Waiting Between Web Service Requests In Power Query

Often when you’re working with web services in Power Query you’ll be making a large number of requests to the web service in a small amount of time – and you’ll notice that your query seems to be a lot slower than you would expect. The reason is, of course, that many public websites and APIs implement throttling to prevent you from making too many requests all at once, I guess to prevent denial-of-service attacks.

While looking into this issue I found a very interesting post containing the solution to this problem on the Power Query forum, from Alejandro Lopez-Lago. In it he shows how to create a function that creates a delay of a specified number of seconds in a Power Query query. Here’s an example of how to use it in a query:

let
    //Find current time
    One = DateTime.LocalNow(),
    //Define Wait function
    Wait = (seconds as number, action as function) =>
            if (List.Count(
             List.Generate(
              () => DateTimeZone.LocalNow() + #duration(0,0,0,seconds),
              (x) => DateTimeZone.LocalNow() < x,
              (x) => x)
              ) = 0)
               then null else action(),
    //Call Wait function to wait 5 seconds
    //then return the current time again
    Two = Wait(5,DateTime.LocalNow),
    //Display start time, finish time, and duration
    Output = "Start Time: " & DateTime.ToText(One) &
                   " Finish Time:" & DateTime.ToText(Two) &
                   " Difference: " & Duration.ToText(Two-One)
in
    Output

Here’s an example of the output:

The important step in this query is Wait, which defines the function. It works by using List.Generate() to keep on generating a list until a given number of seconds (passed in though the parameter seconds) have elapsed; when that has happened, it will execute the function that is passed in through the parameter action. Very clever stuff!