Nonces e ordenação

Imagine sending two transactions: first deposit, then withdraw. If they execute out of order, the withdrawal could fail.

The nonce guarantees your transactions run in the exact order you intend.

An account nonce is a counter tracking how many transactions an account has sent. It starts at 0 and increases by 1 with each confirmed transaction.

Note: this is different from the mining nonce in Proof of Work.

The network only executes a transaction whose nonce matches the account's next expected value.

A transaction with nonce 5 will not run until nonces 0 through 4 have been processed.

Because each nonce is used once, an attacker cannot replay (resubmit) an old signed transaction.

The network already processed that nonce, so the duplicate is rejected automatically.

If a transaction with a given nonce gets stuck (too low a fee), all later nonces are blocked behind it.

Nothing can proceed until that gap is filled, because nonces must be sequential.

To unstick things, you can replace a pending transaction by resubmitting it with the same nonce and a higher fee.

This is often called speed-up or cancel — validators take the higher-paying version.

A nonce gap happens when you submit nonce 7 while nonce 6 is missing.

The transaction sits unprocessed until the gap is filled, since the network requires a continuous sequence.

The nonce orders transactions from one account, but ordering between different accounts within a block is decided by the validator.

Validators typically order by fee, which is why ordering can be exploited (see MEV).

Because validators choose transaction order, they can profit by reordering, inserting, or censoring transactions. This is called Maximal Extractable Value (MEV).

It shows that ordering is economically valuable, not just technical bookkeeping.

Wallets query the network for the next nonce using eth_getTransactionCount.

This returns the count of confirmed transactions, which equals the next nonce to use.

The account nonce sequences your transactions, prevents replays, and must be continuous.

Validators decide cross-account ordering, which introduces fee-based prioritization and MEV.

You learned that:

• The nonce counts an account's transactions and enforces order
• It prevents replay attacks
• A stuck low nonce blocks later ones until replaced
• Validators decide cross-account ordering, enabling MEV

Next course: EVM fundamentals.