diff --git a/src/transaction/buffer.rs b/src/transaction/buffer.rs
index 1d7f61b..1287a28 100644
--- a/src/transaction/buffer.rs
+++ b/src/transaction/buffer.rs
@@ -6,7 +6,6 @@ use std::{
     future::Future,
 };
 use tikv_client_proto::kvrpcpb;
-use tokio::sync::{Mutex, MutexGuard};
 
 #[derive(Default)]
 struct InnerBuffer {
@@ -31,17 +30,17 @@ impl InnerBuffer {
 /// A caching layer which buffers reads and writes in a transaction.
 #[derive(Default)]
 pub struct Buffer {
-    mutations: Mutex<InnerBuffer>,
+    inner: InnerBuffer,
 }
 
 impl Buffer {
     /// Get the primary key of the buffer.
     pub async fn get_primary_key(&self) -> Option<Key> {
-        self.mutations.lock().await.primary_key.clone()
+        self.inner.primary_key.clone()
     }
     /// Get the primary key of the buffer, if not exists, use `key` as the primary key.
-    pub async fn get_primary_key_or(&self, key: &Key) -> Key {
-        self.mutations.lock().await.get_primary_key_or(key).clone()
+    pub async fn get_primary_key_or(&mut self, key: &Key) -> Key {
+        self.inner.get_primary_key_or(key).clone()
     }
 
     /// Get a value from the buffer.
@@ -55,7 +54,7 @@ impl Buffer {
 
     /// Get a value from the buffer. If the value is not present, run `f` to get
     /// the value.
-    pub async fn get_or_else<F, Fut>(&self, key: Key, f: F) -> Result<Option<Value>>
+    pub async fn get_or_else<F, Fut>(&mut self, key: Key, f: F) -> Result<Option<Value>>
     where
         F: FnOnce(Key) -> Fut,
         Fut: Future<Output = Result<Option<Value>>>,
@@ -64,8 +63,7 @@ impl Buffer {
             MutationValue::Determined(value) => Ok(value),
             MutationValue::Undetermined => {
                 let value = f(key.clone()).await?;
-                let mut mutations = self.mutations.lock().await;
-                Self::update_cache(&mut mutations, key, value.clone());
+                Self::update_cache(&mut self.inner, key, value.clone());
                 Ok(value)
             }
         }
@@ -76,7 +74,7 @@ impl Buffer {
     ///
     /// only used for snapshot read (i.e. not for `batch_get_for_update`)
     pub async fn batch_get_or_else<F, Fut>(
-        &self,
+        &mut self,
         keys: impl Iterator<Item = Key>,
         f: F,
     ) -> Result<impl Iterator<Item = KvPair>>
@@ -85,7 +83,6 @@ impl Buffer {
         Fut: Future<Output = Result<Vec<KvPair>>>,
     {
         let (cached_results, undetermined_keys) = {
-            let mutations = self.mutations.lock().await;
             // Partition the keys into those we have buffered and those we have to
             // get from the store.
             let (undetermined_keys, cached_results): (
@@ -93,7 +90,8 @@ impl Buffer {
                 Vec<(Key, MutationValue)>,
             ) = keys
                 .map(|key| {
-                    let value = mutations
+                    let value = self
+                        .inner
                         .entry_map
                         .get(&key)
                         .map(BufferEntry::get_value)
@@ -111,11 +109,10 @@ impl Buffer {
         };
 
         let fetched_results = f(Box::new(undetermined_keys)).await?;
-        let mut mutations = self.mutations.lock().await;
         for kvpair in &fetched_results {
             let key = kvpair.0.clone();
             let value = Some(kvpair.1.clone());
-            Self::update_cache(&mut mutations, key, value);
+            Self::update_cache(&mut self.inner, key, value);
         }
 
         let results = cached_results.chain(fetched_results.into_iter());
@@ -124,7 +121,7 @@ impl Buffer {
 
     /// Run `f` to fetch entries in `range` from TiKV. Combine them with mutations in local buffer. Returns the results.
     pub async fn scan_and_fetch<F, Fut>(
-        &self,
+        &mut self,
         range: BoundRange,
         limit: u32,
         f: F,
@@ -134,8 +131,7 @@ impl Buffer {
         Fut: Future<Output = Result<Vec<KvPair>>>,
     {
         // read from local buffer
-        let mut mutations = self.mutations.lock().await;
-        let mutation_range = mutations.entry_map.range(range.clone());
+        let mutation_range = self.inner.entry_map.range(range.clone());
 
         // fetch from TiKV
         // fetch more entries because some of them may be deleted.
@@ -166,7 +162,7 @@ impl Buffer {
 
         // update local buffer
         for (k, v) in &results {
-            Self::update_cache(&mut mutations, k.clone(), Some(v.clone()));
+            Self::update_cache(&mut self.inner, k.clone(), Some(v.clone()));
         }
 
         let mut res = results
@@ -179,10 +175,10 @@ impl Buffer {
     }
 
     /// Lock the given key if necessary.
-    pub async fn lock(&self, key: Key) {
-        let mutations = &mut self.mutations.lock().await;
-        mutations.primary_key.get_or_insert(key.clone());
-        let value = mutations
+    pub async fn lock(&mut self, key: Key) {
+        self.inner.primary_key.get_or_insert(key.clone());
+        let value = self
+            .inner
             .entry_map
             .entry(key)
             // Mutated keys don't need a lock.
@@ -194,25 +190,19 @@ impl Buffer {
     }
 
     /// Insert a value into the buffer (does not write through).
-    pub async fn put(&self, key: Key, value: Value) {
-        self.mutations
-            .lock()
-            .await
-            .insert(key, BufferEntry::Put(value));
+    pub async fn put(&mut self, key: Key, value: Value) {
+        self.inner.insert(key, BufferEntry::Put(value));
     }
 
     /// Mark a value as Insert mutation into the buffer (does not write through).
-    pub async fn insert(&self, key: Key, value: Value) {
-        self.mutations
-            .lock()
-            .await
-            .insert(key, BufferEntry::Insert(value));
+    pub async fn insert(&mut self, key: Key, value: Value) {
+        self.inner.insert(key, BufferEntry::Insert(value));
     }
 
     /// Mark a value as deleted.
-    pub async fn delete(&self, key: Key) {
-        let mut mutations = self.mutations.lock().await;
-        let value = mutations
+    pub async fn delete(&mut self, key: Key) {
+        let value = self
+            .inner
             .entry_map
             .entry(key.clone())
             .or_insert(BufferEntry::Del);
@@ -224,14 +214,12 @@ impl Buffer {
             new_value = BufferEntry::Del
         }
 
-        mutations.insert(key, new_value);
+        self.inner.insert(key, new_value);
     }
 
     /// Converts the buffered mutations to the proto buffer version
     pub async fn to_proto_mutations(&self) -> Vec<kvrpcpb::Mutation> {
-        self.mutations
-            .lock()
-            .await
+        self.inner
             .entry_map
             .iter()
             .filter_map(|(key, mutation)| mutation.to_proto_with_key(key))
@@ -239,16 +227,14 @@ impl Buffer {
     }
 
     async fn get_from_mutations(&self, key: &Key) -> MutationValue {
-        self.mutations
-            .lock()
-            .await
+        self.inner
             .entry_map
             .get(&key)
             .map(BufferEntry::get_value)
             .unwrap_or(MutationValue::Undetermined)
     }
 
-    fn update_cache(buffer: &mut MutexGuard<InnerBuffer>, key: Key, value: Option<Value>) {
+    fn update_cache(buffer: &mut InnerBuffer, key: Key, value: Option<Value>) {
         match buffer.entry_map.get(&key) {
             Some(BufferEntry::Locked(None)) => {
                 buffer
@@ -378,7 +364,7 @@ mod tests {
     #[tokio::test]
     #[allow(unreachable_code)]
     async fn set_and_get_from_buffer() {
-        let buffer = Buffer::default();
+        let mut buffer = Buffer::default();
         buffer
             .put(b"key1".to_vec().into(), b"value1".to_vec())
             .await;
@@ -411,7 +397,7 @@ mod tests {
     #[tokio::test]
     #[allow(unreachable_code)]
     async fn insert_and_get_from_buffer() {
-        let buffer = Buffer::default();
+        let mut buffer = Buffer::default();
         buffer
             .insert(b"key1".to_vec().into(), b"value1".to_vec())
             .await;
@@ -453,13 +439,13 @@ mod tests {
         let v2: Value = b"value2".to_vec();
         let v2_ = v2.clone();
 
-        let buffer = Buffer::default();
+        let mut buffer = Buffer::default();
         let r1 = block_on(buffer.get_or_else(k1.clone(), move |_| ready(Ok(Some(v1_)))));
         let r2 = block_on(buffer.get_or_else(k1.clone(), move |_| ready(panic!())));
         assert_eq!(r1.unwrap().unwrap(), v1);
         assert_eq!(r2.unwrap().unwrap(), v1);
 
-        let buffer = Buffer::default();
+        let mut buffer = Buffer::default();
         let r1 = block_on(
             buffer.batch_get_or_else(vec![k1.clone(), k2.clone()].into_iter(), move |_| {
                 ready(Ok(vec![(k1_, v1__).into(), (k2_, v2_).into()]))
diff --git a/src/transaction/snapshot.rs b/src/transaction/snapshot.rs
index 28711c4..3267ccd 100644
--- a/src/transaction/snapshot.rs
+++ b/src/transaction/snapshot.rs
@@ -19,18 +19,18 @@ pub struct Snapshot {
 
 impl Snapshot {
     /// Get the value associated with the given key.
-    pub async fn get(&self, key: impl Into<Key>) -> Result<Option<Value>> {
+    pub async fn get(&mut self, key: impl Into<Key>) -> Result<Option<Value>> {
         self.transaction.get(key).await
     }
 
     /// Check whether the key exists.
-    pub async fn key_exists(&self, key: impl Into<Key>) -> Result<bool> {
+    pub async fn key_exists(&mut self, key: impl Into<Key>) -> Result<bool> {
         self.transaction.key_exists(key).await
     }
 
     /// Get the values associated with the given keys.
     pub async fn batch_get(
-        &self,
+        &mut self,
         keys: impl IntoIterator<Item = impl Into<Key>>,
     ) -> Result<impl Iterator<Item = KvPair>> {
         self.transaction.batch_get(keys).await
@@ -38,7 +38,7 @@ impl Snapshot {
 
     /// Scan a range, return at most `limit` key-value pairs that lying in the range.
     pub async fn scan(
-        &self,
+        &mut self,
         range: impl Into<BoundRange>,
         limit: u32,
     ) -> Result<impl Iterator<Item = KvPair>> {
@@ -47,7 +47,7 @@ impl Snapshot {
 
     /// Scan a range, return at most `limit` keys that lying in the range.
     pub async fn scan_keys(
-        &self,
+        &mut self,
         range: impl Into<BoundRange>,
         limit: u32,
     ) -> Result<impl Iterator<Item = Key>> {
@@ -56,7 +56,7 @@ impl Snapshot {
 
     /// Unimplemented. Similar to scan, but in the reverse direction.
     #[allow(dead_code)]
-    fn scan_reverse(&self, range: impl RangeBounds<Key>) -> BoxStream<Result<KvPair>> {
+    fn scan_reverse(&mut self, range: impl RangeBounds<Key>) -> BoxStream<Result<KvPair>> {
         self.transaction.scan_reverse(range)
     }
 }
diff --git a/src/transaction/transaction.rs b/src/transaction/transaction.rs
index 1fe8be4..3b5fb97 100644
--- a/src/transaction/transaction.rs
+++ b/src/transaction/transaction.rs
@@ -93,7 +93,7 @@ impl<PdC: PdClient> Transaction<PdC> {
     /// txn.commit().await.unwrap();
     /// # });
     /// ```
-    pub async fn get(&self, key: impl Into<Key>) -> Result<Option<Value>> {
+    pub async fn get(&mut self, key: impl Into<Key>) -> Result<Option<Value>> {
         self.check_allow_operation().await?;
         let timestamp = self.timestamp.clone();
         let rpc = self.rpc.clone();
@@ -170,7 +170,7 @@ impl<PdC: PdClient> Transaction<PdC> {
     /// txn.commit().await.unwrap();
     /// # });
     /// ```
-    pub async fn key_exists(&self, key: impl Into<Key>) -> Result<bool> {
+    pub async fn key_exists(&mut self, key: impl Into<Key>) -> Result<bool> {
         let key = key.into();
         Ok(self.scan_keys(key.clone()..=key, 1).await?.next().is_some())
     }
@@ -202,7 +202,7 @@ impl<PdC: PdClient> Transaction<PdC> {
     /// # });
     /// ```
     pub async fn batch_get(
-        &self,
+        &mut self,
         keys: impl IntoIterator<Item = impl Into<Key>>,
     ) -> Result<impl Iterator<Item = KvPair>> {
         self.check_allow_operation().await?;
@@ -299,7 +299,7 @@ impl<PdC: PdClient> Transaction<PdC> {
     /// # });
     /// ```
     pub async fn scan(
-        &self,
+        &mut self,
         range: impl Into<BoundRange>,
         limit: u32,
     ) -> Result<impl Iterator<Item = KvPair>> {
@@ -333,7 +333,7 @@ impl<PdC: PdClient> Transaction<PdC> {
     /// # });
     /// ```
     pub async fn scan_keys(
-        &self,
+        &mut self,
         range: impl Into<BoundRange>,
         limit: u32,
     ) -> Result<impl Iterator<Item = Key>> {
@@ -589,7 +589,7 @@ impl<PdC: PdClient> Transaction<PdC> {
     }
 
     async fn scan_inner(
-        &self,
+        &mut self,
         range: impl Into<BoundRange>,
         limit: u32,
         key_only: bool,
diff --git a/tests/integration_tests.rs b/tests/integration_tests.rs
index 74e7c9b..7f37f9f 100644
--- a/tests/integration_tests.rs
+++ b/tests/integration_tests.rs
@@ -102,7 +102,7 @@ async fn crud() -> Result<()> {
     txn.commit().await?;
 
     // Read again from TiKV
-    let snapshot = client.snapshot(
+    let mut snapshot = client.snapshot(
         client.current_timestamp().await?,
         // TODO needed because pessimistic does not check locks (#235)
         TransactionOptions::new_optimistic(),
@@ -341,9 +341,9 @@ async fn txn_bank_transfer() -> Result<()> {
             .await?;
         let chosen_people = people.iter().choose_multiple(&mut rng, 2);
         let alice = chosen_people[0];
-        let mut alice_balance = get_txn_u32(&txn, alice.clone()).await?;
+        let mut alice_balance = get_txn_u32(&mut txn, alice.clone()).await?;
         let bob = chosen_people[1];
-        let mut bob_balance = get_txn_u32(&txn, bob.clone()).await?;
+        let mut bob_balance = get_txn_u32(&mut txn, bob.clone()).await?;
         if alice_balance == 0 {
             txn.rollback().await?;
             continue;
@@ -362,7 +362,7 @@ async fn txn_bank_transfer() -> Result<()> {
     let mut new_sum = 0;
     let mut txn = client.begin_optimistic().await?;
     for person in people.iter() {
-        new_sum += get_txn_u32(&txn, person.clone()).await?;
+        new_sum += get_txn_u32(&mut txn, person.clone()).await?;
     }
     assert_eq!(sum, new_sum);
     txn.commit().await?;
@@ -716,7 +716,7 @@ async fn get_u32(client: &RawClient, key: Vec<u8>) -> Result<u32> {
 }
 
 // helper function
-async fn get_txn_u32(txn: &Transaction, key: Vec<u8>) -> Result<u32> {
+async fn get_txn_u32(txn: &mut Transaction, key: Vec<u8>) -> Result<u32> {
     let x = txn.get(key).await?.unwrap();
     let boxed_slice = x.into_boxed_slice();
     let array: Box<[u8; 4]> = boxed_slice