# Rollups Use Cases

## Cast
- Alice and Bob interact with each other on a blockchain.
- Charlie creates and operates a rollup.
- Dave is a DApp developer.
- Eve challenges rollup commitment.

## Michelson SCORU minimal use case

(no deposits/withdrawals)

- Charlie creates a new Michelson rollup by executing Tezos Client command, which moves the required deposit in XTZ from Charlie’s account into ???.
- just usual fees for cost of storage
- system shouldn't be sensitive to number of rollups
- only active rollups should consume L1 resources
- maybe amortise future common-good ops? no
- behaviour is similar to smart contract
- storage for rollup is small
- artificially bump fee just in case? to limit number of rollups?

- Charlie starts an instance of the Michelson Rollup Node process on his computer to operate the rollup, configuring it with the private key of his L1 account (for submitting operations to L1), and address of a Tezos Node (for submitting L1 operations and observing the chain). Rollup Node exposes RPC which can be used to submit operations.

- Alice, Bob and Dave create accounts in the rollup (BLS addresses), using Michelson Rollup Client (or a wallet application). They share their L2 public keys with Charlie, keeping private keys secret.

- Charlie runs a command with Rollup Client to deposit small amounts of L2 tokens into L2 accounts of Alice, Bob and Dave - so that they could pay for transactions in the rollup. (??? TODO: detail this operation is processed ???)

- Dave takes Michelson code of DApp he developed earlier for Tezos mainchain (??? what kind of DApp would be a good example??? QuipuSwap? Hic et Nunc? Carbon Credits?) and deploys it into the rollup using Michelson Rollup Client. Dave pays for the origination with L2 tokens that Charlie deposited into his account. The Rollup Client prints the BLS address of the contract.

- Dave starts a new web site for the DApp, based on his prior work, configuring it with the BLS address of the deployed contract and RPC address of the Rollup Node. He gives the address of the web site to Alice and Bob.

- Alice opens the DApp web site and presses a button to purchase something (??? game tokens). That connects a wallet application in her web browser to the DApp. Alice selects her L2 account in the wallet app - to pay for the operation, and signs the operation.

- Web site submits the operation (signed by Alice’s private key) to the Rollup Node via RPC.

- There is no mempool in Rollup Node. Upon receiving the L2 operation, the Rollup Node wraps and forwards the operation to a Tezos Node. L1 fees are paid from Charlie’s L1 account. Wrapped operation is included into an L1 block by a baker.

- Note that Charlie is compensated for L1 fees by some L2 tokens he receieves as part of L2 operation, and Rollup Node checks that fees are sufficient before forwarding operation to L1 (rejecting it otherwise).

- Rollup Node observes new L1 block with wrapped operation, extracts L2 operation from it and executes it, updating Rollup State.

- Alice checks back later the DApp web site, and can see that she now has a balance of game tokens. For this to be displayed DApp fetches corresponding part of Rollup State from Rollup Node via RPC and interprets it. Alice can also see that this information is not final because Rollup State is not committed.

- At some point Rollup Node computes hash of the current Rollup State and submits committment operation to L1. Charlie's account pays for the operation again, including some deposit for the commitment. This operation is included by a baker into an L1 block.

- Eve is also running a Rollup Node (for the same rollup), but her code has a bug (she hasn't upgraded from an older version). Eve's Rollup Node observes commitment operation on L1, tries to validate it and gets a different hash value.

- Eve's Rollup Node automatically submits refutation operation to L1, including some deposing (which is paid together with feeds from Eve's L1 account). Refutation operation is included into an L1 block by a baker.

- This starts a refutation game where Chalie's Rollup Node and Eve's Rollup Node automatically submit corresponding moves as L1 operations to the main chain (each move takes at least one L1 block). Eve eventually loses and her deposit is slashed.

- After some time Charlie's commitment is finalised - L1 block produced by baker automatically labels hash of the Rollup State in L1 context as final.

- Alice checks the DApp web site again, and can see that her ownership of the game tokens is final.