Working with constraints

Multiple constraint types are available in LYNX, like "Start no earlier than"  (SNET) or "Finish no later than" (FNLT). The behavior of these constraints are described in this article. 

A special type of constraint is a "deadline".  A deadline can be set on a task, in case it is required to enter a "Milestone Buffer"  or protect the deadline set. 

The available constraints in LYNX vary. There are three types:

• Controlled by LYNX (CCPM) Scheduling Settings
  • As late as possible (ALAP)
  • As soon as possible (ASAP)
  • Start no earlier than
  • Start no later than
  • Finish no earlier than
  • Finish no later than
• Fixed position (Must) constraints
  • Must finish on
  • Must start on
• Other possible constraints
  • Do not level
  • Ignore Task

Activating/Limiting additional constraints

There is a space-wide setting which limits the constraint options available in the projects. When this setting is activated (checkbox is selected), only the following constraints are possible:

• Finish no later than
• Start no later than

Note: this also impacts the dependency options available. When limited, only Finish -> Start relation is possible.

Controlled by LYNX (CCPM) Scheduling Settings

In the CCPM Mode, the ALAP and ASAP constraints are controlled by the defined project properties for scheduling behavior for the critical chain and feeding chains. Therefore, they cannot be controlled by the user.

A "Start no earlier than" constraint pushes the task and its subsequent chain towards the future without pinning the task to a fixed position. Similar behavior is made for Finish no earlier than.

A "Start no later than" constraints pushes the task towards the green line (current date) to make sure the task is not started earlier than the constraint. This constraint does not pin the task to a fixed position. Similar behavior is made for Finish no later than.

Must Constraints

Must finish on/Must start on constraints pin the task to a fixed position in the plan. This impact means the scheduler cannot move tasks. Therefore, we recommend to avoid using must constraints in CCPM plans.

Note: If the project manager is not monitoring the position and timing of a must constraint and does not take action, the scheduler may be forced to push project tasks in the past!

Other possible constraints

Do not level and Ignore Task are the remaining constraints possible in LYNX.

Tasks with the constraint "Ignore Task" are not included in the resource load anymore. It is recommended to remove such a task from the plan instead of using this constraint.

The "Do not level" constraint means possible resource dependencies will be ignored when calculating a critical chain. The normal dependencies (predecessors and successors) remain.

Recommended Constraints in CCPM

Finish no later than and a Deadline have the same scheduling effect. LYNX will try to push all tasks before these constraints. The difference is for a task with a deadline, a milestone buffer will be generated to protect the deadline.

The recommended constraints are therefore Finish no later than, Start no earlier than, and deadlines.

The "Must" constraints are not recommended as they pin tasks to fixed positions that cannot move. This eliminates flow and flexibility from the plan.