diff --git a/src/merkle-tree-large.ts b/src/merkle-tree-large.ts
index 459d531..560c63d 100644
--- a/src/merkle-tree-large.ts
+++ b/src/merkle-tree-large.ts
@@ -1,11 +1,7 @@
 import { Field } from "./field";
 import { poseidon } from "./poseidon";
-import {
-  default_snapshot_id,
-  local_uri,
-  MerkleTreeDb
-} from "./db";
-import LRUCache = require('lru-cache');
+import { default_snapshot_id, local_uri, MerkleTreeDb } from "./db";
+import LRUCache = require("lru-cache");
 
 const hash = poseidon;
 export const MaxHeight = 16;
@@ -22,23 +18,60 @@ const cacheOptions: any = {};
 cacheOptions.max = 100;
 cacheOptions.maxAge = 60 * 1000;
 
-export class MerkleTree {
+interface MerkleTreeStorage {
+  getNode(mtIndex: string): Promise<Field | undefined>;
+  setNode(mtIndex: string, value: Field): void;
+  startSnapshot(id: string): void;
+  endSnapshot(): void;
+  lastestSnapshot(): Promise<string>;
+  loadSnapshot(latestSnapshot: string): void;
+  close(): void;
+}
+
+class MerkleTreeMemory implements MerkleTreeStorage {
+  private inMemoryMerkleTree: Map<string, Field>;
+
+  constructor() {
+    this.inMemoryMerkleTree = new Map();
+  }
+
+  async getNode(mtIndex: string) {
+    return this.inMemoryMerkleTree.get(mtIndex + "I");
+  }
+
+  setNode(mtIndex: string, value: Field) {
+    return this.inMemoryMerkleTree.set(mtIndex + "I", value);
+  }
+
+  startSnapshot(id: string) {
+    return;
+  }
+
+  endSnapshot() {
+    return;
+  }
+
+  async lastestSnapshot() {
+    return "0";
+  }
+
+  loadSnapshot(_latestSnapshot: string) {
+    return;
+  }
+
+  close() {
+    return;
+  }
+}
+
+class MerkleTreeDB implements MerkleTreeStorage {
+  private cache: LRUCache<string, Field>;
+  private db: MerkleTreeDb;
   private currentSnapshotIdx: string | undefined = undefined;
-  private cache = new LRUCache<string, Field>(10000);
-  private db_name = "delphinus";
-  private db = new MerkleTreeDb(local_uri, this.db_name);
 
-  static emptyHashes: Field[] = [];
-  static emptyNodeHash(height: number) {
-    if (this.emptyHashes.length === 0) {
-      this.emptyHashes.push(new Field(0));
-      for (let i = 0; i < MaxHeight; i++) {
-        const last = this.emptyHashes[i];
-        this.emptyHashes.push(hash([last, last, last, last]));
-      }
-      this.emptyHashes = this.emptyHashes.reverse();
-    }
-    return this.emptyHashes[height];
+  constructor(dbName: string, cacheSize: number) {
+    this.cache = new LRUCache<string, Field>(cacheSize);
+    this.db = new MerkleTreeDb(local_uri, dbName);
   }
 
   private async getRawNode(mtIndex: string) {
@@ -46,9 +79,6 @@ export class MerkleTree {
   }
 
   async getNode(mtIndex: string) {
-    if (mtIndex.startsWith("-")) {
-      throw new Error(mtIndex);
-    }
     let field = this.cache.get(mtIndex);
     if (field !== undefined) {
       return field;
@@ -77,12 +107,16 @@ export class MerkleTree {
     this.cache.set(mtIndex, value);
   }
 
-  async startSnapshot(id: string) {
+  startSnapshot(id: string) {
     this.currentSnapshotIdx = id;
   }
 
   async endSnapshot() {
-    this.db.updateLatestSnapshotId(this.currentSnapshotIdx!);
+    if (this.currentSnapshotIdx === undefined) {
+      throw new Error("snapshot not set");
+    }
+
+    await this.db.updateLatestSnapshotId(this.currentSnapshotIdx);
     this.currentSnapshotIdx = undefined;
   }
 
@@ -90,14 +124,71 @@ export class MerkleTree {
     return this.db.queryLatestSnapshotId();
   }
 
-  async loadSnapshot(latest_snapshot: string) {
-    await this.db.restoreMerkleTree(latest_snapshot);
+  async loadSnapshot(latestSnapshot: string) {
+    await this.db.restoreMerkleTree(latestSnapshot);
     this.cache.reset();
   }
 
-  async closeDb() {
+  async close() {
     await this.db.closeMongoClient();
   }
+}
+
+export class MerkleTree {
+  private storage: MerkleTreeStorage;
+
+  constructor(isMemData = false) {
+    if (isMemData) {
+      this.storage = new MerkleTreeMemory();
+    } else {
+      this.storage = new MerkleTreeDB("delphinus", 10000);
+    }
+  }
+
+  static emptyHashes: Field[] = [];
+  static emptyNodeHash(height: number) {
+    if (this.emptyHashes.length === 0) {
+      this.emptyHashes.push(new Field(0));
+      for (let i = 0; i < MaxHeight; i++) {
+        const last = this.emptyHashes[i];
+        this.emptyHashes.push(hash([last, last, last, last]));
+      }
+      this.emptyHashes = this.emptyHashes.reverse();
+    }
+    return this.emptyHashes[height];
+  }
+
+  async getNode(mtIndex: string) {
+    if (mtIndex.startsWith("-")) {
+      throw new Error(mtIndex);
+    }
+
+    return await this.storage.getNode(mtIndex);
+  }
+
+  async setNode(mtIndex: string, value: Field) {
+    await this.storage.setNode(mtIndex, value);
+  }
+
+  async startSnapshot(id: string) {
+    this.storage.startSnapshot(id);
+  }
+
+  async endSnapshot() {
+    await this.storage.endSnapshot();
+  }
+
+  async lastestSnapshot() {
+    return await this.storage.lastestSnapshot();
+  }
+
+  async loadSnapshot(latestSnapshot: string) {
+    await this.storage.loadSnapshot(latestSnapshot);
+  }
+
+  async closeDb() {
+    await this.storage.close();
+  }
 
   private async getNodeOrDefault(mtIndex: string) {
     let value = await this.getNode(mtIndex);
diff --git a/test/db.test.ts b/test/db.test.ts
index 4adfc6d..b83de1d 100644
--- a/test/db.test.ts
+++ b/test/db.test.ts
@@ -3,7 +3,7 @@ import { MerkleTree } from "../src/merkle-tree-large";
 
 var assert = require('assert');
 
-async function main() {
+async function testDBMerkleTree() {
     const merkle_tree = new MerkleTree();
 
     await merkle_tree.loadSnapshot("0");
@@ -55,4 +55,32 @@ async function main() {
     await merkle_tree.closeDb();
 }
 
-main().then(() => console.log("done"))
\ No newline at end of file
+async function testInMemoryMerkleTree() {
+    const merkle_tree = new MerkleTree(true);
+
+    await merkle_tree.getNode("0001").then((node) => {
+        assert.ok(node === undefined)
+    });
+
+    await merkle_tree.setNode("0001", new Field(1));
+    await merkle_tree.getNode("0001").then((node) => {
+        assert.ok(node!.v.eq(new Field(1).v))
+    });
+
+    // lastestSnapshot should always return string "0"
+    await merkle_tree.lastestSnapshot().then((node) => {
+        assert.equal(node, "0");
+    });
+
+    // should do nothing
+    await merkle_tree.loadSnapshot("0");
+    await merkle_tree.endSnapshot();
+    await merkle_tree.closeDb();
+}
+
+async function main() {
+    await testDBMerkleTree();
+    await testInMemoryMerkleTree();
+}
+
+main().then(() => console.log("done"))