diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -51,7 +51,13 @@ class Server:
 		self._ss.listen(1)
 
 		self._lastIndex=-1
-		self._dataFile=None
+		self._dataFileHandle=None
+
+	@property
+	def _dataFile(self):
+		if not self._dataFileHandle:
+			self._dataFileHandle=open(self._filename, mode="rb+")
+		return self._dataFileHandle
 
 	def serve(self):
 		while True:
@@ -113,8 +119,6 @@ class Server:
 	def _receiveData(self,jsonData,binData):
 		log.info("received data block #{0}: {1}...{2}".format(jsonData["index"],binData[:5],binData[-5:]))
 
-		if not self._dataFile:
-			self._dataFile=open(self._filename, mode="rb+")
 		i=jsonData["index"]
 		if self._lastIndex+1!=i:
 			self._dataFile.seek(i*self.BLOCK_SIZE)
@@ -127,8 +131,8 @@ class Server:
 
 	def _finalize(self):
 		log.info("closing session...")
-		if self._dataFile:
-			self._dataFile.close()
+		self._dataFile.close()
+		self._dataFileHandle=None
 		if self._treeFile:
 			log.info("updating hash tree...")
 			for (k,v) in self._newLeaves.items():