There are many ways in which blockchains choose to scale their networks. Each with their own unique take on how to solve the trilemma. Here we are discussing the differences between Avalanche Subnets and blockchain sidechains.
Subnets are completely sovereign and don't share their state with the Primary Network so transactions are final and immutable in less than 2 seconds w/the Avalanche Consensus.
Sidechains need to continuously send a checkpoint/save state to the 'parent' chain (Polygon to Ethereum, for example). Transactions on sidechains are not final until the checkpoint is accepted and finalized on the parent chain. So it must be recorded on the sidechain, then sent to the ‘parent’ chain which must then also record it. This leaves a point of failure where a chain may experience a reorg, as a failsafe.
Subnets transactions do not affect the speed of the Primary Network, and the Primary Networks transactions do not affect the speed of the Subnet transactions. Both are allowed to operate at maximum speed without affecting one another.
Sidechains transactions are not affected by the ‘parent’ chain and likewise the other way around. This allows both to work at their optimal speeds. However, gas price fluctuations on the ‘parent’ chain can affect the speed of checkpointing, and congestion can prevent them altogether.
Subnet nodes are REQUIRED to validate the Primary Network, thereby creating a common coordination environment. This can be used for registering metadata, check-pointing proofs, atomic transfers, and other similar uses.
Not all sidechains use the same nodes as their parent chain but can, depending on the sidechain.
Subnets provide maximum flexibility. You are free to create your own economics (such as custom tokens for gas), security, and also your choice of VM (virtual machine). So a project is not handcuffed to only using the Primary Network’s tokenomics and technical structure. This allows projects to customize all aspects of their environment. This means you can use other execution environments besides the EVM. Also Subnets can be permissioned or permissionless.
Sidechains, however, require users to use their token for gas, their VM, and abide by their security structure.
Subnets have the added benefit of being risk isolated from the Primary Network since subnets don't share their state with the Primary Network. They don’t share risk with other Subnets either.
With Subnets, if the Primary network were to be maliciously attacked and successful in the attack, it will not affect the Subnet.
Avalanche security is parameterizable. The safety threshold is at 80% (59% of nodes can be "Byzantine"). In e.g. Ethereum the threshold is 50% (33% Byzantine).
Sidechains that are compromised will not affect their ‘parent’ chain. However, if the ‘parent’ chain is affected, the sidechain will be impacted as well.
How many Subnets can you have?
Unlike other networks, Subnets are not limited. Anyone can build a subnet, no need for auction slots or limited caps.