Supporto dello streaming in SqlClient
Si applica a: 
.NET Framework .NET .NET Standard
Il supporto dello streaming tra SQL Server e un'altra applicazione supporta i dati non strutturati nel server (documenti, immagini e file multimediali). Un database SQL Server può archiviare oggetti binari di grandi dimensioni (BLOB), ma il recupero dei BLOB può impegnare una quantità consistente di memoria.
Il supporto del streaming da e verso SQL Server semplifica la creazione di applicazioni che trasmettono i dati, senza dover caricare completamente i dati in memoria, con conseguente riduzione del numero di eccezioni di overflow di memoria.
Il supporto dello streaming consentirà anche una migliore scalabilità delle applicazioni di livello intermedio, soprattutto in scenari in cui gli oggetti business si connettono ad Azure SQL per inviare, recuperare e gestire BLOB di grandi dimensioni.
Avviso
I membri che supportano lo streaming vengono usati per recuperare i dati dalle query e per passare i parametri a query e stored procedure. La funzionalità di streaming è rivolta agli scenari di migrazione dei dati e OLTP di base ed è applicabile agli ambienti di migrazione dei dati locali e off-premise.
Supporto dello streaming da SQL Server
Il supporto dello streaming da SQL Server introduce la nuova funzionalità nelle classi DbDataReader e SqlDataReader per ottenere gli oggetti Stream, XmlReader e TextReader e rispondere ad essi. Queste classi vengono usate per recuperare i dati dalle query. Di conseguenza, il supporto dello streaming da SQL Server è destinato agli scenari OLTP e si applica agli ambienti locali e off-premise.
I seguenti membri sono stati aggiunti a SqlDataReader per abilitare il supporto dello streaming da SQL Server:
I seguenti membri sono stati aggiunti a DbDataReader per abilitare il supporto dello streaming da SQL Server:
Supporto dello streaming verso SQL Server
Il supporto del flusso verso SQL Server è disponibile nella classe SqlParameter cosicché può accettare e rispondere agli oggetti XmlReader, Stream e TextReader. SqlParameter viene usato per passare i parametri a query e stored procedure.
Nota
L'eliminazione di un oggetto SqlCommand o la chiamata di Cancel deve annullare qualsiasi operazione di flusso. Se un'applicazione invia CancellationToken, l'annullamento non è garantito.
I seguenti tipi SqlDbType accetteranno Value di Stream:
Binario
VarBinary
I seguenti tipi SqlDbType accetteranno Value di TextReader:
Char
NChar
NVarChar
Xml
Il tipo XmlSqlDbType accetterà Value di XmlReader.
SqlValue può accettare valori di tipo XmlReader, TextReader e Stream.
Gli oggetti XmlReader, TextReader e Stream verranno trasferiti fino al valore definito da Size.
Esempio: streaming da SQL Server
Usare il codice Transact-SQL seguente per creare il database di esempio:
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
Nell'esempio vengono descritte le operazioni seguenti:
Evitare di bloccare un thread di interfaccia utente fornendo una modalità asincrona per recuperare i file di grandi dimensioni.
Trasferire un file di testo di grandi dimensioni da SQL Server in .NET.
Trasferire un file con estensione XML di grandi dimensioni da SQL Server in .NET.
Recuperare i dati da SQL Server.
Trasferire file di grandi dimensioni (BLOB) da un database SQL Server a un altro senza esaurire la memoria.
using System;
using System.Data;
using Microsoft.Data.SqlClient;
using System.IO;
using System.Threading.Tasks;
using System.Xml;
namespace StreamingFromServer
{
class Program
{
private const string connectionString = @"Server=localhost;Database=Demo;Integrated Security=true";
static void Main(string[] args)
{
CopyBinaryValueToFile().Wait();
PrintTextValues().Wait();
PrintXmlValues().Wait();
PrintXmlValuesViaNVarChar().Wait();
Console.WriteLine("Done");
}
// Application retrieving a large BLOB from SQL Server in .NET 4.5 using the new asynchronous capability
private static async Task CopyBinaryValueToFile()
{
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("binarydata.bin", 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);
}
}
}
}
}
}
}
}
// Application transferring a large Text File from SQL Server
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();
}
}
}
}
}
// Application transferring a large Xml Document from SQL Server
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;
}
}
}
}
}
}
}
}
}
// Application transferring a large Xml Document from SQL Server
// 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://docs.microsoft.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
// We are not keeping 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;
}
}
}
}
}
}
}
}
}
}
}
Esempio: streaming verso SQL Server
Usare il codice Transact-SQL seguente per creare il database di esempio:
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
Nell'esempio vengono descritte le operazioni seguenti:
Trasferimento di un BLOB di grandi dimensioni verso SQL Server in .NET.
Trasferimento di un file di testo di grandi dimensioni verso SQL Server in .NET.
Utilizzo della nuova funzionalità asincrona per trasferire un BLOB di grandi dimensioni.
Uso della nuova funzionalità asincrona e della parola chiave await per trasferire un BLOB di grandi dimensioni.
Annullamento del trasferimento di un BLOB di grandi dimensioni.
Streaming da un database SQL Server a un altro usando la nuova funzionalità asincrona.
using System;
using System.Data;
using Microsoft.Data.SqlClient;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
namespace StreamingToServer
{
class Program
{
private const string connectionString = @"Server=localhost;Database=Demo2;Integrated Security=true";
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 - re-throw 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));
}
}
// Application transferring a large BLOB to SQL Server
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();
}
}
}
}
// Application transferring a large Text File to SQL Server
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();
}
}
}
}
// Cancelling 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);
}
}
}
}
}
}
Esempio: streaming da un database SQL Server a un altro database SQL Server
Questo esempio illustra come trasmettere in modo asincrono un BLOB di grandi dimensioni da un database SQL Server a un altro, con il supporto per l'annullamento.
Nota
Prima di eseguire l'esempio seguente, assicurarsi di aver creato i database Demo e Demo2.
using System;
using System.Data;
using Microsoft.Data.SqlClient;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
namespace StreamingFromServerToAnother
{
class Program
{
private const string connectionString = @"Server=localhost;Database=Demo2;Integrated Security=true";
static void Main(string[] args)
{
// For this example, we don't want to cancel
// So we can pass in a "blank" cancellation token
E2EStream(CancellationToken.None).Wait();
Console.WriteLine("Done");
}
// Streaming from one SQL Server to Another One
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);
}
}
}
}
}
}
}
}
}
}