`/update`, `/venue` sometimes return empty list

[DavidZhang73]

It seems that this bug is affected by multiprocessing. When I set num_worker to 1, bug disappears. Global variable is incompatible with multiprocessing:

• multiprocessing global variable updates not returned to parent
• Globals variables and Python multiprocessing

Possible solution:

• Just set num_worker to 1
• Make a wrapper to all global variables following the singleton design pattern