Skip to main content

7 minutes read

# How to Scale a Proof of Work Blockchain

The below post is a compilation of tweets from my Twitter account @KadenaDirEng

## Introduction to scaling

Let’s start with the classic Computer Science parallelization and work from there.

If one Bitcoin blockchain can do 5 transactions per second (using round number approximations for simplicity), two Bitcoin blockchains can do 10 transactions per second, ten blockchains can do 50 tps, etc. But this approach has two major problems:

1. Each of these completely standalone blockchains has a completely different currency. Ten separate currencies…not so great.

2. A 51% attack becomes a 5.1% attack…not good.

Kadena’s Chainweb protocol solves both of these problems by braiding the chains together. For 2 chains, in addition to making each block include the hash of the previous block on the same chain, you also have it include the hash of the previous block on the other chain. This means each Chainweb block in a 2-chain network contains one additional hash — i.e. 256 additional bits of information…not bad! But what benefits do we get in exchange?

Well, if you wait one block after your transaction, it will require the full hash power of both chains to do a 51% attack on that block. And the hash braiding gives us the Merkle Tree structure needed to do cross-chain SPV proofs, yielding a single currency across both chains. So now we’ve solved the two major problems, and the cost is only 256 bits (32 bytes) of additional storage per block. But if we scale this up to, say 10 chains, that starts to add up.

If each block in a 10-chain web contains the hashes of the previous block on all other 9 chains, that’s an additional 288 bytes of overhead per block or 2880 bytes per block height — it gets out of hand fast. But there’s hope! It turns out there’s a brilliant result from an obscure branch of math that can save us and solve the Proof of Work scalability problem! Back in Undergraduate Computer Science, we learned a little bit of this arcane branch of math called graph theory. If you didn’t think it was all that interesting, you’re in good company. I didn’t either. But as it turns out it’s just the thing we need to scale blockchains

## Enter Graph Theory

Earlier I mentioned that with two chains, where each chain has the hash of the other chain, we solve both the multiple currency problem and the 5.1% attack problem. If we visualize the way these two chains are connected, it looks like this.

We need to scale this up to hundreds or potentially thousands of chains. But first we need to figure out a strategy for how the chains should be connected together. Let’s start with 10 chains and explore a few different strategies. The naïve approach would be to have every chain be connected to every other chain. That would look like the image below.

As we calculated before this would use way too much space because we have to store an extra 32 bytes for every line attached to a node. It might be doable with 10 chains but it definitely would be too much with more chains. We need to reduce the number of edges each node has. We could drop down to just two edges for each node. This reduces our storage requirements but it has a different problem. It takes 5 hops to get from any chain to the chain that is farthest away. If we scale this up to 100 chains, that would be 50 hops.

This means that with 100 chains you would have to wait 50 blocks before it would require 51% of the whole network hash power to attack that block. Even in blockchains like Ethereum with a relatively fast block time, that’s still a long time to wait. That’s also how long it would take to transfer coins from one chain to the farthest chain. So we have a dilemma. We want to minimize 1) the number of hops it takes to get to the farthest chain, and 2) the number of chains each chain is connected to.

Here’s where graph theory saves the day. The number of hops is called the diameter of the graph. And the number of edges each node has is called the degree (assuming they all have the same number which is fine in this case). This is a well-known problem in graph theory called the degree diameter problem! That is the brilliant result that Kadena uses to scale proof of work in what we call the Chainweb protocol.

## How Chainweb Works

Graph theory researchers have been studying the degree diameter problem for a long time. Optimal solutions are very difficult to find for large graphs. But it turns out that we know how to construct solutions that are quite good. Here’s the 10-chain graph that Kadena used at launch. If you examine it closely you’ll see that every node has three edges (in graph theory parlance, it’s degree 3) and you can get from any node to any other node in at most two hops (diameter 2).

This means that we only have to store 3 additional hashes per block and after two blocks you can transfer coins from any chain to any other chain and you have to have 51% of the entire network hash power to attack any single chain!

On August 20, 2020, the Kadena network forked from 10 chains to 20 chains. The first 10 chains kept all the same coins and smart contracts that they had before and 10 new chains came into existence. The graph had a new structure though. Here’s the 20 chain graph.

The 20-chain Chainweb graph still has degree three, but it now has diameter 3, so we have to wait 3 blocks after a transaction before the whole network’s hash power is protecting it. The most amazing thing is how much growth potential the degree diameter problem research gives us as we expand beyond 20 chains. Here’s a table that shows the largest known graph size for graphs of degree d and diameter k.

The above table means that using a chain graph with a degree and diameter of 7 Kadena’s blockchain can scale to more than 50,000 chains! And if we increase the degree and diameter one more to 8, we can get well into the hundreds of thousands!!!

## Conclusion

To recap what we’ve learned**, Kadena’s Chainweb protocol leverages graph theory research into the degree diameter problem to scale tried and true proof of work blockchain easily into the realm that we need for the scale of the global financial system**. As Vitalik Buterin has pointed out in the past, “the only solution to high tx fees is scaling”. As we have learned above, Kadena has solved the scaling problem. Join us in building the future of #DeFi!

Credits to @wjmartino who came up with this idea originally. It’s pure genius.