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I am trying to understand how do transactions are actually ordered within a block. I have found something strange specifically which confuses me:

  • Txid 1: 0xab144b6bf8aa7fb6a5927a26a0e556607d33612de814909399fade591f1ef212
  • Txid 2: 0x5e910ca9fea65438c0844b8a397233d4a2663b58af2ca683f3732570fed58f82

These two smart contracts have swapped the same coin within the same block using the same router; Txid 1 has position 1 within the block, while Txid 2 has position 9 in the same block. Nonetheless, Txid 1 has given as input a lower max and priority gas respect to Txid 2.

How is this even possibile? Thanks.

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  • The protocol doesn't define a specific order. The block proposer could order them in any way they want.
    – Ismael
    Oct 24, 2022 at 6:45

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The validator is the one that decides the tx ordering. If it was a swap in a DEX maybe there is something like an MEV sandwich attack. You can read more about it here.

The take away: the block proposer decides the ordering.

Side note: A proposer is the node that proposes the block to be validated by the network.

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  • I know. I am sure it was not a sandwich attack, neither the transaction was 'manipulated' - as when it happens I see within the transaction some ETH moving to Validators/staking pools which "support MEV democratization"
    – Skid11
    Oct 17, 2022 at 18:22

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