Routing

Lightning uses source-based routing: the payer (usually) constructs a path across the public graph plus hints, then onion-encrypts hop instructions.

Why this matters

Routing success rates define UX. Fee-aware, reliability-aware pathfinding separates toy demos from production wallets.

Analogy

You plan a road trip with toll costs and bridge weight limits. If a bridge is “full” (insufficient balance), you detour, often without knowing the exact traffic until you try.

Common mistakes

• Optimizing only for lowest fee and ignoring success probability
• Ignoring max HTLC / capacity constraints
• Forgetting private channel hints from invoices

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

Onion routing: how hop payloads stay private.