diff --git a/api/__init__.py b/api/__init__.py
index b3e9439..5d348c6 100644
--- a/api/__init__.py
+++ b/api/__init__.py
@@ -3,6 +3,7 @@ from .project import is_project_open, open_project, close_project
 
 from .operation import execute_undo as undo
 from .operation import execute_redo as redo
+from .operation import take_snapshot, pick_snapshot
 
 from .s0_base import is_node, is_junction, is_reservoir, is_tank
 from .s0_base import is_link, is_pipe, is_pump, is_valve
diff --git a/api/operation.py b/api/operation.py
index f0bb301..92c8d67 100644
--- a/api/operation.py
+++ b/api/operation.py
@@ -74,3 +74,79 @@ def execute_redo(name: str) -> None:
 
     _execute(name, redo)
     _update_current_operation(name, curr, child)
+
+# snapshot support to check out between different version of database
+# snapshot is persistent
+# since redo always remember the recently undo path
+
+def take_snapshot(name: str, tag: str) -> None:
+    if tag == None or tag == '':
+        print('Non empty tag is expected!')
+        return
+
+    curr = _get_current_operation(name)
+
+    with conn[name].cursor() as cur:
+        parent = _get_current_operation(name)
+        cur.execute(f"insert into snapshot_operation (id, tag) values ({curr}, '{tag}')")
+
+
+def _get_parents(name: str, id: int) -> list[int]:
+    ids = [id]
+    with conn[name].cursor(row_factory=dict_row) as cur:
+        while ids[-1] != 0:
+            cur.execute(f"select parent from operation where id = {ids[-1]}")
+            ids.append(int(cur.fetchone()['parent']))
+    return ids
+
+def pick_snapshot(name: str, tag: str) -> None:
+    if tag == None or tag == '':
+        print('Non empty tag is expected!')
+        return
+
+    curr = _get_current_operation(name)
+    curr_parents = _get_parents(name, curr)
+
+    with conn[name].cursor(row_factory=dict_row) as cur:
+        cur.execute(f"select id from snapshot_operation where tag = '{tag}'")
+        if cur.rowcount < 1:
+            print('No such snapshot!')
+            return
+        target = int(cur.fetchone()['id'])
+        if target in curr_parents: # target -> curr
+            for i in range(curr_parents.index(target)):
+                execute_undo(name)
+        else:
+            target_parents = _get_parents(name, target)
+            if curr in target_parents: # curr -> target
+                for i in range(target_parents.index(curr)):
+                    execute_redo(name)
+            else:
+                ancestor_index = -1
+                while curr_parents[ancestor_index] == target_parents[ancestor_index]:
+                    ancestor_index -= 1
+
+                # ancestor -> curr
+                ancestor = curr_parents[ancestor_index + 1] # ancestor_index + 1 is common parent
+                for i in range(curr_parents.index(ancestor)):
+                    execute_undo(name)
+                # ancestor -> redo, need assign redo_child
+                while target_parents[ancestor_index] != target:
+                    cur.execute(f"update operation set redo_child = '{target_parents[ancestor_index]}' where id = '{target_parents[ancestor_index + 1]}'")
+                    execute_redo(name)
+                    ancestor_index -= 1
+                cur.execute(f"update operation set redo_child = '{target}' where id = '{target_parents[1]}'")
+                execute_redo(name)
+
+# transaction is volatile, commit/rollback will destroy transaction.
+# can not undo a committed transaction or redo a rollback transaction.
+# it may remove snapshot tag if snapshot in a rollback transaction
+
+def start_transaction(name: str) -> None:
+    pass
+
+def commit_transaction(name: str) -> None:
+    pass
+
+def rollback_transaction(name: str) -> None:
+    pass
diff --git a/new_demo.py b/new_demo.py
index 553c9d4..5974d8f 100644
--- a/new_demo.py
+++ b/new_demo.py
@@ -1,5 +1,40 @@
 from tjnetwork_new import *
 
+def demo_snapshot():
+    p = "demo_snapshot"
+
+    if is_project_open(p):
+        close_project(p)
+
+    if have_project(p):
+        delete_project(p)
+
+    create_project(p)
+    open_project(p)
+
+    add_junction(p, 'j-1', 10.0, 20.0, 30.0)
+    add_junction(p, 'j-2', 10.0, 20.0, 30.0) 
+    add_junction(p, 'j-3', 10.0, 20.0, 30.0) 
+    add_junction(p, 'j-4', 10.0, 20.0, 30.0)
+    take_snapshot(p, "1-2-3-4")
+
+    undo(p)
+    undo(p)
+    undo(p)
+    undo(p)
+
+    add_junction(p, 'j-5', 10.0, 20.0, 30.0)
+    add_junction(p, 'j-6', 10.0, 20.0, 30.0) 
+    add_junction(p, 'j-7', 10.0, 20.0, 30.0) 
+    add_junction(p, 'j-8', 10.0, 20.0, 30.0)
+    take_snapshot(p, "5-6-7-8")
+
+    pick_snapshot(p, "1-2-3-4")
+
+    close_project(p)
+    # delete_project(p)
+
+
 def demo_1_title():
     p = "demo_1_title"
 
@@ -27,7 +62,8 @@ def demo_1_title():
     print(get_title(p)) # 
 
     close_project(p)
-    delete_project(p)
+    # delete_project(p)
+
 
 def demo_2_junctions():
     p = "demo_2_junctions"
@@ -77,9 +113,15 @@ def demo_2_junctions():
     undo(p)
     print(get_junction_coord(p, j)) # {'x': 10.0, 'y': 20.0}
 
+    redo(p)
+    print(get_junction_coord(p, j)) # {'x': 100.0, 'y': 200.0}
+
     close_project(p)
-    delete_project(p)
+    # delete_project(p)
+
 
 if __name__ == "__main__":
-    demo_1_title()
-    demo_2_junctions()
+    demo_snapshot()
+    # demo_1_title()
+    # demo_2_junctions()
+    pass
diff --git a/tjnetwork_new.py b/tjnetwork_new.py
index 73375fa..4e8632e 100644
--- a/tjnetwork_new.py
+++ b/tjnetwork_new.py
@@ -46,6 +46,12 @@ def undo(name: str) -> None:
 def redo(name: str) -> None:
     return api.redo(name)
 
+def take_snapshot(name: str, tag: str) -> None:
+    return api.take_snapshot(name, tag)
+
+def pick_snapshot(name: str, tag: str) -> None:
+    return api.pick_snapshot(name, tag)
+
 
 ############################################################
 # type