Track 1 · Mechanics · lesson 4

Nodes and the mempool

8 min

"The network" is not an abstraction. It is thousands of ordinary computers, each running software that keeps a copy of the chain and refuses to accept anything that breaks the rules.

Each one is a node.

Transactions wait here until a validator picks them up.

Nobody is in charge, and everyone checks

A node's job is unglamorous: receive blocks and transactions, verify every rule against its own copy, pass on what is valid, discard what is not.

There is no central list of participants. A node that decides to accept an invalid block does not corrupt the chain — it just disagrees with everyone else, and the rest of the network carries on without it.

This is what "decentralised" concretely means. Not that the data is spread around, but that the rules are enforced independently, thousands of times over, by parties who never agreed to trust each other.

Running a node is how you stop trusting anyone's answer about your balance and start checking it.

The mempool is a waiting room, not a queue

A transaction that has been broadcast but not yet included sits in each node's mempool. Two things about it surprise people:

It is not one shared pool. Every node keeps its own, and they differ. There is no canonical ordering and no fair queue.

It is public. Anyone can watch pending transactions before they are executed. Your intent is visible for seconds before it becomes real.

Predict

You submit a trade that will move a price. It sits visibly in the mempool for eight seconds. What can someone else do with that information?

Nothing — it is already signed and cannot be changedPay a higher fee to go first, then sell into your tradeCopy your signature to send it from their account

Who chooses what goes in

Validators pick from their mempool, and they are not obliged to be fair. Ordinary practice is to take the most profitable transactions, which usually means the highest tip.

Nothing forces a validator to include your transaction at all. A high fee is a strong incentive, not a guarantee.

Check

Why is there no single, canonical mempool?

Because it would be too large to storeBecause every node keeps its own view, and there is no authority to make one canonicalBecause transactions are encrypted until inclusion

Worth remembering

A node independently verifies every rule; decentralisation means the rules are enforced thousands of times over.
There is no single mempool — each node keeps its own view, and they differ.
Pending transactions are public before they execute, which is what enables front-running and MEV.
Validators choose what to include, usually by profitability. A high fee is an incentive, not a guarantee.