处理Web API返回的大型JSON数据

Question

We are building an web API that receives the array of strings as input parameter which queries the oracle database and returns the result as a JSON file.

So the code is like

 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;
                }
            }

        }
    }
}
}

The data returned for the API is in the below format

{"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"}]}

We are sometimes having issue in returning the large amount of data it throws the OutOfMemory Exception. It was suggested to use the JSON property, parallel to the “data” property: like “next_data”, with a value of the value you need to pass into the SQL OFFSET (which works in MySQL, I am not sure if this works in oracle),if there no data remaining then set the value of “next_data” to 0.I am not sure how to implement this.Not sure if this can be implemented. Any help with this is greatly appreciated.

Answer 1

Your problem is that you are running an Oracle query that is returning a very large number of results, and then loading that entire result set into memory before serializing it out to the HttpResponseMessage .

To reduce your memory usage, you should find and eliminate all cases where the entire set of results from the query is loaded into a temporary intermediate representation (eg a DataTable or JSON string), and instead stream the data out using a DataReader . This avoids pulling everything into memory at once according to this answer .

First, from your traceback, it appears you have Enable Browser Link checked. Since this apparently tries to cache the entire response in a MemoryStream , you will want to disable it as explained in FilePathResult thrown an OutOfMemoryException with large file .

Next, you can stream the contents of an IDataReader directly to JSON using Json.NET with following class and converter:

[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();
    }
}

Then modify your code to look something like:

    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;
    }

This avoids the in-memory DataSet representation of the results.

Incidentally, I reckon the line

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

instead should be:

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

I believe you're trying to peel off the trailing comma in the query, which will be present if and only if inconditions.Length > 0

Please note - I'm not an Oracle developer and I don't have Oracle installed. For testing I mocked up the OracleClient classes using an underlying OleDbConnection and it worked fine.

Answer 2

Are you allowed to change your method to get that data? I mean, if you are using RESTful services its not a good idea to traffic so much data on a single request. Maybe downloading a file for that purpose or maybe getting the data by pagination.

You can also try to change the max request lenght like this answer: Change max request lenght

But again, it's not good for a web application to traffic and/or process so much data at once.