So by using the consensus mechanism the blockchain network gets prevention from external malicious attacks i.e. adding a random block in the blockchain. If any miner or validator of the blockchain network tries to add the malicious block(collection of invalid transactions) in the blockchain then the consensus protocol used in the blockchain network didn't allow such activities in the blockchain. Let's talk about the algorithms about consensus mechanism. Mostly there are 2 consensus protocols used these days:

- Proof of work

- Proof of stake

Proof of Work

Proof of work is first applied in Bitcoin to validate the transactions in the blockchain which is also used in the Ethereum and other blockchain networks for transaction validation purposes. In this method, the miners have to solve complex mathematical problems to get the hash(Tha hash should include the values inside the target value which is defined by the algorithm in the blockchain. The target values changes in a certain period of time.) of the block with the valid set of transactions. After calculating the valid hash and adding the block in the blockchain the miners get the rewards. In proof of work, the miners should have a rich set of resources for calculating the mathematical problem which costs a large amount of money.

Proof of Stake

Proof of stake is first introduced by the Cardano blockchain network in which the validation process of a block is done by staking. In proof of stake, there are validators instead of miners the validators should deposit certain amount of crypto to take part in the validation process and the system selects the validators based on the deposited value of crypto. The more crypto validator deposits the more chances of him/her to get selected to validate the blocks and earn rewards. In this method, the validation can be done with any kind of device. The only thing that is needed to take part in validation is the stake(money or coins or tokens).

How does Blockchain Prevent Double Spending?

To understand more about double spending we need to know about the working of block creation in Blockchain. In the blockchain, the coin will only be deducted from the sender's account only after the transaction is verified and included in the Block by using a consensus mechanism i.e proof of work till then the user can send that coin to multiple addresses. Because In blockchain transactions aren't directly included in the block initially transactions are placed in the memory pool which is also known as unconfirmed transactions, Once the miner picks one of the transactions initiated by the user and adds it to the blockchain then the user's wallet will be updated with the balance 0. After that, every other transaction initiated by that user is rejected because he has 0 balance in his wallet.