Persist backend per database id in admin metadata so restarts and lazy opens always use the correct engine (Sled/Redb)

src/admin_meta.rs

@@ -69,9 +69,38 @@ pub fn open_admin_storage(
         if let Some(st) = w.get(base_dir) {
             return Ok(st.clone());
         }
-        let st = init_admin_storage(base_dir, backend, admin_secret)?;
+
+        // Detect existing 0.db backend by filesystem, if present.
+        let admin_path = PathBuf::from(base_dir).join("0.db");
+        let detected = if admin_path.exists() {
+            if admin_path.is_file() {
+                Some(options::BackendType::Redb)
+            } else if admin_path.is_dir() {
+                Some(options::BackendType::Sled)
+            } else {
+                None
+            }
+        } else {
+            None
+        };
+
+        let effective_backend = match detected {
+            Some(d) if d != backend => {
+                eprintln!(
+                    "warning: Admin DB 0 at {} appears to be {:?}, but process default is {:?}. Using detected backend.",
+                    admin_path.display(),
+                    d,
+                    backend
+                );
+                d
+            }
+            Some(d) => d,
+            None => backend, // First boot: use requested backend to initialize 0.db
+        };
+
+        let st = init_admin_storage(base_dir, effective_backend, admin_secret)?;
         w.insert(base_dir.to_string(), st.clone());
-        return Ok(st);
+        Ok(st)
     }
 }
 
@@ -122,7 +151,39 @@ pub fn open_data_storage(
         return Ok(st.clone());
     }
 
-    // Determine per-db encryption
+    // Resolve effective backend for this db id:
+    // 1) Try admin meta "backend" field
+    // 2) If missing, sniff filesystem (file => Redb, dir => Sled), then persist into admin meta
+    // 3) Fallback to requested 'backend' (startup default) if nothing else is known
+    let meta_backend = get_database_backend(base_dir, backend.clone(), admin_secret, id).ok().flatten();
+    let db_path = PathBuf::from(base_dir).join(format!("{}.db", id));
+    let sniffed_backend = if db_path.exists() {
+        if db_path.is_file() {
+            Some(options::BackendType::Redb)
+        } else if db_path.is_dir() {
+            Some(options::BackendType::Sled)
+        } else {
+            None
+        }
+    } else {
+        None
+    };
+    let effective_backend = meta_backend.clone().or(sniffed_backend).unwrap_or(backend.clone());
+
+    // If we had to sniff (i.e., meta missing), persist it for future robustness
+    if meta_backend.is_none() {
+        let _ = set_database_backend(base_dir, backend.clone(), admin_secret, id, effective_backend.clone());
+    }
+
+    // Warn if caller-provided backend differs from effective
+    if effective_backend != backend {
+        eprintln!(
+            "notice: Database {} backend resolved to {:?} (caller requested {:?}). Using resolved backend.",
+            id, effective_backend, backend
+        );
+    }
+
+    // Determine per-db encryption (from admin meta)
     let enc = get_enc_key(base_dir, backend.clone(), admin_secret, id)?;
     let should_encrypt = enc.is_some();
 
@@ -134,8 +195,8 @@ pub fn open_data_storage(
         })?;
     }
 
-    // Open storage
+    // Open storage using the effective backend
-    let storage: Arc<dyn StorageBackend> = match backend {
+    let storage: Arc<dyn StorageBackend> = match effective_backend {
         options::BackendType::Redb => Arc::new(Storage::new(&db_file, should_encrypt, enc.as_deref())?),
         options::BackendType::Sled => Arc::new(SledStorage::new(&db_file, should_encrypt, enc.as_deref())?),
     };
@@ -217,6 +278,39 @@ pub fn set_database_public(
     Ok(())
 }
 
+// Persist per-db backend type in admin metadata (module-scope)
+pub fn set_database_backend(
+    base_dir: &str,
+    backend: options::BackendType,
+    admin_secret: &str,
+    id: u64,
+    db_backend: options::BackendType,
+) -> Result<(), DBError> {
+    let admin = open_admin_storage(base_dir, backend, admin_secret)?;
+    let mk = k_meta_db(id);
+    let val = match db_backend {
+        options::BackendType::Redb => "Redb",
+        options::BackendType::Sled => "Sled",
+    };
+    let _ = admin.hset(&mk, vec![("backend".to_string(), val.to_string())])?;
+    Ok(())
+}
+
+pub fn get_database_backend(
+    base_dir: &str,
+    backend: options::BackendType,
+    admin_secret: &str,
+    id: u64,
+) -> Result<Option<options::BackendType>, DBError> {
+    let admin = open_admin_storage(base_dir, backend, admin_secret)?;
+    let mk = k_meta_db(id);
+    match admin.hget(&mk, "backend")? {
+        Some(s) if s == "Redb" => Ok(Some(options::BackendType::Redb)),
+        Some(s) if s == "Sled" => Ok(Some(options::BackendType::Sled)),
+        _ => Ok(None),
+    }
+}
+
 // Set database name
 pub fn set_database_name(
     base_dir: &str,

src/options.rs

@@ -1,4 +1,4 @@
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub enum BackendType {
     Redb,
     Sled,

src/rpc.rs

@@ -158,14 +158,42 @@ impl RpcServerImpl {
             ));
         }
 
-        // Create server instance with default options
+        // Resolve effective backend for this db from admin meta or filesystem; fallback to default
+        let meta_backend = admin_meta::get_database_backend(&self.base_dir, self.backend.clone(), &self.admin_secret, db_id)
+            .ok()
+            .flatten();
+        let db_path = std::path::PathBuf::from(&self.base_dir).join(format!("{}.db", db_id));
+        let sniffed_backend = if db_path.exists() {
+            if db_path.is_file() {
+                Some(crate::options::BackendType::Redb)
+            } else if db_path.is_dir() {
+                Some(crate::options::BackendType::Sled)
+            } else {
+                None
+            }
+        } else {
+            None
+        };
+        let effective_backend = meta_backend.clone().or(sniffed_backend).unwrap_or(self.backend.clone());
+        if effective_backend != self.backend {
+            eprintln!(
+                "notice: get_or_create_server: db {} backend resolved to {:?} (server default {:?})",
+                db_id, effective_backend, self.backend
+            );
+        }
+        // If we had to sniff (no meta), persist the resolved backend
+        if meta_backend.is_none() {
+            let _ = admin_meta::set_database_backend(&self.base_dir, self.backend.clone(), &self.admin_secret, db_id, effective_backend.clone());
+        }
+
+        // Create server instance with resolved backend
         let db_option = DBOption {
             dir: self.base_dir.clone(),
             port: 0, // Not used for RPC-managed databases
             debug: false,
             encryption_key: None,
             encrypt: false,
-            backend: self.backend.clone(),
+            backend: effective_backend,
             admin_secret: self.admin_secret.clone(),
         };
 
@@ -308,17 +336,22 @@ impl RpcServer for RpcServerImpl {
             return Err(jsonrpsee::types::ErrorObjectOwned::owned(-32000, format!("Failed to ensure base dir: {}", e), None::<()>));
         }
 
-        // Create server instance using base_dir and admin secret
+        // Map RPC backend to options backend and persist it in admin meta for this db id
+        let opt_backend = match backend {
+            BackendType::Redb => crate::options::BackendType::Redb,
+            BackendType::Sled => crate::options::BackendType::Sled,
+        };
+        admin_meta::set_database_backend(&self.base_dir, self.backend.clone(), &self.admin_secret, db_id, opt_backend.clone())
+            .map_err(|e| jsonrpsee::types::ErrorObjectOwned::owned(-32000, e.0, None::<()>))?;
+
+        // Create server instance using base_dir, chosen backend and admin secret
         let option = DBOption {
             dir: self.base_dir.clone(),
             port: 0, // Not used for RPC-managed databases
             debug: false,
             encryption_key: None,  // per-db key is stored in admin DB 0
             encrypt: false,        // encryption decided per-db at open time
-            backend: match backend {
+            backend: opt_backend,
-                BackendType::Redb => crate::options::BackendType::Redb,
-                BackendType::Sled => crate::options::BackendType::Sled,
-            },
             admin_secret: self.admin_secret.clone(),
         };