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hardBlockchain Developer interview question

What is MEV, and how would you protect your protocol's users from harmful MEV?

What the interviewer wants to hear

  • Maximal extractable value from ordering, including or excluding transactions
  • Harmful forms: sandwich attacks, front-running
  • Benign forms: arbitrage, liquidations
  • Private orderflow and MEV-protected RPCs
  • Protocol designs: slippage limits, batch auctions, commit-reveal, intent-based systems

Model answer

MEV, maximal extractable value, is profit that block builders and searchers can make by choosing which transactions go into a block and in what order. Some MEV is useful, like arbitrage that keeps prices aligned across DEXs and liquidations that keep lenders solvent. Harmful MEV hurts users directly, mainly sandwich attacks, where a bot buys before a user's swap and sells after it, and front-running of valuable actions like NFT mints. To protect users, I set sensible default slippage and deadlines in the frontend and route transactions through private or MEV-protected RPCs, so they skip the public mempool. At the protocol level, designs like batch auctions with a uniform clearing price, as CoW Protocol uses, intent-based systems where solvers compete, or commit-reveal schemes for actions that should not be visible early, remove the advantage of ordering. Some protocols also redistribute MEV back to users through order flow auctions.

Can you answer it under time pressure?

Take a timed hard round and get scored on every answer.

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