How does Sei run a parallel EVM?

The Ethereum Virtual Machine was designed to run transactions one at a time, in block order. Sei runs the same EVM bytecode but executes a block's transactions in parallel, without asking developers to declare which state each transaction touches. It does this optimistically: run everything at once, record what each transaction read and wrote, detect the ones that collided, and re-execute those. The final state is exactly what sequential execution would have produced, so contracts behave as they would on Ethereum, just with more throughput and blocks that arrive in a few hundred milliseconds.

Optimistic parallel execution

Parallel execution is hard because transactions share state. If two transfers both debit the same account, running them simultaneously against the same starting balance produces the wrong answer. There are two broad ways to handle this:

Approach	How conflicts are avoided	Used by
Declared access	Each transaction lists the state it will touch up front, and the scheduler only runs non-overlapping ones together	Solana, Sui (owned objects), Sei v1
Optimistic	Run everything in parallel, track actual reads and writes, re-execute on conflict	Aptos Block-STM, Sei v2, Monad

Sei v1 used declared dependencies for its Cosmos modules and CosmWasm contracts. That doesn't work for the EVM, where a contract can compute storage keys at runtime and call other contracts dynamically. So Sei v2 switched to optimistic concurrency control. Per block, it works roughly like this:

1. Execute in parallel. Every transaction runs concurrently against a multi-version view of state. Each one records a read set (keys and the versions it read) and a write set (keys it wrote), without committing anything.
2. Validate in block order. For each transaction, check whether any key in its read set was written by a transaction earlier in the block. If not, its result is valid.
3. Re-execute conflicts. Transactions that read stale data run again against the updated values, and validation repeats until everything is consistent.
4. Write sets are applied in block order.