REST API upgrade

source/coordinator/database/connection.js

@@ -0,0 +1,17 @@
+const { MongoClient } = require("mongodb")
+const { mongodb_url, database_name } = require('../config.json')
+
+const client = new MongoClient(mongodb_url)
+
+let database
+
+// throw exception
+module.exports.get_db = async () => {
+    try {
+        await client.connect()
+        database = client.db(database_name)
+    } catch (e) {
+        throw e
+    }
+    return database
+}