In Plasma MVP, users must download all transactions for a given child chain in order to use it. This does not appear to be an improvement over normal light client mode, where (I presume) one can download only transactions for a given contract in each block.
The definition of scaling as a goal pursued on Ethereum is that one wants to allow users to validate less than everything securely, but how does MVP actually fulfil that goal?
I know most people have moved on to Plasma Cash, which does not require this, but I suspect I am still wrong on this point some how, and would love to get some clarity.
Plasma allows the users within the Plasma chain to securely make transactions off-chain while ensuring that everyone else has no need to validate the Plasma chain.
Also, I think it’s important that Plasma Cash isn’t a magic bullet. Plasma Cash is good for non-fungible tokens. It’s not really that good at making arbitrary payments or handling exchanges. The per-coin proof sizes are (currently) quite large, so it isn’t that good at being denominated cash either.
Plasma allows transactions to be run off-chain. While it’s true that a “light client” doesn’t need to download transactions, it still requires there to be “full clients” that process the transactions on-chain, whereas plasma transaction do not need to be processed at all by other ethereum full clients.
but how does [plasma] MVP actually fulfill that goal?
Plasma (usually) requires nodes using the plasma chain to do a lot of heavy validation, HOWEVER nodes that don’t care about the plasma chain can ignore all of the plasma transactions and still validate the main-chain plasma contract properly, so there can potentially be many plasma chains all running independently.
The main innovation of plasma chains is that in the case that a plasma chain operator misbehaves, users of the chain have at least some limited recourse relying on only the main chain contract, in a manner that is validated by other ethereum nodes, even those that don’t care about and don’t track any of the individual plasma chain transactions. A mistreated plasma chain user can exit the plasma chain with all their $$ intact.