DAG (Directed acyclic graph) - Statement reordering

Statement reordering

Suppose we have two Transformations:

1. Create an intermediate dataset DS_np
2. Use DS_np to calculate another dataset DS_p

Even if we write them in "reverse" order, the VTL standard requires executing them in the correct dependency order.

-- Transformation Scheme Example

-- (i) This depends on DS_np
DS_p <- if DS_np >= 0 then DS_np else DS_1;

-- (ii) This produces DS_np
DS_np := (DS_1 - DS_2) * 2;

Execution Order (resolved by the engine)

1. (ii) must run first because DS_np is required before evaluating (i).

2. (i) runs afterwards, since it consumes DS_np.

So even though we wrote (i) before (ii), the engine reorders them automatically, when reordering is activated.

Activate reordering

The current behavior of Trevas is that statement reordering is activated per default, as this is the behavior required by the VTL standard. Statement reordering can be deactivated via the following config flag ("$vtl.engine.use_dag")

ScriptEngine engine = new ScriptEngineManager().getEngineByName("vtl");
engine.put("$vtl.engine.use_dag", "false");