Lightning Ldk

LDK (Lightning Dev Kit)

LDK is a library, not a daemon. Provides Rust crates with C/Swift/ Kotlin/JS bindings. Used by Mutiny Wallet, Cash App, Casa, ldk-node, Phoenix (parts), etc.

Crates (selected)

• lightning — core protocol logic.
• lightning-net-tokio — async TCP transport.
• lightning-persister — file-based persistence.
• lightning-background-processor — background task driver.
• lightning-block-sync — chain sync.
• lightning-transaction-sync — alternative tx-only sync.
• lightning-rapid-gossip-sync — fast gossip via signed snapshot.
• lightning-invoice — BOLT11 invoice handling.
• ldk-node — opinionated bundling for quick-start.
• lightning-liquidity — LSP client/server primitives.
• bdk integration crates.

Core primitives

// Persistence trait
trait Persist<ChannelSigner> { ... }

// Channel manager — owns all channels
let chan_mgr = ChannelManager::new(...);

// Chain monitoring — watches outputs, force-close events
let chain_mon = ChainMonitor::new(...);

// Router — pathfinding
let router = DefaultRouter::new(...);

// Background processor — drives async tasks
BackgroundProcessor::start(...);

User code provides:

• KeysManager — produces/derives keys.
• NetworkGraph — channel graph storage.
• EventHandler — react to events (payment received, channel opened).
• Persister — save state to disk / DB.
• FeeEstimator — fee rate source.
• BroadcasterInterface — broadcast txs.

ldk-node (quick-start)

use ldk_node::{Builder, Network};

let builder = Builder::new()
    .set_network(Network::Bitcoin)
    .set_chain_source_esplora("https://blockstream.info/api".into())
    .set_storage_dir_path("/path/to/data".into());
let node = builder.build().unwrap();
node.start().unwrap();

// Use the node
let address = node.onchain_payment().new_address().unwrap();
let invoice = node.bolt11_payment().receive(amount_msat, "desc", 3600).unwrap();
node.bolt11_payment().send(&invoice, None).unwrap();

ldk-node exposes a single API surface across Rust + Swift/Kotlin/JS bindings.

Async signer

For hardware wallets / remote signers:

trait NodeSigner { fn ecdh(&self, ...) -> Result<...>; ... }
trait ChannelSigner { fn sign_counterparty_commitment(&self, ...) -> ...; ... }
trait SignerProvider { fn derive_channel_signer(&self, ...) -> ChannelSigner; ... }

LDK supports async signing via EventHandler::handle_event returning deferred sigs. Useful for hardware-wallet-backed Lightning.

Persistence model

LDK doesn't dictate storage. Common backends:

• File: lightning-persister writes channel state to disk.
• Database: SQLite, PostgreSQL — implement Persist trait.
• Cloud: persist channel state encrypted to cloud storage (Mutiny does this).
• Trustless replication: VSS (Versioned Storage Service) — Lightning Labs' encrypted cloud storage protocol.

Chain sync options

• bitcoind RPC — lightning-block-sync polls / ZMQs.
• Esplora API — lightning-transaction-sync via Esplora REST.
• Electrum — same crate, via Electrum protocol.
• Neutrino (BIP157/158) — light-client; for mobile.

Mobile uses Neutrino: lightning-block-sync downloads filters, scans locally for UTXO matches, downloads only relevant blocks.

Bindings

LDK provides:

• Rust native crates.
• C via lightning-c-bindings.
• Swift / iOS via LDKSwift and LDKNode.
• Kotlin / Android via LDKKotlin and ldk-node-kotlin.
• JS / WASM via ldk-node-js (experimental).

ldk-node specifically targets Swift/Kotlin/JS so mobile devs get a single API.

Use cases

• Mutiny Wallet — browser-based LN via WASM + LDK.
• Cash App — backend LN.
• Casa — vault + LN integration.
• Phoenix (partial) — uses LDK components.
• Greenlight (Blockstream) — alternative; CLN-based.

Memory footprint

LDK is light: ~10-20 MB RAM for a typical mobile node. Bitcoin Core

• LND can use 1+ GB. This is the main reason LDK dominates mobile.

Comparing LDK to LND/CLN

Common issues

• Persist trait misimplementation: missed updates → state corruption on restart.
• Async signer race conditions: signing requests racing with channel updates.
• Chain sync gaps: skipped blocks → channel state diverges.
• ldk-node migration: upgrading minor versions sometimes requires state migration; back up first.

See also

• bolts/SKILL.md
• channels/SKILL.md
• ../../libraries/ldk/SKILL.md
• consumer-wallets/SKILL.md