bug fix: btconfig will query mongodb for its attributes if not cached…

…, so it's better to avoid accessing the same btconfig attribute in a parallel scenario, otherwise mongodb will raise an error of 'too many logical sessions'

src/hub/databuild/mapper.py

@@ -1,6 +1,7 @@
 import biothings.hub.databuild.mapper as mapper
 from biothings import config as btconfig
 logging = btconfig.logger
+max_id_length = btconfig.MAX_ID_LENGTH
 
 class TagObserved(mapper.BaseMapper):
 
@@ -19,7 +20,7 @@ def load(self, *args, **kwargs):
 
     def process(self,docs):
         for doc in docs:
-            if len(doc["_id"]) > btconfig.MAX_ID_LENGTH:
+            if len(doc["_id"]) > max_id_length:
                 logging.debug("Skip doc, _id too long: %s" % doc["_id"])
                 continue
             yield doc