How Bitcoin restricts the mining time?

Question

How Bitcoin restricts the mining time?

We know cryptocurrency systems restrict the mining time of a block to a certain limit for security reasons. For example, mining time for Bitcoin is ~10 minutes. Some other cryptocurrency system have different mining time limit like 2-5 mins and so on. That restriction is imposed with a constraint i.e. precondition of number of leading ZEROs in the calculating hash for miners.

The question is how the cryptocurrency algorithm becomes certain that the mining time will be exactly as the imposed limit for miners? Like, theoretically it is never possible to get other miners strength in the network, no? Is it just based on approximation?

If it is based on approximation, then, What if all the miners in a group for a particular block are accidentally chosen poor? In that case the mining time will be more than 10 mins. It can even be 20 mins. Won't it violate the limit constraint?

Answer 1

How Bitcoin restricts the mining time?

The mining time is not restricted at all. Blocks are simply found at random times with an expected interval of roughly ten minutes. Miners perform quintillions of hashes of block candidates per second, each of which has a minuscule chance of resulting in a sufficiently low block hash that is required for a valid block. Every 2016 blocks, the difficulty level gets adjusted to reset the expected interval to 10 minutes: if blocks were faster, difficulty increases, if blocks were slower, difficulty decreases.

What if all the miners in a group for a particular block are accidentally chosen poor?

There is no pre-selection of the miners, all miners are working all the time. Each miner has a separate set of block templates they're trying: the first transaction in a block pays the mining reward to the block's author. Since each miner is trying to pay themselves and they have different recipient addresses, each miner's blocktemplates are unique to themselves and thusly never redo work of another miner.

In that case the mining time will be more than 10 mins. It can even be 20 mins. Won't it violate the limit constraint?

There is no time limit. The next block is found whenever someone finds the next block. This varies wildly, sometimes the network produces five blocks in one minute, sometimes it takes ninety minutes until the next block is found.

Answer 2

Yes, It's based on approximation. Mining difficulty periodically updated to make sure the block time stay around 10 minutes.