In a previous post, I provided a simple explanation on Ethereum’s Sharding. I also mentioned how Sharding was one of the driving factors for Ethereum’s switch to Proof of Stake consensus model.
In this post, I will provide a brief & simple explanation on why Proof Of Work was not the ideal choice for Sharding, and how Proof Of Stake tackles the problems posed by Sharding a blockchain.
Why Shard To Begin With?
Sharding is an initiative to tackle the scalability issues that Ethereum faces. All blockchains are limited by their architecture & design. How? Because every node in the blockchain processes every single transaction. While this enforces a high degree of security/reliability (explained in this post), it also limits the scalability.
Sharding improves the scalability of the Ethereum blockchain by splitting the network into smaller “groups/pieces”.
What’s wrong with PoW?
The Ethereum blockchain currently runs on a Proof Of Work consensus algorithm.
Remember from our post on Consensus Methods, that consensus or “agreement” is achieved by a majority agreeing on the state of the ledger.
Proof of Work essentially relies on hash-power to validate the chain. Blockchain is a trustless system, so we need to dis-incentivise people from being a bad actor. As explained, a bad actor would have to have a majority of the hash-power to manipulate the network. That’s a lot of hash-power. The electricity bill & hardware costs will be monumentally expensive.
Hence, in Proof of Work, the hash-power expense makes it cost-prohibitive to be an attacker.
Remember that if we SHARD in PoW, we are breaking up the network into "pieces". Hence, to achieve a “majority” hashpower in a particular shard would suddenly become feasible (since each shard will have only a “piece” of the total hash power”)
Compared to attacking the network, each shard will require only a fraction of the hash-power. An attack is no longer cost-prohibitive.
In fact, if you split the network into 100 pieces, you only need 1% of the total hash-power to takeover a shard. To keep this post simple, I explain how in another post
Validators can collude their hashpower on a single shard, and takeover control of that shard. One way to prevent this attack is to prevent attackers from focussing their hash-power onto a single shard.
However, this is difficult/not-possible to do so in a Proof Of Work protocol. In PoW you cannot stop miners from applying their work on a given shard - This is where Proof Of Stake steps in!
Proof Of Stake
PoS removes the extrinsic cost of validating the chain: hash power/electricity costs. Here's how:
In, Proof Of Stake super-nodes are required to “deposit” (stake) their Ether in order to perform validation functions. Again, remember that:
- COLLATORS (TEACHERS-ASSISTANTS): Gather mini-descriptions of transactions & the current state of the shard – and send it up to the node validators.
- SUPER NODES (PROFESSORS): Collect the data from the Collators. Process the transactions & Validate the blocks.
Proof Of Stake – along with this structure – allows Ethereum to do something that it cannot do (easily) in PoW. And that is the ability conduct “random-sampling”.
Since Super-Nodes are depositing their Ether, we can use that to randomly assign them to a shard. The assignments are reshuffled regularly as well.
This will ensure that attackers:
As you can see, Proof Of Stake makes random-sampling pretty trivial. Sampled validators are reshuffled regularly.
A quote from Vitalik on my previous post:
“ It's really important to mention that validators are super-frequently reshuffled between shards (possibly even once per block), so it's actually quite hard to "target" one specific shard for an attack. This is a large part of where sharding's at least theoretical success in breaking the trilemma comes from.”
When Vitalik mentions “breaking the trilemma”, he is talking about how typically only 2 out of 3 of the following can be achieved in blockchain:
PoS allows for easy sampling & reshuffling which ensures the “Security” of the trilemma – while the sharding structure gives us a level of Scalability and Decentralization. Hence, theoretically breaking the blockchain trilemma. This is a huge feat!
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