Fix blank Assets dependency graph from missing Dag nodes

[davidnzhang]

What

The scheduling dependencies endpoint is currently able to return an edge with a dag: endpoint that is never materialised as a node. This appears to be causing problems for the ELK renderer when creating the dependency graph in the Assets UI.

This PR adds a loop to backfill any dag: nodes that are referenced by an edge but are missing from the nodes set.

Why

The scheduling view of the Assets dependency graph fails to render whenever it contains a Dag referenced by another Dag (e.g. via TriggerDagRunOperator or ExternalTaskSensor) and has no scheduling dependencies of its own.

The get_scheduling_dependencies function iterates over each Dag's dependencies to build a list of nodes and edges. A Dag is only materialised as a dag: node if it has at least one dependency of its own, but it is possible for the same Dag to be an endpoint of an edge if it is referenced by another Dag. This leaves a dangling edge whose endpoint is missing from the node set.

Reproduction

Parent Dag

from datetime import datetime

from airflow.providers.standard.operators.empty import EmptyOperator
from airflow.providers.standard.operators.trigger_dagrun import TriggerDagRunOperator
from airflow.sdk import DAG, Asset

PARENT_ASSET = Asset(name="parent_dag_asset", uri="s3://some-bucket/parent_dag_asset")

with DAG(dag_id="parent_dag", schedule=None, start_date=datetime(2021, 1, 1), catchup=False):
    emit = EmptyOperator(task_id="emit", outlets=[PARENT_ASSET])
    trigger = TriggerDagRunOperator(
        task_id="trigger_child_dag",
        trigger_dag_id="child_dag",
        wait_for_completion=False,
    )
    emit >> trigger

Child Dag

from datetime import datetime

from airflow.providers.standard.operators.empty import EmptyOperator
from airflow.sdk import DAG

with DAG(dag_id="child_dag", schedule=None, start_date=datetime(2021, 1, 1), catchup=False):
    EmptyOperator(task_id="some_task")

Before

Scheduling view

/ui/dependencies?dependency_type=scheduling Response

{
    "edges": [
        {
            "source_id": "dag:parent_dag",
            "target_id": "asset:1"
        },
        {
            "source_id": "dag:parent_dag",
            "target_id": "trigger:parent_dag:child_dag:trigger_child_dag"
        },
        {
            "source_id": "trigger:parent_dag:child_dag:trigger_child_dag",
            "target_id": "dag:child_dag"
        }
    ],
    "nodes": [
        {
            "id": "dag:parent_dag",
            "label": "parent_dag",
            "type": "dag",
            "team": null
        },
        {
            "id": "asset:1",
            "label": "parent_dag_asset",
            "type": "asset",
            "team": null
        },
        {
            "id": "trigger:parent_dag:child_dag:trigger_child_dag",
            "label": "trigger_child_dag",
            "type": "trigger",
            "team": null
        }
    ]
}

After

Scheduling view

/ui/dependencies?dependency_type=scheduling Response

{
    "edges": [
        {
            "source_id": "dag:parent_dag",
            "target_id": "asset:1"
        },
        {
            "source_id": "dag:parent_dag",
            "target_id": "trigger:parent_dag:child_dag:trigger_child_dag"
        },
        {
            "source_id": "trigger:parent_dag:child_dag:trigger_child_dag",
            "target_id": "dag:child_dag"
        }
    ],
    "nodes": [
        {
            "id": "dag:parent_dag",
            "label": "parent_dag",
            "type": "dag",
            "team": null
        },
        {
            "id": "asset:1",
            "label": "parent_dag_asset",
            "type": "asset",
            "team": null
        },
        {
            "id": "trigger:parent_dag:child_dag:trigger_child_dag",
            "label": "trigger_child_dag",
            "type": "trigger",
            "team": null
        },
        {
            "id": "dag:child_dag",
            "label": "child_dag",
            "type": "dag",
            "team": null
        }
    ]
}

Note node set now correctly includes id: dag:child_dag, so edge below is no longer dangling:

{
      "source_id": "trigger:parent_dag:child_dag:trigger_child_dag",
      "target_id": "dag:child_dag"
  }

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[SameerMesiah97]

It seems like this fix is just papering over a gap in the existing algorithm. It might be that is the only reasonable solution so you are free to push back.