Memory pool. Waiting room for transactions.

Submit a transaction on Ethereum or Bitcoin. Doesn't confirm instantly. Broadcasts to the network first. Sits in the mempool. Waits for a validator to pick it up and include it in the next block.

Simple concept. Real implications for anyone doing anything on-chain during busy periods.

How Transactions Get Selected

Validators aren't picking transactions randomly. Picking by fee.

Higher fee per unit of computation, higher priority. Transaction offering more gets included first. Transaction offering less waits. Transaction offering almost nothing might wait hours. Might get dropped entirely after sitting too long.

Normal conditions: most transactions confirm in seconds to minutes. Gas fees manageable.

Popular NFT mint dropping. New token launching. Market crashing and everyone moving at once. Mempool fills up. Validators still processing same number of transactions per block. Queue gets long. Fee competition starts. Gas prices spike. People bidding higher and higher to jump the queue.

That's why gas fees go from $2 to $50 in minutes during chaotic events. Everyone trying to get to the front of the same line simultaneously.

The Public Mempool Problem

Ethereum's mempool is public. Every pending transaction visible to anyone monitoring.

Not just validators watching. MEV bots. Arbitrageurs. Sandwich attackers.

Large swap sitting in mempool waiting to confirm. Bot sees it. Knows that swap will move the price on the DEX. Bot front-runs. Buys before the swap executes. Price moves up. Original swap fills at worse price. Bot sells immediately after. Pocket the difference.

Entire sandwich attack depends on mempool visibility. Transaction visible. Bot acts. User pays. Happens in milliseconds. Completely automated.

Billions extracted from regular users through MEV annually. Not a rounding error. Real money taken from people who don't know their transactions are being watched before they confirm.

Private Mempools

Solution that emerged: hide transactions from the public mempool entirely.

Flashbots on Ethereum. Jito on Solana. Private RPC endpoints that route transactions directly to validators without broadcasting publicly first. Transaction never visible in public mempool. MEV bots can't see it. Can't front-run what they can't see.

Most wallets now offer private RPC options. MetaMask has Flashbots Protect integration. Worth using for any swap of meaningful size. Doesn't cost extra. Just routes differently.

Doesn't eliminate all MEV. Validators themselves can still reorder transactions within a block. But eliminates the sandwich attack vector that hits retail most commonly.

Stuck Transactions

Low fee set. Mempool congested. Transaction sitting there going nowhere.

Not lost. Just waiting. Two options.

Speed it up. Resubmit same transaction with higher fee. On Bitcoin this is Replace by Fee. RBF. New transaction replaces the old one. Higher fee gets it picked up faster.

On Ethereum: cancel by sending a zero ETH transaction to yourself with the same nonce but higher gas. Same nonce tells the network these are competing transactions. Higher fee version wins. Old one drops.

Most wallets handle this through speed up and cancel buttons on pending transactions. Don't need to do it manually. Useful to know it's possible when a transaction is stuck and time matters.

Mempool Size as a Signal

Bitcoin mempool.space shows pending transaction backlog in real time. Useful for timing non-urgent transactions.

Mempool large and fees elevated: bad time for low-priority transactions. Wait.

Mempool quiet and fees low: good window. Weekday early mornings in US and European time zones typically quieter. Weekend evenings sometimes. No consistent pattern. Just check before submitting.

Ethereum gas trackers show the same thing. Etherscan gas tracker. Blocknative. MetaMask shows live gas estimates before confirming. Always worth glancing at before a large transaction.