lnd-navigate

The user needs help navigating or understanding the lnd codebase at https://github.com/lightningnetwork/lnd. Use the reference below to locate code quickly and explain how subsystems relate.

---

lnd Codebase Navigation Guide

Entry Points

File	Role
main.go	Binary entry point. Parses config, calls lnd.Main().
lnd.go	Main() function. Initialises chain backend, wallet, then starts server.
server.go	server struct. Owns all subsystem handles. NewServer() wires everything together; Start() / Stop() lifecycle.
rpcserver.go	rpcServer — implements the LightningServer gRPC interface. All external API calls enter here.
config.go	Top-level Config struct. Parsed from flags + lnd.conf.
subrpcserver_config.go	Aggregates configs for all gRPC subservers.
accessman.go	Per-peer slot/rate-limit manager; controls who may open channels or send messages.

Start reading at server.go:NewServer() to understand how the daemon assembles itself.

---

Key Subsystems

Payment Routing — routing/

• routing/router.go — ChannelRouter: the central router. Pathfinding, mission control, payment lifecycle state machine (SendPayment, SendToRoute).
• routing/pathfind.go — Dijkstra-based pathfinder with probability weighting.
• routing/missioncontrol.go / missioncontrol_state.go — tracks per-pair failure history to bias future path choices.
• routing/payment_lifecycle.go — drives a single payment from initiation through HTLC attempts to settlement or failure.
• routing/payment_session.go — wraps the pathfinder for one payment attempt; handles fee limits, MPP splitting.
• routing/blindedpath/ — BOLT-12 blinded route construction (BuildBlindedPaymentPaths).
• routing/route/ — Route, Hop, Vertex types; ToSphinxPath for onion packet construction.
• routing/chainview/ — FilteredChainView interface; backends for btcd, bitcoind, neutrino. Feeds channel-close events to the router.

HTLC Switch — htlcswitch/

• htlcswitch/switch.go — Switch: forwards, settles, and fails HTLC packets between links. Maintains the circuit map.
• htlcswitch/link.go — channelLink: per-channel HTLC processor. Commitment update loop, bandwidth tracking, retransmission on reconnect.
• htlcswitch/interceptable_switch.go — InterceptableSwitch: wraps Switch to support HTLC interception hooks (used by routerrpc).
• htlcswitch/hop/ — Payload, Iterator: TLV hop payload parsing and iteration for onion packets.
• htlcswitch/hodl/ — Flags that let tests hold HTLCs at various pipeline stages.

Channel Wallet — lnwallet/

• lnwallet/wallet.go — LightningWallet: on-chain wallet and channel factory. Coin selection, UTXO management, channel reservation lifecycle.
• lnwallet/channel.go — LightningChannel: low-level channel state machine. Commitment transactions, HTLC updates, revocation, signing.
• lnwallet/interface.go — WalletController, BlockChainIO, Signer, MessageSigner, Keychain interfaces.
• lnwallet/reservation.go — ChannelReservation: in-flight channel open negotiation state.
• lnwallet/btcwallet/ — Production WalletController backed by btcwallet.
• lnwallet/rpcwallet/ — RPCKeyRing remote-signer pattern: all signing delegated to a remote gRPC endpoint; local node is watch-only.
• lnwallet/chanfunding/ — Assembler/Intent interfaces for funding flows: WalletAssembler (normal), CannedAssembler (pre-signed), PsbtAssembler (PSBT external signing).
• lnwallet/chancloser/ — ChanCloser cooperative-close state machine: closeIdle → closingNegotiated → closeComplete.

Channel Funding — funding/

• funding/manager.go — Manager: drives the channel-open state machine from OpenChannel RPC through funding tx broadcast and channel ready.

Channel Database — channeldb/

• channeldb/db.go — Top-level BoltDB/SQL store. Open/close channels, forwarding log, peers.
• channeldb/channel.go — OpenChannel: serialization of all channel state to disk.
• channeldb/invoices.go — Invoice storage (legacy; SQL path in invoices/sql_store.go).
• channeldb/migrations/ — 35 sequential schema migrations (migration_01_to_11 … migration35).

Routing Graph Database — graph/db/

• graph/db/ — ChannelGraph + Store interface. KV store (BoltDB) and SQL implementations.
• graph/db/models/ — LightningNode, ChannelEdgeInfo, ChannelEdgePolicy, CachedEdgePolicy.
• The in-memory GraphCache lives in graph/db/graph_cache.go; it provides O(1) neighbor lookups during pathfinding.

Invoice Registry — invoices/

• invoices/invoiceregistry.go — InvoiceRegistry: central hub for adding, settling, cancelling invoices and notifying subscribers.
• invoices/sql_store.go — SQL-backed invoice store.
• invoices/resolution.go — HtlcResolution types returned to the switch on settle/cancel.

On-Chain Arbitrator — contractcourt/

• contractcourt/ — Watches the chain for unilateral closes and breaches. Drives justice and second-level HTLC transactions.
• Key types: BreachArbitrator, ChannelArbitrator, ChainWatcher.

UTXO Sweeper — sweep/

• sweep/sweeper.go — Batches time-sensitive outputs (HTLC timeout/success, commit outputs). Fee-bumps via CPFP/RBF.
• sweep/fee_bumper.go — TxPublisher: manages bump attempts and monitors confirms.
• sweep/txgenerator.go — Builds the sweep transaction from a set of inputs.

Gossip / Network Discovery — discovery/

• discovery/gossiper.go — AuthenticatedGossiper: validates and propagates channel announcements, node updates, channel updates.
• discovery/syncer.go — GossipSyncer: per-peer gossip sync state machine.

Peer Management — peer/

• peer/brontide.go — Brontide: per-peer connection handler. Reads/writes wire messages, owns channel links, handles reconnect.

Chain Notifications — chainntnfs/

• chainntnfs/txnotifier.go — TxNotifier: central confirmation/spend notification engine shared by all backends.
• chainntnfs/bitcoindnotify/, btcdnotify/, neutrinonotify/ — three ChainNotifier implementations.

Chain Fan-Out — chainio/

• chainio/ — Blockbeat / Consumer interfaces. Delivers new block events to all registered subsystems in priority order before advancing.

Watchtower — watchtower/

• watchtower/wtclient/ — Client-side manager: encrypts and uploads channel state backups, negotiates sessions.
• watchtower/lookout/ — Server-side breach detection: scans each block for breach hints, queues justice txns.
• watchtower/blob/ — JusticeKit encrypted breach-remedy data; BreachHint/BreachKey.
• watchtower/wtserver/ — Tower gRPC server (session create, state update, delete).
• watchtower/wtwire/ — Tower wire protocol message types.

gRPC Subservers — lnrpc/

Each package follows the same pattern:

1. A ServerShell struct (holds config/deps, created before wallet unlock).
2. After unlock: CreateSubServer() returns the live server.
3. RegisterWithRootServer(grpc.Server) and RegisterWithRestServer(mux) register the gRPC and REST handlers.

Subpackage	What it exposes
lnrpc/routerrpc/	SendPaymentV2, BuildRoute, QueryRoutes, HTLC interceptor, MC reset
lnrpc/walletrpc/	UTXOs, PSBT flows, addresses, coin selection, bump fee, account import
lnrpc/invoicesrpc/	AddHoldInvoice, SettleInvoice, CancelInvoice, SubscribeSingleInvoice
lnrpc/signrpc/	SignMessage, ComputeInputScript, MuSig2 multi-round flows
lnrpc/chainrpc/	RegisterConfirmationsNtfn, RegisterSpendNtfn, RegisterBlockEpochNtfn
lnrpc/autopilotrpc/	Autopilot status, scores, enable/disable
lnrpc/peersrpc/	UpdateNodeAnnouncement
lnrpc/neutrinorpc/	Neutrino chain backend control
lnrpc/watchtowerrpc/	Tower server info
lnrpc/wtclientrpc/	Tower client management (add/remove/list towers)
lnrpc/devrpc/	Dev-only: ImportGraph, Quiesce
lnrpc/verrpc/	GetVersion

Storage Backends — kvdb/ and sqldb/

• kvdb/ — walletdb-compatible KV interface; backends: BoltDB (default), etcd (kvdb/etcd/), Postgres (kvdb/postgres/), SQLite (kvdb/sqlite/).
• sqldb/ — SQL connection layer; sqldb/sqlc/ holds SQLC-generated type-safe query code.
• payments/db/ — PaymentControl interface (KV and SQL implementations) for payment/HTLC attempt state.

Wire Protocol — lnwire/

All BOLT wire message structs and their Encode/Decode live here. If you need to find the Go type for any LN protocol message, start in lnwire/messages.go.

---

Common Navigation Tasks

"Where does a payment start?" rpcserver.go:SendPaymentSync → routing/router.go:SendPayment → routing/payment_lifecycle.go

"Where is an HTLC forwarded?" htlcswitch/switch.go:forward → finds outgoing channelLink → htlcswitch/link.go:handleSwitchPacket

"Where does a channel open?" rpcserver.go:OpenChannel → funding/manager.go:InitFundingWorkflow → lnwallet/wallet.go:InitChannelReservation

"Where does a channel close cooperatively?" rpcserver.go:CloseChannel → server.go:CloseChannel → peer/brontide.go → lnwallet/chancloser/chancloser.go

"Where is channel state persisted?" lnwallet/channel.go calls into channeldb/channel.go:OpenChannel.AppendRemoteCommitChain / AdvanceCommitChainTip

"Where does gossip come in?" peer/brontide.go:msgHandler dispatches announcement messages → discovery/gossiper.go:ProcessRemoteAnnouncement

"Where is the routing graph updated?" discovery/gossiper.go validates and writes to graph/db/ via the Store interface; the GraphCache is kept in sync.

"Where does breach detection happen?" contractcourt/breach_arbitrator.go on the client side; watchtower/lookout/ on the tower side.

---

Key Interfaces to Know

Interface	Defined in	Implemented by
WalletController	lnwallet/interface.go	btcwallet/BtcWallet, rpcwallet/RPCKeyRing
ChainNotifier	chainntnfs/interface.go	bitcoindnotify, btcdnotify, neutrinonotify
FilteredChainView	routing/chainview/interface.go	btcd, bitcoind, neutrino chainviews
MessageSigner	lnwallet/interface.go	BtcWallet, RPCKeyRing
Blockbeat / Consumer	chainio/	All block-consuming subsystems
PaymentControl	payments/db/	KVStore, SQLStore
Store (graph)	graph/db/	KVStore, SQL store
Assembler / Intent	lnwallet/chanfunding/	WalletAssembler, CannedAssembler, PsbtAssembler

---

Testing

• Unit tests — alongside each source file (*_test.go).
• Integration harness — lntest/: HarnessNode (wraps a running lnd process), HarnessRPC (typed wrappers for every gRPC call), lntest/mock/ (chain IO, notifier, key ring mocks).
• Integration test cases — itest/: grouped by feature area (itest_wallet_test.go, itest_channel_test.go, etc.). Run with make itest.
• Shared wallet test helpers — lnwallet/test/: exercises any WalletController implementation against the same suite.
• Chain notification tests — chainntnfs/test/: shared suite run against all three ChainNotifier backends.
• Mocks: lnwallet/mock.go, htlcswitch/mock.go, routing/mock_test.go, lntest/mock/.

---

Configuration

• lncfg/ — one struct per config section (lncfg/db.go, lncfg/tor.go, etc.). Matches the [db], [tor] etc. stanzas in lnd.conf.
• config.go — top-level Config embeds all lncfg.* sub-structs.
• subrpcserver_config.go — SubRPCServerConfigs aggregates the config for every gRPC subserver.

---

Useful grep patterns

# Find where an RPC method is implemented
grep -n "func.*rpcServer.*MethodName" lnd/rpcserver.go

# Find all types implementing an interface
grep -rn "func.*WalletController" lnd/

# Find where a channeldb bucket is defined
grep -rn "bucketName\|rootBucket" lnd/channeldb/

# Find all SubServer RegisterWithRootServer implementations
grep -rn "RegisterWithRootServer" lnd/lnrpc/

# Find where a wire message type is handled
grep -rn "case \*lnwire\." lnd/peer/brontide.go