tracing-downstream-lineage skill

---
name: tracing-downstream-lineage
description: Trace downstream data lineage and impact analysis. Use when the user asks what depends on this data, what breaks if something changes, downstream dependencies, or needs to assess change risk before modifying a table or DAG.
---

# Downstream Lineage: Impacts

Answer the critical question: "What breaks if I change this?"

Use this BEFORE making changes to understand the blast radius.

## Impact Analysis

### Step 1: Identify Direct Consumers

Find everything that reads from this target:

**For Tables:**

1. **Search DAG source code**: Look for DAGs that SELECT from this table
   - Use `af dags list` to get all DAGs
   - Use `af dags source <dag_id>` to search for table references
   - Look for: `FROM target_table`, `JOIN target_table`

2. **Check for dependent views**:
   ```sql
   -- Snowflake
   SELECT * FROM information_schema.view_table_usage
   WHERE table_name = '<target_table>'

   -- Or check SHOW VIEWS and search definitions
   ```

3. **Look for BI tool connections**:
   - Dashboards often query tables directly
   - Check for common BI patterns in table naming (rpt_, dashboard_)

**For DAGs:**

1. **Check what the DAG produces**: Use `af dags source <dag_id>` to find output tables
2. **Then trace those tables' consumers** (recursive)

### Step 2: Build Dependency Tree

Map the full downstream impact:

```
SOURCE: fct.orders
    |
    +-- TABLE: agg.daily_sales --> Dashboard: Executive KPIs
    |       |
    |       +-- TABLE: rpt.monthly_summary --> Email: Monthly Report
    |
    +-- TABLE: ml.order_features --> Model: Demand Forecasting
    |
    +-- DIRECT: Looker Dashboard "Sales Overview"
```

### Step 3: Categorize by Criticality

**Critical** (breaks production):
- Production dashboards
- Customer-facing applications
- Automated reports to executives
- ML models in production
- Regulatory/compliance reports

**High** (causes significant issues):
- Internal operational dashboards
- Analyst workflows
- Data science experiments
- Downstream ETL jobs

**Medium** (inconvenient):
- Ad-hoc analysis tables
- Development/staging copies
- Historical archives

**Low** (minimal impact):
- Deprecated tables
- Unused datasets
- Test data

### Step 4: Assess Change Risk

For the proposed change, evaluate:

**Schema Changes** (adding/removing/renaming columns):
- Which downstream queries will break?
- Are there SELECT * patterns that will pick up new columns?
- Which transformations reference the changing columns?

**Data Changes** (values, volumes, timing):
- Will downstream aggregations still be valid?
- Are there NULL handling assumptions that will break?
- Will timing changes affect SLAs?

**Deletion/Deprecation**:
- Full dependency tree must be migrated first
- Communication needed for all stakeholders

### Step 5: Find Stakeholders

Identify who owns downstream assets:

1. **DAG owners**: Check `owners` field in DAG definitions
2. **Dashboard owners**: Usually in BI tool metadata
3. **Team ownership**: Look for team naming patterns or documentation

## Output: Impact Report

### Summary
"Changing `fct.orders` will impact X tables, Y DAGs, and Z dashboards"

### Impact Diagram
```
                    +--> [agg.daily_sales] --> [Executive Dashboard]
                    |
[fct.orders] -------+--> [rpt.order_details] --> [Ops Team Email]
                    |
                    +--> [ml.features] --> [Demand Model]
```

### Detailed Impacts

| Downstream | Type | Criticality | Owner | Notes |
|------------|------|-------------|-------|-------|
| agg.daily_sales | Table | Critical | data-eng | Updated hourly |
| Executive Dashboard | Dashboard | Critical | analytics | CEO views daily |
| ml.order_features | Table | High | ml-team | Retraining weekly |

### Risk Assessment

| Change Type | Risk Level | Mitigation |
|-------------|------------|------------|
| Add column | Low | No action needed |
| Rename column | High | Update 3 DAGs, 2 dashboards |
| Delete column | Critical | Full migration plan required |
| Change data type | Medium | Test downstream aggregations |

### Recommended Actions

Before making changes:
1. [ ] Notify owners: @data-eng, @analytics, @ml-team
2. [ ] Update downstream DAG: `transform_daily_sales`
3. [ ] Test dashboard: Executive KPIs
4. [ ] Schedule change during low-impact window

### Related Skills
- Trace where data comes from: **tracing-upstream-lineage** skill
- Check downstream freshness: **checking-freshness** skill
- Debug any broken DAGs: **debugging-dags** skill
- Add manual lineage annotations: **annotating-task-lineage** skill
- Build custom lineage extractors: **creating-openlineage-extractors** skill