Weird behavior with Sets in App Services Function

MongoDB Atlas App Services & Realm
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1

Hi,
This is my first time using the Atlas App Services, and while working on my app I faced a weird issue with functions which I’m not able to explain & figure out. I’m sure there is an explanation for the behavior, it will be great if someone can explain it.

So here it goes:
The below code is not exactly my app code, but it is sufficient to explain the behavior. I have created a function myFunc which has 2 nested loops for rows & columns. For every row I get 2 column numbers by calling another function getSetWithNumbers in the same file. This function returns these 2 numbers in a Set.

const COLS = 6;
const ROWS = 6;

function getSetWithNumbers(start, end) {
  const set = new Set();

  const num1 = start;
  let num2 = start + 2;
  if (num2 > end) {
    num2 = start + 2 - end;
  } else if (num2 === end) {
    num2 = 0;
  }

  set.add(num1);
  set.add(num2);

  console.log(
    `Added ${num1} & ${num2} to the set for row ${start}, set size: ${set.size}`
  );

  return set;
}

function myFunc(request, response) {
  for (let row = 0; row < ROWS; row++) {
    const mySet = getSetWithNumbers(row, ROWS);

    for (let col = 0; col < COLS; col++) {
      if (mySet.has(col)) {
        console.log(`Row-${row}: mySet has col ${col}: ${mySet.has(col)}`);
      }
    }
  }

  return 'Hello World!';
}

exports = myFunc;

Now here is the weird part:
If I call myFunc using my local node setup I get following logs (everything is as per the expectation):

Added 0 & 2 to the set for row 0, set size: 2
Row-0: mySet has col 0: true
Row-0: mySet has col 2: true
Added 1 & 3 to the set for row 1, set size: 2
Row-1: mySet has col 1: true
Row-1: mySet has col 3: true
Added 2 & 4 to the set for row 2, set size: 2
Row-2: mySet has col 2: true
Row-2: mySet has col 4: true
Added 3 & 5 to the set for row 3, set size: 2
Row-3: mySet has col 3: true
Row-3: mySet has col 5: true
Added 4 & 0 to the set for row 4, set size: 2
Row-4: mySet has col 0: true
Row-4: mySet has col 4: true
Added 5 & 1 to the set for row 5, set size: 2
Row-5: mySet has col 1: true
Row-5: mySet has col 5: true

But when I call this function by pushing it to App Services I get the following logs:

Added 0 & 2 to the set for row 0, set size: 2 
Row-0: mySet has col 0: true 
Row-0: mySet has col 2: true 
Added 1 & 3 to the set for row 1, set size: 2 
Row-1: mySet has col 1: true 
Added 2 & 4 to the set for row 2, set size: 2 
Row-2: mySet has col 2: true 
Added 3 & 5 to the set for row 3, set size: 2 
Row-3: mySet has col 3: true 
Added 4 & 0 to the set for row 4, set size: 2 
Row-4: mySet has col 0: true 
Row-4: mySet has col 4: true 
Added 5 & 1 to the set for row 5, set size: 2 
Row-5: mySet has col 5: true

As you can see there are missing entries / logs. What is causing this behavior?

If I replace the Set with a List (with the corresponding changes for add → push & has → includes), everything works perfectly.

This is bugging me for last 3-4 days, so better to have an explanation :slight_smile:

Thanks

2

Hi @Rajeev_R_Sharma,

Thank you for raising this: we could reproduce the behaviour, and indeed it looks like Set.has(…) isn’t returning the expected results.

We’re opening an internal ticket about the matter, and will keep this post updated.

3

Thank you @Paolo_Manna.

I was thinking that may be there is some limitation with functions I’m not aware about. Good to know that I found a valid issue.

4

Hi @Rajeev_R_Sharma,

The Team responsible of the underlying function engine has confirmed that the error is due to mishandling of integer values. Set.has(…) should still work for other types, though.

(To clarify: the problem is that the addition of two integers is returning a float, that Set.has() doesn’t compare properly)

For the time being, you can use your workaround with List: we don’t have a timeline for the resolution yet, if there’s one soon we’ll update this post.

Thank you for your help in identifying this bug!

5

Thanks for the update @Paolo_Manna.

Glad that I could help in finding the bug. Came across it so shared with the right people :-). Now that the team has identified the root cause, it will be fixed in due course.

Just wanted to add one more observation with regards to the same. I think this issue happens only if the added integers are not consecutive (index wise). If you add numbers which are consecutive then it works fine. I tried with the following changes:

function getSetWithNumbersNew(start, end) {
  const set = new Set();
  let logContent = ['Added'];
  for (let i = start; i < end; i++) {
    set.add(i);
    logContent.push(i);
  }

  logContent.push(`to the set for row ${start}, set size: ${set.size}`);

  console.log(logContent.join(' '));

  return set;
}

And called this function instead (also changed ROWS & COLUMNS to 4 to reduce the number of logs), and I get the same result with my local node setup as well as the atlas function.

Added 0 1 2 3 to the set for row 0, set size: 4 
Row-0: mySet has col 0: true 
Row-0: mySet has col 1: true 
Row-0: mySet has col 2: true 
Row-0: mySet has col 3: true 
Added 1 2 3 to the set for row 1, set size: 3 
Row-1: mySet has col 1: true 
Row-1: mySet has col 2: true 
Row-1: mySet has col 3: true 
Added 2 3 to the set for row 2, set size: 2 
Row-2: mySet has col 2: true 
Row-2: mySet has col 3: true 
Added 3 to the set for row 3, set size: 1 
Row-3: mySet has col 3: true

That is all from my side.

Thanks

6

Indeed the problematic operation is the addition

not the insertion in the set, per se: if you don’t have an addition, you don’t get into the issue.

7

Oh! my bad. I didn’t read the sentence properly.

And that explains the actual issue which I faced (not the above dummy code)

const randomInt = (min, max) => {
  return Math.floor(Math.random() * (max - min)) + min;
};

In my actual code I was inserting random entries to the Set using the above function. So float is getting returned from that addition here as well. If I moved that outside min to inside Math.floor then probably it would have worked.

Thanks for explaining :pray: :slight_smile: