The MEV supply chain: builders, relays, and the reversal

Graduate · enrolled learners

This lesson opens with MEV-boost relay censorship after the Tornado Cash sanctions, 2022 to 2023.

What happened

After Ethereum's move to proof of stake, most validators source their blocks through MEV-boost, which splits block production into a supply chain: searchers submit profitable transaction bundles, builders assemble them into full ordered blocks, relays pass the best block to the validator, and the validator, the proposer, signs whatever the relay hands over. This was meant to democratize access to MEV, and it concentrated power in the middle. After the US sanctioned the Tornado Cash mixer in August 2022, several relays chose to become OFAC-compliant and excluded sanctioned transactions from the blocks they relayed, and for a period a large share of Ethereum blocks, at times a majority, were built through censoring relays. No funds were stolen; what the episode exposed was structural. The proposer holds the ordering right in principle, but in practice a handful of builders and relays decide what goes into blocks and in what order, so they are the real ordering power and a genuine censorship surface.

The decision point

Individual-block MEV analysis, reading a sandwich or a liquidation, is necessary and not sufficient, because the ordering power those extractions depend on has an owner, and after MEV-boost that owner is a small set of builders and relays rather than the diffuse crowd of validators. The relay-censorship episode is the demonstration: a proposer nominally chooses the order, but if it takes blocks from a relay that censors, the relay's policy, not the proposer's, decides what is included, and when a few relays sit in the path of most blocks, their choices shape the whole chain. The April 2023 reversal sits in the same structure from the other side: the relay is a trusted intermediary, and a proposer exploited the relay software to betray that trust and reorder searchers' bundles. So the analyst reads MEV at two levels: the block, where a specific extraction happens, and the market, where a handful of builders and relays hold the ordering power that every extraction and every censorship decision runs through.

Orientation and Year One are open: anyone can read them without an account. From Year Two onward the lessons are for enrolled learners, because progress through the later years only means anything if it is tracked against a record.

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