处理Web API返回的大型JSON数据

Question

我们正在构建一个Web API,它接收字符串数组作为输入参数,该参数查询oracle数据库并将结果作为JSON文件返回.

所以代码就像

 namespace PDataController.Controllers
{
  public class ProvantisDataController : ApiController
  {
    public HttpResponseMessage Getdetails([FromUri] string[] id)
    {

       List<OracleParameter> prms = new List<OracleParameter>();
        string connStr = ConfigurationManager.ConnectionStrings["PDataConnection"].ConnectionString;
        using (OracleConnection dbconn = new OracleConnection(connStr))
        {
            var inconditions = id.Distinct().ToArray();
            var srtcon = string.Join(",", inconditions);
            DataSet userDataset = new DataSet();
            var strQuery = @"SELECT 
                           STCD_PRIO_CATEGORY_DESCR.DESCR AS CATEGORY, 
                           STCD_PRIO_CATEGORY_DESCR.SESSION_NUM AS SESSION_NUMBER, 
                           Trunc(STCD_PRIO_CATEGORY_DESCR.START_DATE) AS SESSION_START_DATE, 
                           STCD_PRIO_CATEGORY_DESCR.START_DATE AS SESSION_START_TIME , 
                           Trunc(STCD_PRIO_CATEGORY_DESCR.END_DATE) AS SESSION_END_DATE, 
                             FROM 
                             STCD_PRIO_CATEGORY_DESCR, 
                             WHERE 
                            STCD_PRIO_CATEGORY_DESCR.STD_REF IN(";
            StringBuilder sb = new StringBuilder(strQuery);
             for(int x = 0; x < inconditions.Length; x++)
                 {
                   sb.Append(":p" + x + ",");
                   OracleParameter p = new OracleParameter(":p" + x,OracleDbType.NVarchar2);
                   p.Value = inconditions[x];
                   prms.Add(p);
                 }
            if(sb.Length > 0) sb.Length--;
            strQuery = sb.ToString() + ")"; 
            using (OracleCommand selectCommand = new OracleCommand(strQuery, dbconn))
              {
               selectCommand.Parameters.AddRange(prms.ToArray());
                 using (OracleDataAdapter adapter = new OracleDataAdapter(selectCommand))
                {
                    DataTable selectResults = new DataTable();
                    adapter.Fill(selectResults);
                    var returnObject = new { data = selectResults };
                    var response = Request.CreateResponse(HttpStatusCode.OK, returnObject, MediaTypeHeaderValue.Parse("application/json"));
                    ContentDispositionHeaderValue contentDisposition = null;
                    if (ContentDispositionHeaderValue.TryParse("inline; filename=ProvantisStudyData.json", out contentDisposition))
                    {
                        response.Content.Headers.ContentDisposition = contentDisposition;
                    }
                    return response;
                }
            }

        }
    }
}
}

为API返回的数据采用以下格式

{"data":[{"CATEGORY":"Internal Study","SESSION_NUMBER":7,"SESSION_START_DATE":"2015-02-13T00:00:00","SESSION_START_TIME":"2015-02-13T10:33:59.288394","SESSION_END_DATE":"2015-02-13T00:00:00"}]}

我们有时会在返回大量数据时遇到问题,它会抛出OutOfMemory Exception. 有人建议使用与"data"属性并行的JSON属性:如"next_data",其值为您需要传递到SQL OFFSET的值(在MySQL中有效,我不确定这是否适用于oracle),如果没有剩余数据,则将"next_data"的值设置为0.我不知道如何实现它.不确定是否可以实现.非常感谢任何帮助.

Answer 1

您的问题是您运行的Oracle查询返回了大量结果,然后将整个结果集加载到内存中,然后再将其序列化为HttpResponseMessage.

为了减少内存使用量,您应该找到并消除将查询的整个结果集加载到临时中间表示(例如,DataTable或JSON字符串)中的所有情况,而是使用a将数据流出DataReader.根据这个答案,这可以避免一次将所有内容都拉入内存.

首先,从您的回溯中,您似乎已选中" 启用浏览器链接".由于这显然会尝试将整个响应缓存在a中MemoryStream,因此您需要将其禁用,如FilePathResult中所述,抛出带有大文件的OutOfMemoryException.

接下来,您可以IDataReader使用Json.NET通过以下类和转换器直接将内容流式传输到JSON:

[JsonConverter(typeof(OracleDataTableJsonResponseConverter))]
public sealed class OracleDataTableJsonResponse
{
    public string ConnectionString { get; private set; }
    public string QueryString { get; private set; }
    public OracleParameter[] Parameters { get; private set; }

    public OracleDataTableJsonResponse(string connStr, string strQuery, OracleParameter[] prms)
    {
        this.ConnectionString = connStr;
        this.QueryString = strQuery;
        this.Parameters = prms;
    }
}

class OracleDataTableJsonResponseConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(OracleDataTableJsonResponse);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException("OracleDataTableJsonResponse is only for writing JSON.  To read, deserialize into a DataTable");
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var response = (OracleDataTableJsonResponse)value;

        using (var dbconn = new OracleConnection(response.ConnectionString))
        {
            dbconn.Open();
            using (var selectCommand = new OracleCommand(response.QueryString, dbconn))
            {
                if (response.Parameters != null)
                    selectCommand.Parameters.AddRange(response.Parameters);
                using (var reader = selectCommand.ExecuteReader())
                {
                    writer.WriteDataTable(reader, serializer);
                }
            }
        }
    }
}

public static class JsonExtensions
{
    public static void WriteDataTable(this JsonWriter writer, IDataReader reader, JsonSerializer serializer)
    {
        if (writer == null || reader == null || serializer == null)
            throw new ArgumentNullException();
        writer.WriteStartArray();
        while (reader.Read())
        {
            writer.WriteStartObject();
            for (int i = 0; i < reader.FieldCount; i++)
            {
                writer.WritePropertyName(reader.GetName(i));
                serializer.Serialize(writer, reader[i]);
            }
            writer.WriteEndObject();
        }
        writer.WriteEndArray();
    }
}

然后修改您的代码,如下所示:

    public HttpResponseMessage Getdetails([FromUri] string[] id)
    {
        var prms = new List<OracleParameter>();
        var connStr = ConfigurationManager.ConnectionStrings["PDataConnection"].ConnectionString;
        var inconditions = id.Distinct().ToArray();
        var strQuery = @"SELECT 
                       STCD_PRIO_CATEGORY_DESCR.DESCR AS CATEGORY, 
                       STCD_PRIO_CATEGORY_DESCR.SESSION_NUM AS SESSION_NUMBER, 
                       Trunc(STCD_PRIO_CATEGORY_DESCR.START_DATE) AS SESSION_START_DATE, 
                       STCD_PRIO_CATEGORY_DESCR.START_DATE AS SESSION_START_TIME , 
                       Trunc(STCD_PRIO_CATEGORY_DESCR.END_DATE) AS SESSION_END_DATE, 
                         FROM 
                         STCD_PRIO_CATEGORY_DESCR, 
                         WHERE 
                        STCD_PRIO_CATEGORY_DESCR.STD_REF IN(";
        var sb = new StringBuilder(strQuery);
        for (int x = 0; x < inconditions.Length; x++)
        {
            sb.Append(":p" + x + ",");
            var p = new OracleParameter(":p" + x, OracleDbType.NVarchar2);
            p.Value = inconditions[x];
            prms.Add(p);
        }
        if (sb.Length > 0)// Should this be inconditions.Length > 0  ?
            sb.Length--;
        strQuery = sb.Append(")").ToString();

        var returnObject = new { data = new OracleDataTableJsonResponse(connStr, strQuery, prms.ToArray()) };
        var response = Request.CreateResponse(HttpStatusCode.OK, returnObject, MediaTypeHeaderValue.Parse("application/json"));
        ContentDispositionHeaderValue contentDisposition = null;
        if (ContentDispositionHeaderValue.TryParse("inline; filename=ProvantisStudyData.json", out contentDisposition))
        {
            response.Content.Headers.ContentDisposition = contentDisposition;
        }
        return response;
    }

这避免DataSet了结果的内存表示.

顺便说一下,我认为这条线

        if (sb.Length > 0)
            sb.Length--;

相反应该是:

        if (inconditions.Length > 0)
            sb.Length--;

我相信你正试图剥离查询中的尾随逗号,当且仅当如此 inconditions.Length > 0

请注意 - 我不是Oracle开发人员,我没有安装Oracle.为了测试我OracleClient使用底层模拟了类OleDbConnection,它运行良好.