Blockchain Developer Certification Practice

What happens to blocks in the blockchain if a previous block is altered?

1. All subsequent blocks become valid

2. Subsequent blocks need to be validated by nodes

3. They need to be re-mined

4. The blockchain is entirely reset

The correct answer is: They need to be re-mined

When a previous block in the blockchain is altered, it creates an important consequence for all subsequent blocks. The blockchain operates on a consensus mechanism where each block is tied to the previous one through cryptographic hashes. If any information in a previous block is changed, its hash also changes, leading to a mismatch with the hash stored in the next block. This chain reaction means that all subsequent blocks will no longer be valid unless they are adjusted to reflect the change made in the earlier block. In this context, "re-mined" refers to the process by which blocks are recalculated to produce new hashes that align with the altered previous block. Thus, the integrity of the blockchain relies on its immutability, and altering a block necessitates that all following blocks must be re-mined to maintain the security and consistency of the entire chain. This is a crucial feature of blockchain technology that ensures reliability and trustworthiness in a decentralized system.