# Here-and-now and hazard-decision

*This tutorial was generated using Literate.jl.* *Download the source as a .jl file*.

*Download the source as a*.

`.ipynb`

fileSDDP.jl assumes that the agent gets to make a decision *after* observing the realization of the random variable. This is called a *wait-and-see* or *hazard-decision* model. In contrast, you might want your agent to make decisions *before* observing the random variable. This is called a *here-and-now* or *decision-hazard* model.

The terms decision-hazard and hazard-decision from the French *hasard*, meaning chance. It could also have been translated as uncertainty-decision and decision-uncertainty, but the community seems to have settled on the transliteration *hazard* instead. We like the hazard-decision and decision-hazard terms because they clearly communicate the order of the decision and the uncertainty.

The purpose of this tutorial is to demonstrate how to model here-and-how decisions in SDDP.jl.

This tutorial uses the following packages:

```
using SDDP
import HiGHS
```

## Hazard-decision formulation

As an example, we're going to build a standard hydro-thermal scheduling model, with a single hydro-reservoir and a single thermal generation plant. In each of the four stages, we need to choose some mix of `u_thermal`

and `u_hydro`

to meet a demand of `9`

units, where unmet demand is penalized at a rate of $500/unit.

```
hazard_decision = SDDP.LinearPolicyGraph(
stages = 4,
sense = :Min,
lower_bound = 0.0,
optimizer = HiGHS.Optimizer,
) do sp, node
@variables(sp, begin
0 <= x_storage <= 8, (SDDP.State, initial_value = 6)
u_thermal >= 0
u_hydro >= 0
u_unmet_demand >= 0
end)
@constraint(sp, u_thermal + u_hydro == 9 - u_unmet_demand)
@constraint(sp, c_balance, x_storage.out == x_storage.in - u_hydro + 0)
SDDP.parameterize(sp, [2, 3]) do ω_inflow
return set_normalized_rhs(c_balance, ω_inflow)
end
@stageobjective(sp, 500 * u_unmet_demand + 20 * u_thermal)
end
```

```
A policy graph with 4 nodes.
Node indices: 1, 2, 3, 4
```

## Decision-hazard formulation

In the wait-and-see formulation, we get to decide the generation variables *after* observing the realization of `ω_inflow`

. However, a common modeling situation is that we need to decide the level of thermal generation `u_thermal`

*before* observing the inflow.

SDDP.jl can model here-and-now decisions with a modeling trick: a wait-and-see decision in stage *t-1* is equivalent to a here-and-now decision in stage *t*.

In other words, we need to convert the `u_thermal`

decision from a control variable that is decided in stage `t`

, to a state variable that is decided in stage `t-1`

. Here's our new model, with the three lines that have changed:

```
decision_hazard = SDDP.LinearPolicyGraph(
stages = 4,
sense = :Min,
lower_bound = 0.0,
optimizer = HiGHS.Optimizer,
) do sp, node
@variables(sp, begin
0 <= x_storage <= 8, (SDDP.State, initial_value = 6)
u_thermal >= 0, (SDDP.State, initial_value = 0) # <-- changed
u_hydro >= 0
u_unmet_demand >= 0
end)
@constraint(sp, u_thermal.in + u_hydro == 9 - u_unmet_demand) # <-- changed
@constraint(sp, c_balance, x_storage.out == x_storage.in - u_hydro + 0)
SDDP.parameterize(sp, [2, 3]) do ω
return set_normalized_rhs(c_balance, ω)
end
@stageobjective(sp, 500 * u_unmet_demand + 20 * u_thermal.in) # <-- changed
end
```

```
A policy graph with 4 nodes.
Node indices: 1, 2, 3, 4
```

Can you understand the reformulation? In each stage, we now use the value of `u_thermal.in`

instead of `u_thermal`

, and the value of the outgoing `u_thermal.out`

is the here-and-how decision for stage `t+1`

.

(If you can spot a "mistake" with this model, don't worry, we'll fix it below. Presenting it like this simplifies the exposition.)

## Comparison

Let's compare the cost of operating the two models:

```
function train_and_compute_cost(model)
SDDP.train(model; print_level = 0)
return println("Cost = \$", SDDP.calculate_bound(model))
end
train_and_compute_cost(hazard_decision)
```

`Cost = $400.0`

`train_and_compute_cost(decision_hazard)`

`Cost = $650.0`

This suggests that choosing the thermal generation before observing the inflow adds a cost of $250. But does this make sense?

If we look carefully at our `decision_hazard`

model, the incoming value of `u_thermal.in`

in the first stage is fixed to the `initial_value`

of `0`

. Therefore, we must always meet the full demand with `u_hydro`

, which we cannot do without incurring unmet demand.

To allow the model to choose an optimal level of `u_thermal`

in the first-stage, we need to add an extra stage that is deterministic with no stage objective.

## Fixing the decision-hazard

In the following model, we now have five stages, so that stage `t+1`

in `decision_hazard_2`

corresponds to stage `t`

in `decision_hazard`

. We've also added an `if`

-statement, which adds different constraints depending on the node. Note that we need to add an `x_storage.out == x_storage.in`

constraint because the storage can't change in this new first-stage.

```
decision_hazard_2 = SDDP.LinearPolicyGraph(
stages = 5, # <-- changed
sense = :Min,
lower_bound = 0.0,
optimizer = HiGHS.Optimizer,
) do sp, node
@variables(sp, begin
0 <= x_storage <= 8, (SDDP.State, initial_value = 6)
u_thermal >= 0, (SDDP.State, initial_value = 0)
u_hydro >= 0
u_unmet_demand >= 0
end)
if node == 1 # <-- new
@constraint(sp, x_storage.out == x_storage.in) # <-- new
@stageobjective(sp, 0) # <-- new
else
@constraint(sp, u_thermal.in + u_hydro == 9 - u_unmet_demand)
@constraint(sp, c_balance, x_storage.out == x_storage.in - u_hydro + 0)
SDDP.parameterize(sp, [2, 3]) do ω
return set_normalized_rhs(c_balance, ω)
end
@stageobjective(sp, 500 * u_unmet_demand + 20 * u_thermal.in)
end
end
train_and_compute_cost(decision_hazard_2)
```

`Cost = $410.0`

Now we find that the cost of choosing the thermal generation before observing the inflow adds a much more reasonable cost of $10.

## Summary

To summarize, the difference between here-and-now and wait-and-see variables is a modeling choice.

To create a here-and-now decision, add it as a state variable to the previous stage

In some cases, you'll need to add an additional "first-stage" problem to enable the model to choose an optimal value for the here-and-now decision variable. You do not need to do this if the first stage is deterministic. You must make sure that the subproblem is feasible for all possible incoming values of the here-and-now decision variable.