import time
import os


class JobDoneError(Exception):
    pass


class Job:
    """A file backed job, completed steps are commited to disk so it's resumable"""

    def __init__(self, job_id, steps, filestore):
        self._job_id = job_id
        self._steps = steps
        self._offset = 0
        self._filestore = filestore
        self._rebuild_from_disk()

    def run(self):
        while True:
            try:
                self._run_next_step()
            except JobDoneError:
                print('Job complete!')
                break

    def _run_next_step(self):
        current_step = self._dequeue_step()
        current_step.run()
        self._commit_current_step()

    def _dequeue_step(self):
        if self._offset >= len(self._steps):
            raise JobDoneError()
        return self._steps[self._offset]

    def _commit_current_step(self):
        next_offset = self._offset + 1
        with open(self._filestore, 'a') as fout:
            fout.write('{}:{}\n'.format(self._job_id, next_offset))
            fout.flush()
        self._offset = next_offset

    def _rebuild_from_disk(self):
        # Ensure file exists
        if not os.path.exists(self._filestore):
            with open(self._filestore, 'w'):
                pass
        with open(self._filestore) as fin:
            for line in fin:
                # Ignore empty lines
                if len(line.strip()) == 0:
                    continue
                job_id, offset = line.split(':')
                if job_id == self._job_id:
                    self._offset = int(offset)


class Step:
    def __init__(self, name):
        self._name = name

    def run(self):
        print(self._name)
        time.sleep(1)


step1 = Step('trigger query 1')
step2 = Step('trigger query 2')
step3 = Step('transfer results to store')
job = Job(job_id='my_awesome_job', steps=[step1, step2, step3], filestore='db.txt')
job.run()