-- Jobs to be executed as soon as possible in the background (so not to delay HTTP-responses, or triggered by cron-system without associated HTTP-Request)
QueuedJob
    content             Value              -- JSON-encoded description of the work to be done (send an email to "test@example.org", find all recipients for a certain notifications and queue one new job each, distribute all submissions for a sheet to correctors, ...)
    creationInstance    InstanceId         -- multiple uni2work-instances access the same database, record which instance created this job for debugging purposes
    creationTime        UTCTime
    lockInstance        InstanceId Maybe   -- instance that has started to execute this job
    lockTime            UTCTime Maybe      -- time when execution had begun
    writeLastExec       Bool default=false -- record successful execution to CronLastExec
    deriving Eq Read Show Generic Typeable

-- Jobs are deleted from @QueuedJob@ after they are executed successfully and recorded in @CronLastExec@
-- There is a Cron-system that, at set intervals, queries the database for work to be done in the background (i.e. if a lecturer has set a sheet's submissions to be automatically distributed and the submission deadline passed since the last check, then queue a new job to actually do the distribution)
-- For the cron-system to determine whether a job needs to be done it needs to know if and when it was last (or ever) executed (i.e. a sheet's submissions should not be distributed twice)
CronLastExec
    job                 Value -- JSON-encoded description of work done
    time                UTCTime -- When was the job executed
    instance            InstanceId -- Which uni2work-instance did the work
    UniqueCronLastExec job
