Sidechain

A sidechain is an independent blockchain that operates alongside a main blockchain (parent chain) and is connected to it through a two-way bridge, allowing assets to move between the two networks. Unlike Layer 2 solutions that derive their security from the main chain, sidechains have their own consensus mechanism, validator set, and security assumptions — they are essentially separate blockchains that maintain a bridge to the parent chain.

The most notable sidechain is Polygon PoS (formerly Matic), which runs alongside Ethereum. Users bridge their ETH or tokens from Ethereum to Polygon, where they can transact at a fraction of the cost (~$0.01 per transaction vs. $5-50 on Ethereum) and much higher speed (~2-second block times). Polygon PoS has its own set of 100+ validators using a PoS consensus mechanism. Other sidechains include Gnosis Chain (formerly xDai), Ronin (Axie Infinity), and Liquid Network (Bitcoin sidechain for faster settlements).

The key trade-off with sidechains is security. Because they have independent validators, sidechains don't inherit the security of the parent chain. If a sidechain's validator set is compromised, funds on the sidechain could be at risk — as demonstrated by the $600 million Ronin bridge hack in 2022, where compromised validator keys allowed attackers to drain the bridge. This is fundamentally different from rollups (Optimistic and ZK), which post transaction data to the parent chain and inherit its security guarantees. The sidechain vs. rollup distinction matters for users evaluating where to store significant value: sidechains offer lower costs but weaker security guarantees, while rollups offer stronger security at somewhat higher costs.