Why TransactionRollbackError:

could not serialize access due to concurrent update , why this happens and what will be the reason? How can we overcome this problem

Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/cybrosys/hilar/odoo-9.0/openerp/service/server.py", line 237, in target
    self.cron_thread(i)
  File "..hilar/odoo-9.0/openerp/service/server.py", line 219, in cron_thread
    acquired = openerp.addons.base.ir.ir_cron.ir_cron._acquire_job(db_name)
  File "../odoo-9.0/openerp/addons/base/ir/ir_cron.py", line 230, in _acquire_job
    (job['id'],), log_exceptions=False)
  File "../odoo-9.0/openerp/sql_db.py", line 139, in wrapper    return f(self, *args, **kwargs)
  File "../odoo-9.0/openerp/sql_db.py", line 218, in execute    res = self._obj.execute(query, params)
TransactionRollbackError: could not serialize access due to concurrent update