Lesson 2 — The mempool and pending state: visibility, MEV, and private submission
Once you've broadcast a transaction, it sits in public view before being included. Today: what's in the mempool, who watches it, and when to bypass it.
Every transaction starts in the mempool — the queue of broadcast-but-not-yet-included transactions visible to anyone running a node. Most users never think about this stage. MEV operators think about little else. Understanding what's visible, to whom, and what it means for your transaction-execution quality is the difference between blindly broadcasting and informed submission.
**What the mempool actually is.** Every Ethereum node maintains its own view of pending transactions. When you broadcast a transaction through MetaMask, your RPC provider (Infura, Alchemy, or a private endpoint) submits it to the network, where it propagates to other nodes. Each node holds these transactions in memory while validators / builders select which to include in the next block. The mempool is *public* in the sense that anyone running a full node can see every pending transaction — which means anyone with bot infrastructure can analyse and react to your transaction before it's included.
**Why the public mempool creates problems.** Three categories of MEV (Maximal Extractable Value) operate on the mempool. **Front-running**: a bot sees your pending DEX swap, submits an identical transaction with higher gas fees, and gets included first — capturing the price impact you would have caused. **Sandwich attacks**: a bot front-runs you with a buy that pushes price against you, then back-runs you with a sell, capturing the slippage. **Generalised arbitrage**: bots watch the mempool for any transaction that creates an arbitrage opportunity (a swap, a liquidation, a mint) and race to capture it in the same or next block.
**Private mempools.** Several systems let you submit transactions privately — they go directly to block builders without appearing in the public mempool first. **Flashbots Protect** is the most-used: your transaction goes to Flashbots' bundle stream and is included in a block without ever being publicly visible. **MEV-Share** is a more nuanced variant that allows transaction senders to share information about their trades with searchers in exchange for a portion of the MEV captured — turning the front-running problem into a negotiated revenue split. **MEV-Blocker** and **CoW Swap** offer similar private-flow options on top of solver-based execution. For large DEX swaps (anything above a few thousand dollars), private submission is the difference between paying slippage to a bot and not paying it.
**What you can see in the mempool yourself.** Tools like Blocknative's mempool explorer, Eden's mempool dashboard, and mevscan.io let you watch the public mempool in real time. You can search for pending transactions from specific addresses, see gas-price distributions, and observe MEV bots reacting to opportunities within milliseconds. For research purposes, watching the mempool during a high-volatility event (a token launch, a large liquidation, a protocol exploit in progress) is extremely educational — you see the bot ecosystem operate at full speed.
**Pending vs confirmed transactions.** A transaction in the mempool is not on the blockchain yet. It can be: (a) included in a block — becoming a confirmed transaction; (b) dropped from the mempool — usually because its gas price was too low and it sat too long; (c) replaced by the sender — submitting a new transaction with the same nonce but higher gas (in MetaMask, the 'speed up' button does this). The distinction matters when reading addresses on Etherscan: the 'Pending Txns' tab shows what's in the mempool; the regular transaction list shows confirmed history.
**Builder-extractable value (BEV).** As Ethereum's MEV landscape has matured, the value-capture point has moved from searchers (independent bots) to block builders themselves. Builders like Beaver Build, Titan Builder, and rsync-builder run sophisticated MEV-extraction in-house and either capture it for themselves or compete on which builder pays validators the most. This is what proposer-builder separation (PBS) and MEV-Boost were designed to manage. For users, the practical consequence is that even private-mempool submission can leak information to builders who then extract MEV on the order flow.
**Solana and other chains.** Solana doesn't have a public mempool in the same way — transactions are sent to the current leader (block producer) and either included or dropped. This eliminates traditional front-running but introduces different MEV dynamics: priority-fee auctions, leader spam, and validator-level extraction. Each chain's transaction-submission model has its own MEV profile, and the analysis is chain-specific.
Example
Watch a real DEX swap in two paths. Path A: user broadcasts a $50,000 USDC → ETH swap on Uniswap V3 via MetaMask's default RPC. The transaction lands in the public mempool. A sandwich bot detects it within 50 ms, front-runs with a buy that pushes ETH up 0.4 percent, the user's swap executes at the moved price (paying ~0.4 percent extra), and the bot back-runs with a sell capturing the difference. Net user loss to the sandwich: ~$200. Path B: same user submits the same swap via Flashbots Protect RPC. The transaction goes directly to a builder, never appears in the public mempool, and executes at the unmoved price. Same gas cost, same execution, no sandwich loss. The Flashbots Protect path costs nothing extra and is a one-line MetaMask configuration change. For any meaningful trade size, it's the default-correct option.
Common mistakes
- Assuming the public mempool is some kind of authoritative state. It's a transient queue; transactions can be dropped, replaced, or reordered.
- Ignoring MEV for 'small' trades. The threshold at which sandwich attacks become profitable is lower than most users assume — a few thousand dollars on a thin pool is enough.
- Treating Flashbots Protect as 'paranoid.' It's standard infrastructure used by serious traders for years; the marginal cost is zero and the benefit is real.
- Forgetting that builders see your transaction even when the mempool doesn't. Private submission reduces but does not eliminate information leakage.
- Generalising EVM mempool dynamics to Solana or Bitcoin. The submission models are different; the MEV profiles are different.
Check your understanding
You are about to execute a $30,000 DEX swap on Uniswap V3. Which submission method most directly reduces your exposure to sandwich attacks?
Key terms covered
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