A sidechain is a separate blockchain which runs in parallel to Ethereum Mainnet and operates independently. It has its own consensus algorithm (e.g. proof-of-authority, Delegated proof-of-stake, Byzantine fault tolerance). It is connected to Mainnet by a two-way bridge.
What makes a sidechain particularly exciting is that the chain works the same as the main Ethereum chain because it's based on the EVM. It doesn't use Ethereum, it is Ethereum. This means if you want to use your dapp on a sidechain, it's just a matter of deploying your code to this sidechain. It looks, feels, and acts just like Mainnet – you write contracts in Solidity, and interact with the chain via the Web3 API.
PROS&CONS
Pros
Cons
Pros
Cons
1
Established technology.
Less decentralized.
2
Supports general computation, EVM compatibility.
Uses a separate consensus mechanism. Not secured by layer 1 (so technically it’s not layer 2).
3
A quorum of sidechain validators can commit fraud.
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How Do Sidechains work?
Nodes within a sidechain network are responsible for confirming & processing transactions, writing transactions to blocks, and maintaining consensus across the network. Security is the responsibility of each sidechain; it is not directly inherited from Ethereum. Sidechains often incorporate alternate validator selection and consensus mechanisms to provide faster transaction times.