Mempool & Fees

The mempool is each node’s holding area for valid unconfirmed transactions. Miners (and other nodes) choose what to relay and mine based on policy and fees, not every consensus-valid transaction is welcome under default policy.

Why this matters

Users experience Bitcoin through confirmation delays and fee spikes. Wallets that cannot bump fees create support nightmares.

Analogy

The mempool is an auction house lobby. Anyone can submit a bid (transaction). Higher fee rates tend to get seated (mined) first when space is scarce. Some items are rejected at the door for policy reasons even if they are “legal.”

Fee rate basics

const feeRateSatVb = feeSats / vsize;

Estimate using node RPCs (estimatesmartfee) or public observers, then add UX affordances for slow/medium/fast targets.

RBF and CPFP

• RBF (BIP-125): replace an unconfirmed tx with a higher-fee variant (signals required)
• CPFP: spend an unconfirmed output with a high-fee child that incentivizes mining both

Common mistakes

• Broadcasting without RBF signaling when users may need to bump
• Overpaying dramatically due to stale fee estimates
• Creating long unconfirmed chains that hit ancestor/descendant limits

Worked mental model

Re-read the diagrams in this lesson once out loud in plain language. If you cannot explain the flow to a friend without jargon, pause and revisit Mastering Bitcoin / Mastering Lightning chapters linked in Resources. Chapter references are intentional, not decorative.

Hands-on habit

Every protocol idea should be paired with one local experiment:

1. Reproduce the happy path on regtest (or Polar for Lightning)
2. Break it on purpose (wrong fee, expired invoice, offline peer)
3. Write down what error you saw and which layer produced it (wallet, node, mempool, peer)

That habit turns reading into builder instinct.

Glossary check

Pick three terms from this lesson and define them in one sentence each without opening notes. Weak definitions mean the lesson is not finished yet.

Resource order

Use Resources in order: narrative book chapter first, then BIP/BOLT for precision, then implementation docs for commands. Jumping straight to RPC flags without the mental model creates brittle knowledge.

Next steps

Study the P2P network: how transactions and blocks actually propagate, then mining and consensus.