feat(mintd): allow multiple lightning backends per currency unit

crates/cdk-integration-tests/src/bin/start_regtest_mints.rs

@@ -302,14 +302,15 @@ fn create_ldk_settings(
         },
         mint_info: cdk_mintd::config::MintInfo::default(),
         limits: cdk_mintd::config::Limits::default(),
-        ln: cdk_mintd::config::Ln {
+        ln: vec![cdk_mintd::config::Ln {
             ln_backend: cdk_mintd::config::LnBackend::LdkNode,
+            unit: cdk::nuts::CurrencyUnit::Sat,
             invoice_description: None,
             min_mint: 1.into(),
             max_mint: 500_000.into(),
             min_melt: 1.into(),
             max_melt: 500_000.into(),
-        },
+        }],
         cln: None,
         lnbits: None,
         lnd: None,

crates/cdk-integration-tests/src/shared.rs

@@ -220,14 +220,15 @@ pub fn create_fake_wallet_settings(
         },
         mint_info: cdk_mintd::config::MintInfo::default(),
         limits: cdk_mintd::config::Limits::default(),
-        ln: cdk_mintd::config::Ln {
+        ln: vec![cdk_mintd::config::Ln {
             ln_backend: cdk_mintd::config::LnBackend::FakeWallet,
+            unit: cdk::nuts::CurrencyUnit::Sat,
             invoice_description: None,
             min_mint: DEFAULT_MIN_MINT.into(),
             max_mint: DEFAULT_MAX_MINT.into(),
             min_melt: DEFAULT_MIN_MELT.into(),
             max_melt: DEFAULT_MAX_MELT.into(),
-        },
+        }],
         cln: None,
         lnbits: None,
         lnd: None,
@@ -276,14 +277,15 @@ pub fn create_cln_settings(
         },
         mint_info: cdk_mintd::config::MintInfo::default(),
         limits: cdk_mintd::config::Limits::default(),
-        ln: cdk_mintd::config::Ln {
+        ln: vec![cdk_mintd::config::Ln {
             ln_backend: cdk_mintd::config::LnBackend::Cln,
+            unit: cdk::nuts::CurrencyUnit::Sat,
             invoice_description: None,
             min_mint: DEFAULT_MIN_MINT.into(),
             max_mint: DEFAULT_MAX_MINT.into(),
             min_melt: DEFAULT_MIN_MELT.into(),
             max_melt: DEFAULT_MAX_MELT.into(),
-        },
+        }],
         cln: Some(cln_config),
         lnbits: None,
         lnd: None,
@@ -327,14 +329,15 @@ pub fn create_lnd_settings(
         },
         mint_info: cdk_mintd::config::MintInfo::default(),
         limits: cdk_mintd::config::Limits::default(),
-        ln: cdk_mintd::config::Ln {
+        ln: vec![cdk_mintd::config::Ln {
             ln_backend: cdk_mintd::config::LnBackend::Lnd,
+            unit: cdk::nuts::CurrencyUnit::Sat,
             invoice_description: None,
             min_mint: DEFAULT_MIN_MINT.into(),
             max_mint: DEFAULT_MAX_MINT.into(),
             min_melt: DEFAULT_MIN_MELT.into(),
             max_melt: DEFAULT_MAX_MELT.into(),
-        },
+        }],
         cln: None,
         lnbits: None,
         ldk_node: None,

crates/cdk-mintd/README.md

@@ -186,6 +186,32 @@ engine = "postgres"
 url = "postgresql://mint_user:password@localhost:5432/cdk_mint"
 ```
 
+### With Multiple Lightning Backends
+
+A single mint can serve more than one currency unit by configuring a separate backend per unit. Replace the single `[ln]` block with one `[[ln]]` block per `(unit, method)` pair, and keep the existing per-backend config sections (`[cln]`, `[lnbits]`, etc.) as-is.
+
+```toml
+[[ln]]
+ln_backend = "cln"
+unit = "sat"
+
+[[ln]]
+ln_backend = "lnbits"
+unit = "eur"
+
+[cln]
+rpc_path = "/home/bitcoin/.lightning/bitcoin/lightning-rpc"
+
+[lnbits]
+admin_api_key = "..."
+invoice_api_key = "..."
+lnbits_api = "https://lnbits.example.com"
+```
+
+Each `[[ln]]` block carries its own `min_mint`, `max_mint`, `min_melt`, `max_melt` if you want different limits per unit. Two entries colliding on the same `(unit, method)` pair are rejected at startup.
+
+The legacy single `[ln]` form is still accepted; it's equivalent to one `[[ln]]` entry with `unit = "sat"` (the default). `CDK_MINTD_LN_*` environment variables only apply when there is exactly one (or zero) `[[ln]]` entry — multi-backend setups must be configured via the file.
+
 ## Directory Structure
 
 After setup and first run, your directory will look like:

crates/cdk-mintd/example.config.toml

@@ -93,9 +93,19 @@ max_connections = 20
 # Connection timeout in seconds (optional, defaults to 10)
 connection_timeout_seconds = 10
 
+# Lightning backends. Use [ln] for a single backend, or repeat [[ln]] for one
+# backend per (unit, method) pair, e.g.:
+#   [[ln]]
+#   ln_backend = "cln"
+#   unit = "sat"
+#   [[ln]]
+#   ln_backend = "lnbits"
+#   unit = "eur"
+
 [ln]
 # Required ln backend `cln`, `lnd`, `fakewallet`, 'lnbits', 'ldknode'
 ln_backend = "fakewallet"
+# unit = "sat"          # Optional, defaults to "sat"
 # min_mint=1
 # max_mint=500000
 # min_melt=1
@@ -175,7 +185,6 @@ ln_backend = "fakewallet"
 # webserver_port = 0  # 0 = auto-assign available port
 
 [fake_wallet]
-supported_units = ["sat"]
 fee_percent = 0.02
 reserve_fee_min = 1
 min_delay_time = 1
@@ -202,7 +211,6 @@ max_delay_time = 3
 
 # [grpc_processor]
 # gRPC Payment Processor configuration
-# supported_units = ["sat"]
 # addr = "127.0.0.1"
 # port = 50051
 # tls_dir = "/path/to/tls"

crates/cdk-mintd/src/config.rs

@@ -177,17 +177,33 @@ impl std::str::FromStr for LnBackend {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Ln {
     pub ln_backend: LnBackend,
+    #[serde(default)]
+    pub unit: CurrencyUnit,
+    #[serde(default)]
     pub invoice_description: Option<String>,
+    #[serde(default = "default_ln_min")]
     pub min_mint: Amount,
+    #[serde(default = "default_ln_max")]
     pub max_mint: Amount,
+    #[serde(default = "default_ln_min")]
     pub min_melt: Amount,
+    #[serde(default = "default_ln_max")]
     pub max_melt: Amount,
 }
 
+fn default_ln_min() -> Amount {
+    1.into()
+}
+
+fn default_ln_max() -> Amount {
+    500_000.into()
+}
+
 impl Default for Ln {
     fn default() -> Self {
         Ln {
             ln_backend: LnBackend::default(),
+            unit: CurrencyUnit::default(),
             invoice_description: None,
             min_mint: 1.into(),
             max_mint: 500_000.into(),
@@ -197,6 +213,23 @@ impl Default for Ln {
     }
 }
 
+fn deserialize_ln<'de, D>(deserializer: D) -> Result<Vec<Ln>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    #[derive(Deserialize)]
+    #[serde(untagged)]
+    enum LnOneOrMany {
+        Many(Vec<Ln>),
+        One(Ln),
+    }
+
+    match LnOneOrMany::deserialize(deserializer)? {
+        LnOneOrMany::Many(v) => Ok(v),
+        LnOneOrMany::One(l) => Ok(vec![l]),
+    }
+}
+
 #[cfg(feature = "lnbits")]
 #[derive(Clone, Serialize, Deserialize)]
 pub struct LNbits {
@@ -438,8 +471,11 @@ fn default_keyset_version() -> String {
 #[cfg(feature = "fakewallet")]
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct FakeWallet {
+    #[serde(default = "default_fake_wallet_supported_units")]
     pub supported_units: Vec<CurrencyUnit>,
+    #[serde(default = "default_fake_wallet_fee_percent")]
     pub fee_percent: f32,
+    #[serde(default = "default_fake_wallet_reserve_fee_min")]
     pub reserve_fee_min: Amount,
     #[serde(default = "default_min_delay_time")]
     pub min_delay_time: u64,
@@ -476,6 +512,21 @@ fn default_reserve_fee_min() -> Amount {
     2.into()
 }
 
+#[cfg(feature = "fakewallet")]
+fn default_fake_wallet_supported_units() -> Vec<CurrencyUnit> {
+    vec![CurrencyUnit::Sat]
+}
+
+#[cfg(feature = "fakewallet")]
+fn default_fake_wallet_fee_percent() -> f32 {
+    0.02
+}
+
+#[cfg(feature = "fakewallet")]
+fn default_fake_wallet_reserve_fee_min() -> Amount {
+    2.into()
+}
+
 #[cfg(feature = "fakewallet")]
 fn default_min_delay_time() -> u64 {
     1
@@ -647,7 +698,8 @@ fn default_blind() -> AuthType {
 pub struct Settings {
     pub info: Info,
     pub mint_info: MintInfo,
-    pub ln: Ln,
+    #[serde(default, deserialize_with = "deserialize_ln")]
+    pub ln: Vec<Ln>,
     /// Transaction limits for DoS protection
     #[serde(default)]
     pub limits: Limits,
@@ -859,6 +911,95 @@ mod tests {
         assert!(debug_output.contains("<hashed: "));
     }
 
+    #[cfg(feature = "fakewallet")]
+    #[test]
+    fn test_multi_backend_config_parses() {
+        use std::{env, fs};
+
+        let temp_dir = env::temp_dir().join("cdk_test_multi_backend_config");
+        fs::create_dir_all(&temp_dir).expect("Failed to create temp dir");
+        let config_path = temp_dir.join("config.toml");
+
+        let config_content = r#"
+[[ln]]
+ln_backend = "fakewallet"
+unit = "sat"
+min_mint = 1
+max_mint = 500000
+min_melt = 1
+max_melt = 500000
+
+[[ln]]
+ln_backend = "fakewallet"
+unit = "eur"
+min_mint = 1
+max_mint = 1000
+min_melt = 1
+max_melt = 1000
+"#;
+        fs::write(&config_path, config_content).expect("Failed to write config file");
+
+        let settings = Settings::new(Some(&config_path));
+
+        assert_eq!(settings.ln.len(), 2);
+
+        assert_eq!(settings.ln[0].ln_backend, LnBackend::FakeWallet);
+        assert_eq!(settings.ln[0].unit, CurrencyUnit::Sat);
+        let max_mint_0: u64 = settings.ln[0].max_mint.into();
+        assert_eq!(max_mint_0, 500_000);
+
+        assert_eq!(settings.ln[1].ln_backend, LnBackend::FakeWallet);
+        assert_eq!(settings.ln[1].unit, CurrencyUnit::Eur);
+        let max_mint_1: u64 = settings.ln[1].max_mint.into();
+        assert_eq!(max_mint_1, 1_000);
+
+        let _ = fs::remove_dir_all(&temp_dir);
+    }
+
+    #[cfg(feature = "fakewallet")]
+    #[test]
+    fn test_legacy_ln_block_parses() {
+        use std::{env, fs};
+
+        let temp_dir = env::temp_dir().join("cdk_test_legacy_ln_block");
+        fs::create_dir_all(&temp_dir).expect("Failed to create temp dir");
+        let config_path = temp_dir.join("config.toml");
+
+        let config_content = r#"
+[ln]
+ln_backend = "fakewallet"
+min_mint = 1
+max_mint = 500000
+min_melt = 1
+max_melt = 500000
+"#;
+        fs::write(&config_path, config_content).expect("Failed to write config file");
+
+        let settings = Settings::new(Some(&config_path));
+
+        assert_eq!(settings.ln.len(), 1);
+        assert_eq!(settings.ln[0].ln_backend, LnBackend::FakeWallet);
+        assert_eq!(settings.ln[0].unit, CurrencyUnit::Sat);
+
+        let _ = fs::remove_dir_all(&temp_dir);
+    }
+
+    #[cfg(feature = "fakewallet")]
+    #[test]
+    fn test_example_config_loads() {
+        let path = std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("example.config.toml");
+        let settings = Settings::new_from_default(&Settings::default(), Some(&path))
+            .expect("example.config.toml must parse");
+        assert_eq!(
+            settings.ln.len(),
+            1,
+            "example.config.toml ln backend was dropped: {:?}",
+            settings.ln
+        );
+        assert_eq!(settings.ln[0].ln_backend, LnBackend::FakeWallet);
+        assert_eq!(settings.ln[0].unit, CurrencyUnit::Sat);
+    }
+
     /// Test that configuration can be loaded purely from environment variables
     /// without requiring a config.toml file with backend sections.
     ///