Compatibilidad con streaming de SqlClient
Gracias a la compatibilidad con el streaming entre SQL Server y una aplicación (nueva en .NET Framework 4.5), se pueden obtener datos no estructurados en el servidor (documentos, imágenes y archivos multimedia). Una base de datos de SQL Server puede almacenar objetos binarios grandes (BLOB), pero la recuperación de BLOB puede usar mucha memoria.
La compatibilidad con streaming hacia y desde SQL Server simplifica la escritura de aplicaciones que transmiten datos, sin necesidad de cargar totalmente los datos en la memoria, lo que da como resultado menos excepciones de desbordamiento de memoria.
La compatibilidad con streaming también permitirá que las aplicaciones de nivel intermedio escalen mejor, especialmente en escenarios donde los objetos comerciales establezcan conexión con SQL Azure para enviar, recuperar y manipular BLOB grandes.
Advertencia
Las llamadas asincrónicas no se admiten si una aplicación también usa la palabra clave de cadena de conexión Context Connection
.
Los miembros agregados para admitir streaming se usan para recuperar datos de consultas y para pasar parámetros a consultas y procedimientos almacenados. La característica de streaming aborda escenarios básicos de migración de datos y OLTP, y es aplicable a entornos de migración de datos locales y externos.
Compatibilidad con streaming desde SQL Server
La compatibilidad con streaming desde SQL Server presenta una nueva funcionalidad en las clases DbDataReader y SqlDataReader para obtener objetos Stream, XmlReader y TextReader y reaccionar ante ellos. Estas clases se usan para recuperar datos de consultas. Como resultado, la compatibilidad con streaming de SQL Server aborda los escenarios OLTP y se aplica a entornos locales y externos.
Los miembros siguientes se agregaron a SqlDataReader para habilitar la compatibilidad con streaming desde SQL Server:
Los miembros siguientes se agregaron a DbDataReader para habilitar la compatibilidad con streaming desde SQL Server:
Compatibilidad con streaming hacia SQL Server
La compatibilidad con streaming hacia SQL Server presenta una nueva funcionalidad en la clase SqlParameter, por lo que puede aceptar y reaccionar ante objetos XmlReader, Stream y TextReader. SqlParameter se usa para pasar parámetros a consultas y procedimientos almacenados.
Al desechar un objeto SqlCommand o llamar a Cancel se cancela cualquier operación de streaming. Si una aplicación envía CancellationToken, la cancelación no se puede garantizar.
Los siguientes tipos SqlDbType aceptarán Value de Stream:
- Binario
- VarBinary
Los siguientes tipos SqlDbType aceptarán Value de TextReader:
- Char
- NChar
- NVarChar
- Xml
El tipo XmlSqlDbType aceptará Value de XmlReader.
SqlValue puede aceptar valores de tipo XmlReader, TextReader y Stream.
Los objetos XmlReader, TextReader y Stream se transferirán al valor definido por Size.
Ejemplo: Streaming de SQL Server
Use el siguiente Transact-SQL para crear la base de datos de ejemplo:
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
El ejemplo muestra cómo hacer lo siguiente:
- Evitar bloquear un subproceso de interfaz de usuario al proporcionar una manera asincrónica para recuperar archivos grandes.
- Transferir un archivo de texto grande desde SQL Server en .NET Framework 4.5.
- Transferir un gran archivo XML desde SQL Server en .NET Framework 4.5.
- Recuperar datos desde SQL Server.
- Transferir archivos grandes (BLOB) desde una base de datos de SQL Server a otra sin quedarse sin memoria.
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://learn.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.
// 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;
}
}
}
}
}
}
}
}
}
}
}
Ejemplo: Streaming hacia SQL Server
Use el siguiente Transact-SQL para crear la base de datos de ejemplo:
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
El ejemplo muestra cómo hacer lo siguiente:
- Transferir un BLOB grande a SQL Server en .NET Framework 4.5.
- Transferir un archivo de texto grande a SQL Server en .NET Framework 4.5.
- Use la nueva característica asíncrona para transferir un BLOB grande.
- Use la nueva característica asíncrona y la palabra clave await para transferir un BLOB grande.
- Cancele la transferencia de un BLOB grande.
- Streaming de una instancia de SQL Server a otra mediante la nueva característica asíncrona.
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);
}
}
}
}
}
}
Ejemplo: Streaming de una instancia de SQL Server a otra instancia de SQL Server
En este ejemplo se muestra cómo realizar el streaming de un BLOB grande de forma asincrónica desde una instancia de SQL Server a otra, con compatibilidad para cancelación.
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);
}
}
}
}
}
}
}
}
}
}