What is Finality? How does Ethereum PoS achieve Finality?

Okay, no problem. Let's talk about the concept of "finality," and I'll try to explain it in an easy-to-understand way.

What is Finality?

Imagine finality like a judge striking a gavel in court, declaring, "This case is closed! No appeals!"

In the world of blockchain, when a transaction or a block achieves "finality," it means it has been permanently and irreversibly recorded on the ledger. No one, including miners, validators, or even the majority of network participants, can change or revoke it. This block and all the transactions it contains are etched in stone, fixed forever.

A comparison makes it easier to understand:

You might have heard that Bitcoin transactions require "6 block confirmations." Why is that?

Because Bitcoin (and early Ethereum PoW) uses "probabilistic finality."

Probabilistic Finality: This is like writing something on paper with a pencil. Right after you write it, it's easy to erase. If someone writes another layer over your words, it becomes harder to erase. The more layers written (i.e., the more blocks confirmed), the less likely your words are to be erased (your transaction overturned). However, theoretically, that possibility is never zero. It's just "almost impossible" to change.
Deterministic Finality: What we're discussing today, and what Ethereum PoS aims for, is "deterministic finality." This is like stamping words directly onto a metal plate with a steel die. Once it's stamped, it's done. It's not "almost impossible" to change, it's "absolutely impossible" to change.

In a nutshell: Finality is a guarantee that once your transaction is confirmed, it will never "disappear" or be "invalidated."

How Ethereum PoS Achieves Finality

After Ethereum transitioned from Proof of Work (PoW) to Proof of Stake (PoS), it introduced a new consensus mechanism called Gasper. It combines two algorithms, with the part responsible for achieving finality known as Casper FFG.

This might sound complex, but we can imagine it as a "committee voting" process.

The Ethereum network has tens of thousands of validators. They have each staked at least 32 ETH as collateral, becoming members of this "committee." Their job is to validate transactions, propose blocks, and vote on the state of the chain.

The process of achieving finality generally involves the following steps:

1. Units of Time: Slot and Epoch

Slot: Can be thought of as a "second," approximately 12 seconds, which is the time taken to propose a new block.
Epoch: Can be thought of as a "minute," consisting of 32 Slots (approximately 6.4 minutes). Each Epoch is a voting period.

2. Core Concept: Checkpoint

The first block of each Epoch is called a checkpoint. You can view it as the most important milestone within that "voting period." Validators' primary task is to vote on these checkpoints.

3. Voting Process: Justification and Finalization

This process is somewhat like legislative enactment, requiring two rounds of voting to be ultimately settled.

Step One: Justification When a checkpoint (let's call it point A) receives voting support from more than 2/3 of the validators (weighted by their staked ETH), this point A is considered "justified."

This is like a bill passing its first reading. Everyone generally agrees it's fine, but it's not yet final law.
Step Two: Finalization When a "justified" checkpoint A is immediately followed by its next checkpoint B also being "justified" (i.e., B also receives more than 2/3 of the votes), then the preceding checkpoint A is officially marked as "finalized."

This is like a bill passing its second reading and being signed into law by the president. At this point, the bill officially becomes law, set in stone, and cannot be changed.

Typically, a block takes 2 Epochs, or approximately 13 minutes, to go from creation to finalization. Once a block is finalized, all preceding blocks are also considered finalized.

4. Deterrence Mechanism: Slashing

You might ask, what if validators vote maliciously?

This is where PoS truly shines—economic penalties.

If a validator attempts to act maliciously, for instance, by voting on two different chains simultaneously (trying to create a fork), their behavior will be detected by the system. Once discovered, a portion, or even all, of their staked ETH will be slashed.

This penalty is severe. For a validator with 32 ETH, this could mean tens of thousands of dollars in losses. Therefore, for validators, honest voting is the most economically rational choice.

This is like: All "committee members" have signed a military order. If anyone dares to lie or play tricks with their votes, they will not only be expelled from the committee, but all their assets used as collateral will also be confiscated. Under such immense deterrence, almost no one dares to act maliciously.

Summary

Ethereum PoS achieves finality through a "two-phase voting (justification -> finalization)" mechanism. This process involves tens of thousands of validators and is secured by a strict "slashing" penalty system.

Once a block achieves finality, it means that over 2/3 of the network's validators have put their own capital (staked ETH) behind it, declaring that this block is "correct and unchangeable." This is the core of Ethereum PoS finality.