limit_async_parallel_handler_calls.js

// some dummy rows
var rows = [];
for (var i = 0; i < 10000; i++) {
  rows.push("this is row " + i);
}

var index = 0;
var currentlyRunning = 0, maxRunning = 200;

function callHandler(row, cb) {
  console.log("callHandler for row: " + row);
  setTimeout(function() {
    cb(null);
  }, 500 * Math.random());
}

function printStack() {
  var e = new Error('dummy');
  var stack = e.stack.replace(/^[^\(]+?[\n$]/gm, '')
  .replace(/^\s+at\s+/gm, '')
  .replace(/^Object.<anonymous>\s*\(/gm, '{anonymous}()@')
           .split('\n');
           console.log(stack);
}

function run(ix) {
  console.log("run " + ix + ", currentlyRunning=" + currentlyRunning);
  if (ix % 50 == 0) printStack(); // just curious to see the stack from time to time
  currentlyRunning++;
  callHandler(rows[ix], function(err) {
    currentlyRunning--;
    if (index < rows.length && currentlyRunning < maxRunning) {
      run(index++);
    }
  });
};

while (index < rows.length && currentlyRunning < maxRunning) {
  run(index++);
}