Randomly assign comment tree implementation to new links.

r2/example.ini

@@ -555,3 +555,6 @@ sr_discovery_links =
 spotlight_interest_sub_p = .05
 # and for users that have not ever subscribed:
 spotlight_interest_nosub_p = .1
+# map of comment tree version to how frequently it should be chosen relative to
+# the others
+comment_tree_version_weights = 1:9, 2:1

r2/r2/lib/app_globals.py

@@ -217,6 +217,9 @@ class Globals(object):
             'sr_discovery_links',
             'fastlane_links',
         ],
+        ConfigValue.dict(ConfigValue.int, ConfigValue.float): [
+            'comment_tree_version_weights',
+        ],
     }
 
     def __init__(self, global_conf, app_conf, paths, **extra):

r2/r2/models/link.py

@@ -122,6 +122,23 @@ class Link(Thing, Printable):
         submit_url += 'submit?resubmit=true&url=' + url_escape(self.url)
         return submit_url
 
+    @classmethod
+    def _choose_comment_tree_version(cls):
+        try:
+            weights = g.live_config['comment_tree_version_weights']
+        except KeyError:
+            return cls._defaults['comment_tree_version']
+        total = sum(weights.itervalues())
+        r = random.random() * total
+        t = 0
+        for version, weight in weights.iteritems():
+            t += weight
+            if t >= r:
+                return version
+        # this point should never be reached, but if it is, return the default
+        # for old links
+        return cls._defaults['comment_tree_version']
+
     @classmethod
     def _submit(cls, title, url, author, sr, ip, spam=False):
         from r2.models import admintools
@@ -134,7 +151,7 @@ class Link(Thing, Printable):
                 sr_id=sr._id,
                 lang=sr.lang,
                 ip=ip,
-                comment_tree_version=2)
+                comment_tree_version=cls._choose_comment_tree_version())
         l._commit()
         l.set_url_cache()
         if author._spam: