Forward vs Backward Scheduling Comparator

How this works

Once a dispatch rule sets the job sequence, there is still a real choice in when to start: as soon as possible (forward scheduling) or as late as possible while still hitting the due date (backward scheduling). Starting forward when there is slack ties up work-in-process and capacity for no benefit; scheduling backward when there is not enough calendar time left produces a start date that is already in the past, silently promising a date that cannot be hit. This engine checks whether backward scheduling is feasible and recommends it when it is, falling back to an immediate forward start, with an honest lateness figure, when it is not.

Key terms

• Forward scheduling: start as soon as possible and report the earliest completion date.
• Backward scheduling: start from the due date and work back to the latest date the job can start and still finish on time.
• Slack: for a backward schedule, how many days you can wait before you must start. For a late forward schedule, how many days over the due date the job is projected to land.

Assumptions

• One job, one already-summed total lead time across whatever operations it has
• Calendar days, not adjusted for weekends or holidays

Limitations

• Single job, not a multi-job capacity load, see Finite Capacity Load Planner
• Only two real outcomes exist by construction: enough calendar time to schedule backward, or not enough time even starting forward today. There is no calendar-time regime where backward is infeasible but an immediate forward start still makes the due date, because those are the same fact stated two ways, not independent checks.

Sources

Vollmann, Berry, Whybark, and Jacobs, Manufacturing Planning and Control Systems, standard forward and backward scheduling treatment.

Worked example

A job needs 10 days of lead time and is due in 25 days. Backward scheduling only needs to start on day 15, 15 days of slack, so it is preferred, it avoids sitting the job finished and waiting, or tying up capacity, for two weeks it does not need to. If the same job were due in only 6 days, backward scheduling would need to have started 4 days ago, infeasible, so the engine falls back to starting today, and reports the job is still projected 4 days late, a signal to compress the lead time or renegotiate the due date rather than a false promise of an on-time finish.

Common use cases

• Deciding whether to release a job now or hold it to reduce work-in-process
• Catching a due date that cannot be met before committing to it
• Continuing from Dispatch Rule Comparator's sequencing choice into a start-date decision
• Feeding a confirmed schedule into Finite Capacity Load Planner's multi-job check