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In the English Auction example, let's say both wallet 2 and 3 submit a transaction to bid at the same time. (In the simulator, I just don't add a wait time between wallet 1&2 bids). When Wallet 1 closes the auction, nothing happens in the simulator - only the first transaction is seen :
In general, how does Cardano handle a situation when 2 wallets (say from different geographical locations) submit transactions at the same time ? Do both transactions fail or just 1 ?
Whichever transaction the block producer decides to include in block will succeed. Only one can succeed though.
The auction datum exists on a UTxO and each UTxO can only be spent once. In the case of the auction, the validator requires a new UTxO with a new Datum is created to replace the spent TxO. That new Datum isn't visible until the next block though.
Concurrency is a popular topic of conversations in the community and there are many ways a dApp designer can allow participants to interact concurrently. It often requires architectures that are unique to Cardano. Check out this post which includes a lot of helpful resources about concurrency.
