I was looking at the list of M functions supported in custom connectors and not in Power BI Desktop (using the technique I blogged about here) in the latest version of the Power Query SDK when I came across an intriguing new function: List.ParallelInvoke(). It doesn’t seem to be documented anywhere, but I think I’ve worked out what it does and it’s very exciting!

Consider the following M function, declared in a custom connector:

SlowFunction = () as number =>
    Function.InvokeAfter(()=>1, #duration(0,0,0,5));

When you call it, it waits 5 seconds and returns the value 1. If you call it three times and sum up the results, as follows:

List.Sum({SlowFunction(), SlowFunction(), SlowFunction()})

…then after 15 seconds you get the value 3 back.

Now, consider the following expression:

List.Sum(
 List.ParallelInvoke(
  {SlowFunction, SlowFunction, SlowFunction}
 )
)

When this is evaluated in a custom connector, you get the value 3 back after 5 seconds – so it looks like List.ParallelInvoke() allows you to invoke a list of functions in parallel. There’s also an optional second parameter called concurrency, which seems to control the amount of parallelism. So, for example:

List.Sum(
 List.ParallelInvoke(
  {SlowFunction, SlowFunction, SlowFunction},
  2
 )
)

…returns after 10 seconds, suggesting that only two function calls at a time are invoked in parallel.

I can imagine all kinds of uses for this, for example making multiple parallel calls to data sources or doing expensive calculations in parallel. I wonder if it will ever be allowed to be used outside custom connectors?

UPDATE: see Curt Hagenlocher’s comment below for some important information about this function.