If it's just 5 comments, then the computer can quickly get them all from database and count how many of them are there. Now imagine if there is 50 000 comments and suddenly, you me and entire website ask "how many comments are there for this post?"

Suddenly the computer is overwhelmed by the request and you may end up crashing it due to amount of tasks it has to do.

It's way faster if instead of all of that, the computer kept track of a number of all comments and simply adjust it when comment is added or removed. It does not have to get all the comments and count how many are there, just simply return the number and you are done.

But in the essence, you sacriface potential accuracy for speed. You may accidentally "desynchronize" the counter - if an user requests a removal of the same comment twice, and you don't check if that comment was not removed. Or, in theory, if two separate users add or remove a comment at the same time. This is called "race condition", which is common in multi-threaded computing.

[+] PeleSpirit@lemmy.world 1 points 1 year ago* (last edited 10 months ago) (1 children)

[deleted]

[–] athlon@lemm.ee 3 points 1 year ago (1 children)

First method does not store the number itself anywhere. Let's assume that you store apples. I come and ask you "How many apples do you have?". To answer, you go and count every single apple one by one and return me the number. It's very easy if you have a small number of apples, but if you have, let's say, 5000 apples - you can see how long it may take.

Second option is you keeping a track of how many apples you have in stock by having it written down somewhere. If I ask you "How many apples do you have?" you just pull out your notepad and tell me the number. If you give me an apple, you just adjust the number you have written down already.