I started looking at the AWS API and specifically the
Mechanical Turk Requester API the other day. After reading the overview and understanding the basics I wanted to test making a simple call to get a feel for it. I set up an account and got my access credentials (getting credentials is a whole other story as it involves two different websites and seems overly complicated but it's done for now). I figured the GetAccountBalance operation seemed like the best first option as I didn't want to create a HIT (Human Intelligence Task - their term for tasks you can create) and manage that. Since I hadn't added any money it should be easy for them to return a zero to me.
The web services are REST based and calling them seems pretty straightforward. You figure out the operation you want to perform, set some querystring parameters and make a web request. The complicated bit is calculating the signature (a SHA1 hash of the operation, service and current time using your secret access id as the key). Amazon doesn't have a nice simple example of constructing this in C# and even the other language examples don't look simple. There is a
SDK for .NET it looks like it does a good job wrapping the Amazon endpoints but I wanted to figure out the basics for myself. So by starting with the examples from the SDK I was able to simplify the code down to the following:
using System;
using System.Globalization;
using System.IO;
using System.Net;
using System.Security.Cryptography;
using System.Text;
namespace TurkTest {
class Program {
static void Main(string[] args) {
const string SERVICE_NAME = "AWSMechanicalTurkRequester"; // requester service for MTurk
const string TIMESTAMP_FORMAT = "yyyy-MM-ddTHH:mm:ss.fffZ";
// Modify these with your values.
const string operation = "GetAccountBalance";
const string accessKey = "[Your access key]";
const string secretAccessKey = "[Your secret access key]";
// Millisecond values in the timestamp string can result in intermittent BadClaimsSupplied errors.
// Get the current UTC time and use that to create a new time with milliseconds set to zero to avoid this case.
DateTime now = DateTime.UtcNow;
now = new DateTime(now.Year, now.Month, now.Day, now.Hour, now.Minute, now.Second, 0, DateTimeKind.Utc);
string timeStamp = now.ToString(TIMESTAMP_FORMAT, CultureInfo.InvariantCulture);
// Create the hash-based messaged authentication algorithm (SHA1) using our secret access key as the key.
var hmac = new HMACSHA1(Encoding.UTF8.GetBytes(secretAccessKey));
// Combine the service name, operation and timestamp and then hash them to produce the signature.
var dataBytes = Encoding.UTF8.GetBytes(SERVICE_NAME + operation + timeStamp);
string signature = Convert.ToBase64String(hmac.ComputeHash(dataBytes));
// Build the URL to send to Amazon
string url =
@"https://mechanicalturk.amazonaws.com/?Service=AWSMechanicalTurkRequester&AWSAccessKeyId={0}&Version=2012-03-25&Operation={1}&Signature={2}&Timestamp={3}";
url = string.Format(url, accessKey, operation, signature, timeStamp);
// Send the request and write the respose to the console
using (WebClient client = new WebClient()) {
using (StreamReader reader = new StreamReader(client.OpenRead(url))) {
Console.WriteLine(reader.ReadToEnd());
}
}
Console.Read();
}
}
}
After a couple of false starts I was able to get it to return my zero balance. Hope this helps the next person get started.
