What prevents double spends? about plasma-cash HOT 4 CLOSED

This step in the Readme seems to be the relevant one:

Operator calls submitBlock, checkpointing the block merkle root which includes the transaction that gives ownership to charlie. At this point, both the child chain and the root chain, have checkpointed alice's transaction at block number 1000.

But if we have to submit to mainnet on each transaction, don't we lose the speed benefits of using a child chain?

from plasma-cash.

The whole point of Plasma is that every plasma block gets submitted to the main chain. The double spend cases are covered by challengeBetween, https://github.com/loomnetwork/plasma-cash/blob/master/server/test/testChallengeBetween.js. More details here https://karl.tech/plasma-cash-simple-spec/

Plasma does not improve finality. A transaction can be considered finalized after a plasma block root which includes it has been committed to the contract (+some block buffer for potential reorgs, but you parameterize accordingly). Plasma improves throughput/capacity by allowing many transactions of a potentially huge plasma block be finalized in 1 onchain tx. it is closer to a compression mechanism that allows more transactions to settle per block slot, 15s in Ethereum

from plasma-cash.

from plasma-cash.

Closing since the discussion moved in https://ethresear.ch/t/plasma-chain-block-times/3092/. Ideally we should aim for 15s times, but what you say is possible, it's just another parameter of the system

from plasma-cash.