How to use DataTables ajax as a function

Question

I have the following in DataTables 1.10.15

 var myTable = $('#myTable').DataTable( {
    "serverSide": true,
    "ajax": { 
        "url" : "/getData.php",
        "data" : function ( d ) {
            // Search input data
            d.ecNumber = $('#ecNumber').val(),
            d.casNumber = $('#casNumber').val(),
            d.substanceName = $('#substanceName').val()
        },
        "method" : "POST",
        "dataSrc": function ( json ) {
            if (json.form_errors) {
                // ...
            }
            return json.data;
        }
    },
});

My application works with some other JavaScript such that a user has to enter 3 or more characters before the ajax request happens (I use myTable.draw(); to trigger it).

I asked about how to abort ajax requests that may still be running and a reply I got was to set a flag to say whether or not the ajax request was running.

So in normal jquery I could do something like this:

var processing = null;

if (processing !== null) {
    processing.abort();
}

processing = $.ajax({
     // ...
     success: function(result){
         processing = null;
     }
});

I've had a look over the documentation about using ajax as a function. But I can't understand how to add the code - plus the flag - in to something like the example below. I understand that the callback has to be the data to go in the table (presumably json.data in my original code?). But how does the other code fit in to this?

$('#myTable').dataTable( {
  "ajax": function (data, callback, settings) {

  }
});