(1, 1),
[bindata] VARBINARY(MAX))
GO

Das Beispiel erläutert die folgenden Aufgaben:

Übertragen eines großen BLOBs an SQL Server in .NET Framework 4.5.
Übertragen einer großen Textdatei zu SQL Server in .NET Framework 4.5.
Verwenden der neuen asynchronen Funktion zur Übertragung eines großen BLOBs.
Verwenden des neuen asynchronen Features und des await-Schlüsselworts zur Übertragung eines großen BLOBs.
Abbrechen der Übertragung eines großen BLOBs.
Streamen von einer SQL Server-Instanz zu einer anderen mithilfe des neuen asynchronen Features.

using System;
using System.Data;
using System.Data.SqlClient;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

namespace StreamingToServer {
   class Program {
      private const string connectionString = @"...";

      static void Main(string[] args) {
         CreateDemoFiles();

         StreamBLOBToServer().Wait();
         StreamTextToServer().Wait();

         // Create a CancellationTokenSource that will be cancelled after 100ms
         // Typically this token source will be cancelled by a user request (e.g. a Cancel button).
         CancellationTokenSource tokenSource = new CancellationTokenSource();
         tokenSource.CancelAfter(100);
         try {
            CancelBLOBStream(tokenSource.Token).Wait();
         }
         catch (AggregateException ex) {
            // Cancelling an async operation will throw an exception.
            // Since we are using the Task's Wait method, this exception will be wrapped in an AggregateException.
            // If you were using the 'await' keyword, the compiler would take care of unwrapping the AggregateException.
            // Depending on when the cancellation occurs, you can either get an error from SQL Server or from .Net.
            if ((ex.InnerException is SqlException) || (ex.InnerException is TaskCanceledException)) {
               // This is an expected exception.
               Console.WriteLine("Got expected exception: {0}", ex.InnerException.Message);
            }
            else {
               // Did not expect this exception - rethrow it.
               throw;
            }
         }

         Console.WriteLine("Done");
      }

      // This is used to generate the files which are used by the other sample methods.
      private static void CreateDemoFiles() {
         Random rand = new Random();
         byte[] data = new byte[1024];
         rand.NextBytes(data);

         using (FileStream file = File.Open("binarydata.bin", FileMode.Create)) {
            file.Write(data, 0, data.Length);
         }

         using (StreamWriter writer = new StreamWriter(File.Open("textdata.txt", FileMode.Create))) {
            writer.Write(Convert.ToBase64String(data));
         }
      }

      // Transfer a large BLOB to SQL Server in .NET Framework 4.5.
      private static async Task StreamBLOBToServer() {
         using (SqlConnection conn = new SqlConnection(connectionString)) {
            await conn.OpenAsync();
            using (SqlCommand cmd = new SqlCommand("INSERT INTO [BinaryStreams] (bindata) VALUES (@bindata)", conn)) {
               using (FileStream file = File.Open("binarydata.bin", FileMode.Open)) {

                  // Add a parameter which uses the FileStream we just opened
                  // Size is set to -1 to indicate "MAX".
                  cmd.Parameters.Add("@bindata", SqlDbType.Binary, -1).Value = file;

                  // Send the data to the server asynchronously.
                  await cmd.ExecuteNonQueryAsync();
               }
            }
         }
      }

      // Transfer a large Text File to SQL Server in .NET Framework 4.5.
      private static async Task StreamTextToServer() {
         using (SqlConnection conn = new SqlConnection(connectionString)) {
            await conn.OpenAsync();
            using (SqlCommand cmd = new SqlCommand("INSERT INTO [TextStreams] (textdata) VALUES (@textdata)", conn)) {
               using (StreamReader file = File.OpenText("textdata.txt")) {

                  // Add a parameter which uses the StreamReader we just opened.
                  // Size is set to -1 to indicate "MAX".
                  cmd.Parameters.Add("@textdata", SqlDbType.NVarChar, -1).Value = file;

                  // Send the data to the server asynchronously.
                  await cmd.ExecuteNonQueryAsync();
               }
            }
         }
      }

      // Cancel the transfer of a large BLOB.
      private static async Task CancelBLOBStream(CancellationToken cancellationToken) {
         using (SqlConnection conn = new SqlConnection(connectionString)) {
            // We can cancel not only sending the data to the server, but also opening the connection.
            await conn.OpenAsync(cancellationToken);

            // Artificially delay the command by 100ms.
            using (SqlCommand cmd = new SqlCommand("WAITFOR DELAY '00:00:00:100';INSERT INTO [BinaryStreams] (bindata) VALUES (@bindata)", conn)) {
               using (FileStream file = File.Open("binarydata.bin", FileMode.Open)) {

                  // Add a parameter which uses the FileStream we just opened.
                  // Size is set to -1 to indicate "MAX".
                  cmd.Parameters.Add("@bindata", SqlDbType.Binary, -1).Value = file;

                  // Send the data to the server asynchronously.
                  // Pass the cancellation token such that the command will be cancelled if needed.
                  await cmd.ExecuteNonQueryAsync(cancellationToken);
               }
            }
         }
      }
   }
}

Beispiel: Streaming von einer SQL Server-Instanz zu einer anderen SQL Server-Instanz

Dieses Beispiel zeigt, wie ein großes BLOB asynchron von einer SQL Server-Instanz zu einer anderen gestreamt wird, wobei der Abbruch unterstützt wird.

using System;
using System.Data;
using System.Data.SqlClient;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

namespace StreamingFromServerToAnother {
   class Program {
      private const string connectionString = @"...";

      static void Main(string[] args) {
         // For this example, we don't want to cancel,
         // so pass in a "blank" cancellation token.
         E2EStream(CancellationToken.None).Wait();

         Console.WriteLine("Done");
      }

      // Streaming from one SQL Server to Another One using the new Async.NET.
      private static async Task E2EStream(CancellationToken cancellationToken) {
         using (SqlConnection readConn = new SqlConnection(connectionString)) {
            using (SqlConnection writeConn = new SqlConnection(connectionString)) {

               // Note that we are using the same cancellation token for calls to both connections\commands.
               // Also we can start both the connection opening asynchronously, and then wait for both to complete.
               Task openReadConn = readConn.OpenAsync(cancellationToken);
               Task openWriteConn = writeConn.OpenAsync(cancellationToken);
               await Task.WhenAll(openReadConn, openWriteConn);

               using (SqlCommand readCmd = new SqlCommand("SELECT [bindata] FROM [BinaryStreams]", readConn)) {
                  using (SqlCommand writeCmd = new SqlCommand("INSERT INTO [BinaryStreamsCopy] (bindata) VALUES (@bindata)", writeConn)) {

                     // Add an empty parameter to the write command which will be used for the streams we are copying.
                     // Size is set to -1 to indicate "MAX".
                     SqlParameter streamParameter = writeCmd.Parameters.Add("@bindata", SqlDbType.Binary, -1);

                     // The reader needs to be executed with the SequentialAccess behavior to enable network streaming.
                     // Otherwise ReadAsync will buffer the entire BLOB into memory which can cause scalability issues or even OutOfMemoryExceptions.
                     using (SqlDataReader reader = await readCmd.ExecuteReaderAsync(CommandBehavior.SequentialAccess, cancellationToken)) {
                        while (await reader.ReadAsync(cancellationToken)) {
                           // Grab a stream to the binary data in the source database.
                           using (Stream dataStream = reader.GetStream(0)) {

                              // Set the parameter value to the stream source that was opened.
                              streamParameter.Value = dataStream;

                              // Asynchronously send data from one database to another.
                              await writeCmd.ExecuteNonQueryAsync(cancellationToken);
                           }
                        }
                     }
                  }
               }
            }
         }
      }
   }
}

Weitere Informationen

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