When i check my log file then below error are shown in many time and this error are 
automatically created
how to solve this error

ExceTraceback (most recent call last):
  File "/opt/odoo/odoo-server/odoo/http.py", line 640, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/opt/odoo/odoo-server/odoo/http.py", line 677, in dispatch
    result = self._call_function(**self.params)
  File "/opt/odoo/odoo-server/odoo/http.py", line 333, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/opt/odoo/odoo-server/odoo/service/model.py", line 101, in wrapper
    return f(dbname, *args, **kwargs)
  File "/opt/odoo/odoo-server/odoo/http.py", line 326, in checked_call
    result = self.endpoint(*a, **kw)
  File "/opt/odoo/odoo-server/odoo/http.py", line 935, in __call__
    return self.method(*args, **kw)
  File "/opt/odoo/odoo-server/odoo/http.py", line 506, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo-server/addons/bus/controllers/main.py", line 35, in poll
    raise Exception("bus.Bus unavailable")
Exception: bus.Bus unavailable