c# - 解压 SQL Blob 内容并在 PushStreamContent .NET Core 中返回答案

Question

我正在.NET Core 服务中开发一个新的 API，新的 API 应该从 SQL 表中读取一个 BLOB，使用 DeflateStream 解压缩它。然后将其返回（流式传输）给客户端。

为了不消耗太多内存。我正在返回类型和PushStreamContent的响应，以便我可以将 sql 流直接复制到响应流中，而无需将 blob 加载到内存中。所以我最终得到了类似的东西。

return this.ResponseMessage(new HttpResponseMessage
        {
            Content = new PushStreamContent(async (outStream, httpContent, transportContext) =>
            {
                using (SqlConnection connection = new SqlConnection(connectionString))
                {
                    await connection.OpenAsync();
                    using (SqlCommand command = new SqlCommand(query, connection))
                    {

                        // 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() && !(await reader.IsDBNullAsync(0)))
                            {
                                using (Stream streamToDecompress = reader.GetStream(0))
                                using (Stream decompressionStream = new DeflateStream(streamToDecompress, CompressionMode.Decompress))
                                {
                                    // This copyToAsync will take for ever
                                    await decompressionStream.CopyToAsync(outStream);
                                    outStream.close();

                                    return;
                                }
                            }

                            throw new Exception("Couldn't retrieve blob");
                        }
                    }
                }
            },
            "application/octet-stream")
        });

这里的问题是，将 deflateStream 复制到响应输出流的步骤需要永远如代码中所述。尽管我尝试了相同的确切方法，但是将流写入文件而不是将其复制到 resp 流，它就像一个魅力。

所以你们能帮我解决这个问题吗？？使用 PushStreamContent 我错了吗？我应该使用不同的方法吗？问题是我不想将整个 Blob 加载到内存中，我想读取它并即时解压缩它。SqlClient 支持流blob，我想利用它。

标签： c#.net.net-coresqlclientdeflatestream

Answer 1

这是 PushStreamContent 中的一个死锁，我不假装理解。但我复制了它并改变了

await decompressionStream.CopyToAsync(outStream);

至

decompressionStream.CopyTo(outStream);

解决它。

这是完整的复制品：

public ResponseMessageResult Get()
{
    var data =  new string[] { "value1", "value2" };

    var jsonData = Newtonsoft.Json.JsonConvert.SerializeObject(data);

    var msSource = new MemoryStream(Encoding.UTF8.GetBytes(jsonData));
    var msDest = new MemoryStream();
    var compressionStream = new DeflateStream(msDest, CompressionMode.Compress);
    msSource.CopyTo(compressionStream);
    compressionStream.Close();

    var compressedBytes = msDest.ToArray();

    var query = "select @bytes buf";
    var connectionString = "server=localhost;database=tempdb;integrated security=true";

    
    return this.ResponseMessage(new HttpResponseMessage
    {
        Content = new PushStreamContent(async (outStream, httpContent, transportContext) =>
        {
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                await connection.OpenAsync();
                using (SqlCommand command = new SqlCommand(query, connection))
                {
                    command.Parameters.Add("@bytes", SqlDbType.VarBinary, -1).Value = compressedBytes;

                    // 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() && !(await reader.IsDBNullAsync(0)))
                        {
                            using (Stream streamToDecompress = reader.GetStream(0))
                            {
                                //var buf = new MemoryStream();
                                //streamToDecompress.CopyTo(buf);
                                //buf.Position = 0;

                                using (Stream decompressionStream = new DeflateStream(streamToDecompress, CompressionMode.Decompress))
                                {
                                    
                                    // This copyToAsync will take for ever
                                    //await decompressionStream.CopyToAsync(outStream);
                                    decompressionStream.CopyTo(outStream);
                                    outStream.Close();

                                    return;
                                }
                            }
                        }

                        throw new Exception("Couldn't retrieve blob");
                    }
                }
            }
        },
    "application/octet-stream")
    });
}