Please help on performing periodical tasks

Question

Hi all,
we are developing a system in which we store incoming requests in database and the performs commands they carry with themself.
after performing a command then the response will be stored in database as well.
we have three schedulers that every often looks for incoming messages and process and then send the responses.
the problem is that some commands are periodical, in other words, they will run say, every 2 days, or at a specific time. currently, each time, we explore the database table in which incoming messages are stored and if a record has reached the time to be executed we load it and execute that task.
but this seems very messy and not a good solution. I was thinking that each time I run a task store the next time that this task should be executed but don't know how to understand whether this time is that time? because, there is a possibility to down the server for a while and after starting we want to perform the tasks that missed in this period.
I would appreciate any comment if you had such experience in this matter
thank you very much

Answer 1

Each task has an entry that specifies the next time it runs.
When the system comes up you check for expired (past) times and run any that need to be.
You might want to consider that some tasks don't actually need to be caught up. For instance if you clear out old log entries once a day you probably don't need to play catch up on a restart because the next scheduled clean up will do the same thing anyways. Skipping taks like that requires implementating another flag in the task table though.
Rather than polling you can create a scheduled timer where the timer fires on the next available task to be completed. Myself I don't really consider this useful because it takes a lot of work dealing with situations like adding new tasks, firing too often, etc. Conversely just polling every, for example, 5 minutes has very little impact on a system.