Suporte a streaming no SqlClient
Aplicável a: 
.NET Framework .NET .NET Standard
O suporte a streaming entre o SQL Server e um aplicativo aplica-se a dados não estruturados no servidor (documentos, imagens e arquivos de mídia). Um banco de dados do SQL Server pode armazenar BLOBs (objetos binários grandes), mas a recuperação de BLOBS pode usar muita memória.
O suporte a streaming no SQL Server simplifica a escrita de aplicativos que fazem streaming de dados, sem precisar carregar completamente os dados na memória, resultando em menos exceções de estouro de memória.
O suporte a streaming também permitirá que os aplicativos de camada intermediária sejam mais bem escalados, especialmente em cenários em que os objetos empresariais se conectam ao SQL do Azure para enviar, recuperar e manipular BLOBs grandes.
Aviso
Os membros que dão suporte a streaming são usados para recuperar dados de consultas e transmitir parâmetros para consultas e procedimentos armazenados. O recurso de streaming aborda os cenários básicos de OLTP e migração de dados e é aplicável a ambientes de migração de dados locais e externos.
Suporte a streaming do SQL Server
O suporte a streaming do SQL Server apresenta uma nova funcionalidade no DbDataReader e nas classes SqlDataReader para obter os objetos Stream, XmlReader e TextReader e fornecer uma resposta a eles. Essas classes são usadas para recuperar dados de consultas. Como resultado, o suporte a streaming do SQL Server aborda cenários de OLTP e aplica-se a ambientes locais e externos.
Os seguintes membros foram adicionados ao SqlDataReader para habilitar o suporte a streaming do SQL Server:
Os seguintes membros foram adicionados ao DbDataReader para habilitar o suporte a streaming do SQL Server:
Suporte a streaming para o SQL Server
O suporte a streaming para o SQL Server está na classe SqlParameter de modo que ele possa aceitar objetos XmlReader, Stream e TextReader e responder a eles. SqlParameter é usado para transmitir parâmetros para consultas e procedimentos armazenados.
Observação
Descartar um objeto SqlCommand ou chamar Cancel deve cancelar qualquer operação de streaming. Se um aplicativo enviar CancellationToken, o cancelamento não será garantido.
Os seguintes tipos de SqlDbType aceitarão um Value de Stream:
Binary
VarBinary
Os seguintes tipos de SqlDbType aceitarão um Value de TextReader:
Char
NChar
NVarChar
Xml
O tipo XmlSqlDbType aceitará um Value de XmlReader.
SqlValue pode aceitar valores do tipo XmlReader, TextReader e Stream.
O objeto XmlReader, TextReader e Stream serão transferidos até atingir o valor definido pelo Size.
Exemplo – Streaming do SQL Server
Use o seguinte Transact-SQL para criar o banco de dados de exemplo:
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
O exemplo a seguir mostra como fazer o seguinte:
Evite fechar um thread da interface do usuário fornecendo uma maneira assíncrona de recuperar arquivos grandes.
Transferir um arquivo de texto grande do SQL Server no .NET.
Transferir um arquivo XML grande do SQL Server no .NET.
Recuperar dados do SQL Server.
Transferir arquivos grandes (BLOBs) de um banco de dados do SQL Server para outro sem ficar sem memória.
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;
}
}
}
}
}
}
}
}
}
}
}
Exemplo – Streaming para o SQL Server
Use o seguinte Transact-SQL para criar o banco de dados de exemplo:
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
O exemplo a seguir mostra como fazer o seguinte:
Transferir um BLOB grande para o SQL Server no .NET.
Transferir um arquivo de texto grande para o SQL Server no .NET.
Usando o novo recurso assíncrono para transferir um BLOB grande.
Usando o novo recurso assíncrono e a palavra-chave await para transferir um BLOB grande.
Cancelar a transferência de um BLOB grande.
Streaming de um SQL Server para outro usando o recurso assíncrono.
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);
}
}
}
}
}
}
Exemplo – Streaming de um SQL Server para outro
Este exemplo demonstra como transmitir de modo assíncrono um BLOB grande de um SQL Server para outro, com suporte para cancelamento.
Observação
Antes de executar o exemplo a seguir, crie os bancos de dados 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);
}
}
}
}
}
}
}
}
}
}