Struct Cargo 

pub struct Cargo<'a> {
    pub input: &'a Value,
    pub known: &'a [(NodeId, Value)],
    pub keys: &'a [(NodeId, Keys)],
    pub placement: &'a Placement,
    pub memory: &'a Memory,
}

What a plan needs beyond itself in order to run elsewhere.

Fields

input: &'a Value

The graph’s input, for the steps over there that read from nobody.

known: &'a [(NodeId, Value)]

What was already produced here that the plan over there reads and does not produce. Only that: the wire is the expensive part.

keys: &'a [(NodeId, Keys)]

What each of those is called. Without them the slice over there can name nothing it produces, and a cache that stops at the process boundary is a cache nobody can rely on.

placement: &'a Placement

Where each node runs. It travels because a placement is data and the catalog is not.

memory: &'a Memory

What is remembered about each node, for the same reason and by the same rule: it is data. Without it the other side does not know what is frozen, what is worth keeping, or what any of it is called.