diff --git a/src/webapp/models.py b/src/webapp/models.py
index 1b51c1a65..702cf7b24 100644
--- a/src/webapp/models.py
+++ b/src/webapp/models.py
@@ -49,12 +49,17 @@ def __init__(self, data=None, timestamp=0, force_update=True, cache_lifetime=60*
         self.next_update = self.timestamp + self.cache_lifetime
 
     def should_update(self):
+        """Return True if we should update, either because we're past the next update time
+        or because force_update is True.
+        """
         return self.force_update or not self.data or time.time() > self.next_update
 
     def try_again(self):
+        """Set the next update time to now + the retry delay."""
         self.next_update = time.time() + self.retry_delay
 
     def update(self, data):
+        """Cache new data and set the next update time to now + the cache lifetime."""
         self.data = data
         self.timestamp = time.time()
         self.next_update = self.timestamp + self.cache_lifetime