Oraclize handling long response from cryptocompare

Question

I have very custom task and I need to make sure what are limits of each technology. I haven't found nothing similair in this forum yet. I am using Oraclize to get response from this url:

https://min-api.cryptocompare.com/data/histominute?fsym=ETH&tsym=USD&limit=1&aggregate=30&e=Kraken&extraParams=your_app_name.data

and I get such response:

{"Response":"Success","Type":100,"Aggregated":true,"Data":[{"time":1536559200,"close":197.88,"high":197.99,"low":196.45,"open":197.71,"volumefrom":948.44,"volumeto":186888.76},{"time":1536561000,"close":199.19,"high":199.35,"low":196.68,"open":197.88,"volumefrom":448.02,"volumeto":88870.37}], "TimeTo":1536562560,"TimeFrom":1536559200,"FirstValueInArray":true,"ConversionType":{"type":"force_direct","conversionSymbol":""}}

What I need to get is "open" price of first array element from Data which is 197.71 in this example and "close" price of second array element which is 199.19;

However, when I use my code, all I get is [] empty array or empty string. Here is the code:

function updatePriceSingle(uint256 timeStamp) payable {
   if (oraclize_getPrice("URL") > this.balance) {
       emit LogNewOraclizeQuery("Oraclize query was NOT sent, please add some ETH to cover for the query fee");
   } else {
       emit LogNewOraclizeQuery("Oraclize query was sent, standing by for the answer..");
       oraclize_query(timeStamp, "URL", "json(https://min-api.cryptocompare.com/data/histominute?fsym=ETH&tsym=USD&limit=1&aggregate=30e=CCCAGG).Data",200000);
   }

}

and here is callback:

function __callback(bytes32 myid, string result) {
  if (msg.sender != oraclize_cbAddress()) revert();
  emit LogPriceUpdated(result);
}

I know workaround to make 2 calls json(...).Data[0] and json(...).Data[1], but I want to save gas and get results from 1 call in such case above. Is it possible ? Maybe there are solutions using "Test query" in http://app.oraclize.it/home/test_query . Thank you in advance.

Answer 1

The reason you are getting an empty result, is that the URL has some incorrect parameters, which the API complains about, à la:

{"Response":"Error","Message":"aggregate param is not an integer.","Type":1,"Aggregated":false,"Data":[]}

I didn't read the API docs, so for the sake of quickness, just removed the aggregate part being complained about. I was able to achieve parsing of the open/close keys from the response using the following:

json(https://min-api.cryptocompare.com/data/histominute?fsym=ETH&tsym=USD&limit=1).Data[:][open,close]

You can refer to JSONPath docs/testers for more info on achieving this, Oraclize is using a specific standards version referenced in their documentation.

Here's a link to the test_query page with the above: http://app.oraclize.it/home/test_query#VVJMKEdFVCk=:anNvbihodHRwczovL21pbi1hcGkuY3J5cHRvY29tcGFyZS5jb20vZGF0YS9oaXN0b21pbnV0ZT9mc3ltPUVUSCZ0c3ltPVVTRCZsaW1pdD0xKS5EYXRhWzpdW29wZW4sY2xvc2Vd

For more complex parsing, you can also use the computation datasource, which can additionally process various result, potentially saving computation costs on-chain.