解壓 SQL Blob 內容並在 PushStreamContent .NET Core 中返回答案

Question

I am developing a new API in a .NET Core service, the new API is supposed to read a BLOB from SQL table, decompress it using DeflateStream. 然后將其返回（流式傳輸）給客戶端。

為了不消耗太多 memory。 I am returning a response of type and PushStreamContent so that I could copy the sql stream into the response stream directly without loading the blob in memory. 所以我最終得到了類似的東西。

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")
        });

這里的問題是，將 deflateStream 復制到響應 output stream 的步驟永遠如代碼中所述。 盡管我嘗試了相同的確切方法，但是將 stream 寫入文件而不是將其復制到相應的 stream 中，它就像一個魅力。

所以你們能幫我解決這個問題嗎？？ 使用 PushStreamContent 我錯了嗎？ 我應該使用不同的方法嗎？ 問題是我不想在 memory 中加載整個 Blob，我想讀取它並即時解壓縮。 SqlClient 支持流blob，我想利用它。

Answer 1

這是 PushStreamContent 中的一個死鎖，我不假裝理解。 但我復制了它並改變了

await decompressionStream.CopyToAsync(outStream);

至

decompressionStream.CopyTo(outStream);

解決它。

這是完整的復制品：

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")
    });
}