Multiple Constraints On One Joint Node?

Is it possible to have multiple constraints on one joint ?
Because Maya 2019 does allow it. If so, how could that work ?
wouldn’t that cause a dependency loop or double transform ?

It depends…

Technically you can only have a single incoming connection for any given attribute, but you can still have multiple things connected to a transform or joint with a constraint. Maya will create a pairBlend node to blend between the incoming values and spit out the result to a single connection.

Cycles and double transforms are a different beast but are pretty easily avoided

Yes it is possible to have multiple constrains on one joint or transform.

A useful case is for when you want to constrain a transform in between two other transforms. For example, let’s say you have a thigh, knee, and foot joint (all in world space, not in a hierarchy) and you want the knee joint to be exactly in between the thigh and foot. You could point constraint with maintain offset off the knee to the thigh, and then the knee to the foot. Then every time you move the thigh or foot, the knee would be exactly between the two other joints. You could then adjust the point constraint weights to choose where in between the two other joints you would like the knee joint to be located.

At our studio we apply the rule that joints (at least skinned joints) are always driven only by a single parent constraint.

For complex behaviors we build intermediate “rig” nodes ( locator or nurbs box ) and drive these nodes with multiple constraints (or driven keys or space switches, etc…) These “rig” nodes are the constraint parent for the joint. This helps us more easily manage (and debug) the control rig separately from the skinned joints.