When two fix the same thing at once
Two fixing the same thing at once can break the result. One at a time is the answer.
When two fix the same box
A note with a number is stuck on the wall. It says 10 now. Two people each want to add 1 to it.
One reads 10, makes 11 in their head, and writes 11 on the note. The other does the same. But if they start at nearly the same moment, both read the original 10 and both write 11.
Adding 1 twice should give 12, but the result is 11. One add vanished. It is an accident from fixing the same thing at the same moment.
When two fix the same value at once, one of the adds disappears
Fixing the same thing at the same moment can throw the result off.
Same action, different result
The same two people each add 1 the same way, but let us change only the timing.
In turn: one reads 10, writes 11, and finishes, then the other reads that 11 and writes 12. Result 12, correct.
Overlapping: one reads 10 and before writing yet, the other also reads the 10. Both write 11 and the result is 11, wrong.
The actions were literally the same. Only when someone cut in, that one piece of timing, decided right from wrong.
The same adding is right done in turn, wrong when overlapped
What decided the result was not the action but the timing of when someone cut in.
Overlap them yourself
Two workers each add 1 to the same value. Adjust how much they start overlapping.
Spread them far enough and one finishes before the other starts, so the result is right. Bring them close and one reads before the other writes, so an add vanishes.
Move the slider and see from what point the result breaks. The same two actions, yet the moment they overlap the result collapses.
Change the amount of overlap and see when the result breaks
Spreading them apart keeps the result right, and the more they overlap the higher the risk it breaks.
If they take turns one at a time
So how do we prevent it. The answer is simple. While the same thing is being fixed, let only one person touch it.
While one reads, changes, and writes, the others wait. That one bundle must finish before the next starts. Like lining up for a single restroom. If someone is inside, the door locks, and the next goes in only after they come out.
This way there is no overlap, so the result is always right. The cost is some waiting. For safety, you accept a short line.
While the same thing is being fixed, only one touches it and the rest line up
Letting one fix the same thing at a time removes overlap and makes the result safe.
Put it on one page
Just remember three things.
First, fixing the same thing at the same moment causes accidents. As two adding at once lose one add, the result can go wrong.
Second, the problem is timing, not the action. The same work is right in turn, wrong when overlapped. Third, letting one go at a time is safe. While the same thing is fixed, only one touches it and the rest line up, so the result is always right.
At-once plus shared is an accident, timing is the problem, one at a time is safe