Connecting to Google App Engine

The main issue was that you wrote put, not PUT - but otherwise it looks like by default it uses the JSON API. You might be able to append .csv to the URL to fix that though? https://xively.com/dev/docs/api/data/write/multiple_datapoints_to_single_datastream/

counter = 1;
function putXively() {
    content = JSON.stringify({
     version : "1.0.0",
     datastreams : [ { id:"counter", current_value:counter++ } ]
    });
    var options = {
      host: 'api.xively.com',
      port: '80',
      path:'/v2/feeds/1848857974',
      method:'PUT',
      headers: { "X-ApiKey":"RMspyWqYIq1m9vEyuF9Q9BLpngLf4Tr1MlEhmDnHTbAjQvG8", "Content-Length":content.length }
    };
    var req = require("http").request(options, function(res)  {
        res.on('data', function(data) {
        print("->"+data);
        setTimeout(putXively, 2000);
      });
    });
    req.end(content);
}