Support etag headers

web_monitoring/diffing_server.py

@@ -115,8 +115,35 @@ def options(self):
 class DiffHandler(BaseHandler):
     # subclass must define `differs` attribute
 
+    # Compute our own ETag header values.
+    # We're not actually hashing content for this, since that is expensive.
+    def compute_etag(self):
+        query_params = {k: v[-1].decode() for k, v in
+            self.request.arguments.items()}
+        etag = str('W/"' + hashlib.sha256(
+            web_monitoring.__version__.encode('utf-8') + self.request.path.encode('utf-8') + str(query_params).encode('utf-8')
+        ).hexdigest() + '"').encode('utf-8')
+        return etag
+
+    def head(self, differ):
+
+        self.set_etag_header()
+        if self.check_etag_header():
+            self.set_status(304)
+            self.finish()
+            return
+
+
     @tornado.gen.coroutine
     def get(self, differ):
+
+        # Skip a whole bunch of work if possible.
+        self.set_etag_header()
+        if self.check_etag_header():
+            self.set_status(304)
+            self.finish()
+            return
+
         # Find the diffing function registered with the name given by `differ`.
         try:
             func = self.differs[differ]