diff --git a/src/rdbc/embedded.rs b/src/rdbc/embedded.rs
index 54703d8a..0ced2a19 100644
--- a/src/rdbc/embedded.rs
+++ b/src/rdbc/embedded.rs
@@ -20,6 +20,7 @@ mod tests {
         },
         driver::EmbeddedDriver,
     };
+    use crate::server::config::{self, SimpleDBConfig};
 
     #[test]
     fn unit_test() -> Result<()> {
@@ -74,7 +75,13 @@ mod tests {
         ];
 
         // driver
-        let d = EmbeddedDriver::new();
+        let db_config = SimpleDBConfig {
+            block_size: 400,
+            num_of_buffers: 8,
+            buffer_manager: config::BufferMgr::LRU,
+            query_planner: config::QueryPlanner::Heuristic,
+        };
+        let d = EmbeddedDriver::new(db_config);
         // connect database
         let mut conn = d.connect("_test/rdbc")?;
         // init database
diff --git a/src/server/simpledb.rs b/src/server/simpledb.rs
index 40d645fd..83f90859 100644
--- a/src/server/simpledb.rs
+++ b/src/server/simpledb.rs
@@ -70,7 +70,7 @@ pub struct SimpleDB {
     // managers
     fm: Arc<Mutex<FileMgr>>,
     lm: Arc<Mutex<LogMgr>>,
-    bm: Arc<Mutex<dyn BufferMgr>>,
+    bm: Arc<Mutex<dyn BufferMgr + Send>>,
     mdm: Option<Arc<Mutex<MetadataMgr>>>,
     qp: Option<Arc<Mutex<dyn QueryPlanner>>>,
     up: Option<Arc<Mutex<dyn UpdatePlanner>>>,
@@ -132,7 +132,7 @@ impl SimpleDB {
     pub fn log_mgr(&self) -> Arc<Mutex<LogMgr>> {
         Arc::clone(&self.lm)
     }
-    pub fn buffer_mgr(&self) -> Arc<Mutex<dyn BufferMgr>> {
+    pub fn buffer_mgr(&self) -> Arc<Mutex<dyn BufferMgr + Send>> {
         Arc::clone(&self.bm)
     }
     pub fn metadata_mgr(&self) -> Option<Arc<Mutex<MetadataMgr>>> {
@@ -166,7 +166,7 @@ impl SimpleDB {
             ));
             let lm = Arc::new(Mutex::new(LogMgr::new(Arc::clone(&fm), LOG_FILE).unwrap()));
             let (bm_fm, bm_lm, numsbuff) = (Arc::clone(&fm), Arc::clone(&lm), cfg.num_of_buffers);
-            let bm: Arc<Mutex<dyn BufferMgr>> = match cfg.buffer_manager {
+            let bm: Arc<Mutex<dyn BufferMgr + Send>> = match cfg.buffer_manager {
                 config::BufferMgr::Naive => {
                     Arc::new(Mutex::new(NaiveBufferMgr::new(bm_fm, bm_lm, numsbuff)))
                 }
diff --git a/src/tx/bufferlist.rs b/src/tx/bufferlist.rs
index e4d7e133..408b6828 100644
--- a/src/tx/bufferlist.rs
+++ b/src/tx/bufferlist.rs
@@ -1,6 +1,7 @@
 use anyhow::Result;
 use std::{
     collections::HashMap,
+    fmt,
     sync::{Arc, Mutex},
 };
 
@@ -9,15 +10,21 @@ use crate::{
     file::block_id::BlockId,
 };
 
-#[derive(Debug, Clone)]
+#[derive(Clone)]
 pub struct BufferList {
     buffers: HashMap<BlockId, Arc<Mutex<Buffer>>>,
     pins: Vec<BlockId>,
-    bm: Arc<Mutex<dyn BufferMgr>>,
+    bm: Arc<Mutex<dyn BufferMgr + Send>>,
+}
+
+impl fmt::Debug for BufferList {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "BufferList")
+    }
 }
 
 impl BufferList {
-    pub fn new(bm: Arc<Mutex<dyn BufferMgr>>) -> Self {
+    pub fn new(bm: Arc<Mutex<dyn BufferMgr + Send>>) -> Self {
         Self {
             buffers: HashMap::new(),
             pins: vec![],
diff --git a/src/tx/recovery/manager.rs b/src/tx/recovery/manager.rs
index 25b395ed..0f765f2f 100644
--- a/src/tx/recovery/manager.rs
+++ b/src/tx/recovery/manager.rs
@@ -1,7 +1,10 @@
 use anyhow::Result;
 use chrono::NaiveDate;
 use core::fmt;
-use std::sync::{Arc, Mutex};
+use std::{
+    fmt::Debug,
+    sync::{Arc, Mutex},
+};
 
 use super::logrecord::{
     self, checkpoint_record::CheckpointRecord, commit_record::CommitRecord,
@@ -32,20 +35,26 @@ impl fmt::Display for RecoveryMgrError {
     }
 }
 
-#[derive(Debug, Clone)]
+#[derive(Clone)]
 pub struct RecoveryMgr {
     lm: Arc<Mutex<LogMgr>>,
-    bm: Arc<Mutex<dyn BufferMgr>>,
+    bm: Arc<Mutex<dyn BufferMgr + Send>>,
     tx: Arc<Mutex<Transaction>>,
     txnum: i32,
 }
 
+impl fmt::Debug for RecoveryMgr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "RecoveryMgr(Tx = {})", self.txnum)
+    }
+}
+
 impl RecoveryMgr {
     pub fn new(
         tx: Arc<Mutex<Transaction>>,
         txnum: i32,
         lm: Arc<Mutex<LogMgr>>,
-        bm: Arc<Mutex<dyn BufferMgr>>,
+        bm: Arc<Mutex<dyn BufferMgr + Send>>,
     ) -> Result<Self> {
         StartRecord::write_to_log(Arc::clone(&lm), txnum)?;
 
diff --git a/src/tx/transaction.rs b/src/tx/transaction.rs
index dfb0ea94..24f1023c 100644
--- a/src/tx/transaction.rs
+++ b/src/tx/transaction.rs
@@ -1,6 +1,7 @@
 use anyhow::Result;
 use chrono::NaiveDate;
 use std::{
+    fmt,
     sync::{Arc, Mutex},
     usize,
 };
@@ -18,19 +19,25 @@ use crate::{
 
 static END_OF_FILE: i32 = -1;
 
-#[derive(Debug, Clone)]
+#[derive(Clone)]
 pub struct Transaction {
     // static member (shared by all Transaction)
     next_tx_num: Arc<Mutex<i32>>,
 
     recovery_mgr: Option<Arc<Mutex<RecoveryMgr>>>,
     concur_mgr: ConcurrencyMgr,
-    bm: Arc<Mutex<dyn BufferMgr>>,
+    bm: Arc<Mutex<dyn BufferMgr + Send>>,
     fm: Arc<Mutex<FileMgr>>,
     txnum: i32,
     mybuffers: BufferList,
 }
 
+impl fmt::Debug for Transaction {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "Tx: {}", self.txnum)
+    }
+}
+
 impl Transaction {
     pub fn new(
         next_tx_num: Arc<Mutex<i32>>,
@@ -38,7 +45,7 @@ impl Transaction {
 
         fm: Arc<Mutex<FileMgr>>,
         lm: Arc<Mutex<LogMgr>>,
-        bm: Arc<Mutex<dyn BufferMgr>>,
+        bm: Arc<Mutex<dyn BufferMgr + Send>>,
     ) -> Result<Self> {
         let mut tran = Self {
             next_tx_num,