Order JSON keys by default to avoid trashing HTTP caches

CHANGES

@@ -63,6 +63,8 @@ Release date to be decided.
   more reliable in case something handles an exception halfway through
   the error handling process.
 - Added the ``JSONIFY_PRETTYPRINT_REGULAR`` configuration variable.
+- Flask now orders JSON keys by default to not trash HTTP caches due to
+  different hash seeds between different workers.
 
 Version 0.9
 -----------

docs/config.rst

@@ -149,6 +149,17 @@ The following configuration values are used internally by Flask:
                                   unicode strings.  ``jsonfiy`` will
                                   automatically encode it in ``utf-8``
                                   then for transport for instance.
+``JSON_SORT_KEYS``                By default Flask will serialize JSON
+                                  objects in a way that the keys are
+                                  ordered.  This is done in order to
+                                  ensure that independent of the hash seed
+                                  of the dictionary the return value will
+                                  be consistent to not trash external HTTP
+                                  caches.  You can override the default
+                                  behavior by changing this variable.
+                                  This is not recommended but might give
+                                  you a performance improvement on the
+                                  cost of cachability.
 ``JSONIFY_PRETTYPRINT_REGULAR``   If this is set to ``True`` (the default)
                                   jsonify responses will be pretty printed
                                   if they are not requested by an
@@ -197,7 +208,7 @@ The following configuration values are used internally by Flask:
    ``PREFERRED_URL_SCHEME``
 
 .. versionadded:: 0.10
-   ``JSON_AS_ASCII``, ``JSONIFY_PRETTYPRINT_REGULAR``
+   ``JSON_AS_ASCII``, ``JSON_SORT_KEYS``, ``JSONIFY_PRETTYPRINT_REGULAR``
 
 Configuring from Files
 ----------------------

flask/app.py

@@ -291,6 +291,7 @@ class Flask(_PackageBoundObject):
         'TRAP_HTTP_EXCEPTIONS':                 False,
         'PREFERRED_URL_SCHEME':                 'http',
         'JSON_AS_ASCII':                        True,
+        'JSON_SORT_KEYS':                       True,
         'JSONIFY_PRETTYPRINT_REGULAR':          True,
     })
 

flask/json.py

@@ -92,10 +92,12 @@ class JSONDecoder(_json.JSONDecoder):
 
 def _dump_arg_defaults(kwargs):
     """Inject default arguments for dump functions."""
+    kwargs.setdefault('sort_keys', True)
     if current_app:
         kwargs.setdefault('cls', current_app.json_encoder)
         if not current_app.config['JSON_AS_ASCII']:
             kwargs.setdefault('ensure_ascii', False)
+        kwargs.setdefault('sort_keys', current_app.config['JSON_SORT_KEYS'])
 
 
 def _load_arg_defaults(kwargs):

flask/testsuite/helpers.py

@@ -148,6 +148,46 @@ class JSONTestCase(FlaskTestCase):
     if not has_encoding('euc-kr'):
         test_modified_url_encoding = None
 
+    def test_json_key_sorting(self):
+        app = flask.Flask(__name__)
+        app.testing = True
+        self.assert_equal(app.config['JSON_SORT_KEYS'], True)
+        d = dict.fromkeys(range(20), 'foo')
+
+        @app.route('/')
+        def index():
+            return flask.jsonify(values=d)
+
+        c = app.test_client()
+        rv = c.get('/')
+        lines = [x.strip() for x in rv.data.strip().decode('utf-8').splitlines()]
+        self.assert_equal(lines, [
+            '{',
+            '"values": {',
+            '"0": "foo",',
+            '"1": "foo",',
+            '"2": "foo",',
+            '"3": "foo",',
+            '"4": "foo",',
+            '"5": "foo",',
+            '"6": "foo",',
+            '"7": "foo",',
+            '"8": "foo",',
+            '"9": "foo",',
+            '"10": "foo",',
+            '"11": "foo",',
+            '"12": "foo",',
+            '"13": "foo",',
+            '"14": "foo",',
+            '"15": "foo",',
+            '"16": "foo",',
+            '"17": "foo",',
+            '"18": "foo",',
+            '"19": "foo"',
+            '}',
+            '}'
+        ])
+
 
 class SendfileTestCase(FlaskTestCase):