Immutability of blockchain
Blockchain is a database which can ensure high immutability and reliability of data under the distributed network. Does that mean data cannot be modified once they are put into the blockchain?
No! In bitcoin network, miners generate a block every ten minutes. It is not rare for two miners to generate two different blocks at the same time as long as they can solve the mathematical equation. It leads to the creation of two chains with different transaction records. However, as the time goes, the probability of two chains continuing to extend drops. Let’s say the probability of two blocks being generated at the same time is 20%. Then probability of two blocks being generated at the same time in two consecutive 10-min periods is 4% (=20%*20%). For three consecutive 10-min periods, the probability further drops to 0.8% (=20%*20%*20%). In other words, at some point of time, only one chain can grow continuously and another one will be discarded by the nodes. Data inside the abandoned chain are replaced by data from the other chain. Therefore, data can be modified and even deleted entirely and replaced by other data.
That’s why there is a ‘rule of six confirmations’ for a bitcoin transaction. One confirmation refers to a transaction appearing in the block just created. Six confirmation refers to a transaction appearing in the block which is five blocks away from the current block. This can ensure the block including that bitcoin transaction will not be discarded. For example, when Alice wants to buy bitcoin from Bob by using USD, Alice is suggested to wait for six confirmations for Bob’s bitcoin transaction before she sends USD to Bob.
To conclude, data in blockchain is immutable in long term.
