I couldn't find any official documentation specifying what each parameter to a `sla_miss_callback` callable are. This would be a great addition to know how to properly format the messages sent.
airflow/models/dag.py
"""
    Returns the last dag run for a dag, None if there was none.
    Last dag run can be any type of run eg. scheduled or backfilled.
    Overridden DagRuns are ignored.
    """
    DR = DagRun
    query = session.query(DR).filter(DR.dag_id == dag_id)
    if not include_externally_triggered:
        query = query.filter(DR.external_trigger == expression.false())
    query = query.order_by(DR.execution_date.desc())
    return query.first()
@functools.total_ordering
class DAG(LoggingMixin):
    """
    A dag (directed acyclic graph) is a collection of tasks with directional
    dependencies. A dag also has a schedule, a start date and an end date
    (optional). For each schedule, (say daily or hourly), the DAG needs to run
    each individual tasks as their dependencies are met. Certain tasks have
    the property of depending on their own past, meaning that they can't run
    until their previous schedule (and upstream tasks) are completed.