|Exchange settlement on Fuel.|
Non-custodial exchanges can be built by anyone on top of Fuel today. The order book is kept, and matching is done, off-chain, with settlement of each order happening on Fuel as a multi-user transaction.
In this example, Alice and Bob each consume one of their UTXOs in a transaction, which swaps the owner of each asset to produce two new UTXOs. (Fees and change are an implementation detail.) Note that since users sign over an entire transaction, there is no possibility of theft. Alice and Bob can each sign the transaction individually, and any time before the transaction is confirmed, can spend their UTXO in a different transaction to cancel the order.