Blockchain Finality

Finality is the guarantee that past transactions can never change.Blockchain is already immutable, so what does it mean?

Finality is a problem for LOTTERY BASED CONSENSUS algorithms.Examples are; Proof-of-Work, Proof-of-Stake, Proof of Elapsed Time etc.
It means samples with MINING concept.Past transactions may change because of CHAIN FORKS and LONGEST-CHAIN SELECTION
Randomly 2 miners can succeed at the same time. If such issue occurs, there may occur separate chains and it is called FORKING
Solution to chain fork is the longest chain, but if your past transaction was at shorter chain, it will be lost and past transaction list will be different.
So, we can not guarantee that past transaction will stay there forever, so we mention here about PROBABILISTIC FINALITY.
(If your past transaction is not there, it means that transaction is not FINAL, that transaction can not be thought as totally committed.)

But if we use VOTING BASED CONSENSUS, there is always one unique chain.There is never forking and Finality is immediate.
So it is safer to use voting based consensus. Such algorithm samples are ones that do not include MINING.
Examples are Paxos,Raft,Kafka,Zookeeper,PBFT,RBFT,SBFT etc.

For example at bitcoin, a transaction is considered FINAL after 6 blocks from itself.
It means, if a transaction still exists and there is no forking for 6 more transaction period, it is considered that it will be so small ignorable probability that it will be lost after a later chain fork. This 6-block number is calculated due to ECDF algorithm. (ECDF = Emprical Cumulative Distribution Function) ECDF analysis must be done due to block number and average transaction latency time.

Such ECDF analysis at bitcoin is shown below.
It showed that a transaction submitted at block X can be found at block X+2 with %50 chance, in block X+5 with %90 chance, in block X+7 with %99 chance.
X+7 results tells waiting 6-later blocks after that block itself. But waiting for finality also decreases performance.

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