The Blockchain Family Members:

The Parent, the Child, the Uncle, the Nephew, the GRANDPA, and the GHOST.

Though might be considered an extended family member to one another, they are under the big Blockchain family after all.

The Parent and the Child

Let’s imagine a blockchain is a family tree of the generations of a species, called the “block”. What each block carry with them is the “transactions”, something that happened during their lifetime. This is the karma that will follow them forever. There are many different types of blockchain, just like different human races.

There is something strange about this species. Other than the first block, usually called a Genesis Block, which is born without a parent, every block is a parent to ONLY one block, and every block is the ONLY child of ONLY one block.

When a new block is born, it bears a hash passed down by its parent. When it has a child of its own, it will pass its hash, which includes its’s parent’s hash and transactions that happened during its time, to its child. You may say the hash is like a DNA to trace its origin all the way back to its ancestor, the Genesis Block.

Every blockchain race has a different lifespan for each generation, called the “block time”. Like humankind, our average life expectancy is 79 years, but some lived longer, some lived shorter. Eacf blockchain race has an average “block time”, but still, some block times would be shorter, some would be longer.

The Uncle, the Nephew and the GHOST

Another strange thing about blockchain is the child is not produced by the parent, but by people who record the whole history of their generations, kinda like a group of gods. And they send the child to the eligible parent like the stork does.

Every time a blockchain child is born, the god who created the child suppose to inform all other gods. Somehow, sometimes, not all the gods got the information and there could be more than one god who created a child, and ended up a parent has 2 or more children. This is not allowed.

In a blockchain that has a short block time, like Ethereum, there is a higher chance that 2 children, or more, be created at about the same time. In this situation, there will only be one child included in the family tree and the rest become stale blocks and will be disowned. Usually, those disowned children are called orphaned blocks, as they will go without a parent.

This weakens the strength of that blockchain race against attacks by satans and may cause centralization amongst the gods. Both situations are not a favorable one. In order to solve these issues, Ethereum summoned GHOST, the Greedy Heaviest Observed SubTree protocol. You can consider GHOST as the ancestor who looks after the whole blockchain race from above. It likes to keep everyone together as much as possible. With this protocol, the child that didn’t get to be included in the main family tree might be included in a sub-family tree, and is called an Uncle. An Uncle can only be invited to the sub-tree by a Nephew, which is the legitimate child of its sibling on the main family-tree. Only the block has a parent on the main family-tree can be considered an Uncle. if the block’s parent is from the sub-family tree, it will not be invited to be an Uncle. Refer to the figure below for visualised explanation.

Source: https://arxiv.org/pdf/1805.08832.pdf

An Uncle is not necessarily being invited by the immediate next generation. It could be the great-grand Nephew that does the invitation, up to the 7th generation. (They are all referred to as Nephew, “great-grand” is added just to give you the generation concept.) Each Nephew can only invite 2 Uncles to the sub-family tree. Both Uncle and Nephew will be rewarded accordingly.

(You may read more here, under the header “Stales, Efficiency and Centralization” and “GHOST” for the details, as this article is talking about family bonds. Let’s leave this to another time.)

Illustration of block creations and the links between Uncle and Newphew. Source: http://www.ethviewer.live/

The GRANDPA

GRANDPA (GHOST-based Recursive ANcestor Deriving Prefix Agreement) is the finality gadget that is implemented for the Polkadot Relay Chain. — Polkadot Wiki.

GRANDPA is the one that has the final says in Polkadot, another blockchain race. It acts like Casper the Friendly Finality Ghost of the Ethereum blockchain race. A parent has no power to choose which child to disown. In Polkadot, the power lays on GRANDPA’s hands. Once it finalised which child is the one to keep, the extra ones will be evicted from the family tree. Since that point onward, no parent can disown its child, nor a child can runaway; no outsider (attacker/satans) can come and break the family bond either. Finalised generations stay safely together ever after.

(Related reading: Ethereum 2.0 vs Symbol (Part 2): Consensus Protocols)

Bonus: The BABE

BABE from Polkadot stands for Blind Assignment for Block Extension. It does what it says: randomly assign block producers (the gods), basing on the god’s investments and a system called Polkadot randomness cycle. It ensures a child is produced at a given period of time by having a backup plan, where it select an extra producer in the background using a round-robin method to produce a secondary child. If the first god selected fails to produce a child, the secondary child will be included as the legitimate child. If there is a primary child produced, the secondary child will not be evicted too. How would this be handled? Well, that would be another topic.

You may consider BABE as the fertility consultant that is really concern about the continuity of this whole blockchain race.

You have now met some of the family members of different blockchain races. I hope you are interested to find out more about the whole species. Till the next storytelling session.

References:

1. Chapter 7. The Blockchain https://www.oreilly.com/library/view/mastering-bitcoin/9781491902639/ch07.html
2. https://eth.wiki/en/fundamentals/design-rationale
3. https://arxiv.org/pdf/1805.08832.pdf
4. https://blog.ethereum.org/2014/07/11/toward-a-12-second-block-time/
5. https://wiki.polkadot.network/docs/en/learn-consensus
6. https://polkadot.network/polkadot-consensus-part-2-grandpa/

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