Data Quality Gate

Data Quality Gate

Add or extend validation checks that block bad data from propagating through the pipeline.

Inputs

• Stage name where the gate should run (e.g., customer_events)
• Check type: null_rate | range | referential | row_count | custom
• Column(s) affected
• Threshold or reference table (depending on check type)
• Severity: warn (log and continue) or fail (halt the pipeline)

Steps

1. Identify where to add the gate Gates run after ingestion (raw → staging) or after a transform (staging → marts). Most gaps are caught earliest; prefer adding checks at the earliest stage where the data is available.

2. Write the check

   Null-rate check:

   def check_null_rate(df, column, threshold=0.05):
       rate = df[column].isnull().mean()
       if rate > threshold:
           raise DataQualityError(
               f"{column} null rate {rate:.1%} exceeds threshold {threshold:.1%}"
           )
   

   Range check:

   def check_range(df, column, min_val, max_val):
       out_of_range = df[(df[column] < min_val) | (df[column] > max_val)]
       if len(out_of_range) > 0:
           raise DataQualityError(
               f"{column}: {len(out_of_range)} rows outside [{min_val}, {max_val}]"
           )
   

   Referential integrity check:

   def check_referential(df, fk_column, reference_df, pk_column):
       orphans = df[~df[fk_column].isin(reference_df[pk_column])]
       if len(orphans) > 0:
           raise DataQualityError(
               f"{fk_column}: {len(orphans)} rows with no matching {pk_column}"
           )
   

   Row-count sanity check:

   def check_row_count(df, min_rows, max_rows=None):
       n = len(df)
       if n < min_rows:
           raise DataQualityError(f"Only {n} rows; expected at least {min_rows}")
       if max_rows and n > max_rows:
           raise DataQualityError(f"{n} rows exceeds max {max_rows}")
   

3. Register the check in the stage's test suite Add the check to pipelines/ingestion/{stage_name}/tests/test_quality.py or the dbt schema YAML:

   # dbt schema
   - name: {column}
     tests:
       - not_null
       - dbt_utils.accepted_range:
           min_value: {min}
           max_value: {max}
   

4. Set the severity

   • warn: emit a metric + log; allow the pipeline to continue.
   • fail: raise DataQualityError; the scheduler marks the run as failed. Document the choice and rationale in the check's docstring.

5. Test the check Write a test that intentionally violates the threshold and confirms the exception is raised:

   def test_null_rate_fails():
       bad_df = pd.DataFrame({"col": [None] * 10})
       with pytest.raises(DataQualityError):
           check_null_rate(bad_df, "col", threshold=0.05)
   

6. Document in the data dictionary Add a "Quality checks" section to docs/data-dictionary/{stage_name}.md:

   Check	Column	Threshold	Severity
   null_rate	{col}	< 5%	fail

Conventions

• All checks are functions in pipelines/quality/checks.py; imported by stage test files
• DataQualityError is defined in pipelines/quality/errors.py
• Metrics emitted: data_quality.{stage}.{check}.{column} (gauge, 0=pass, 1=fail)
• Null-rate thresholds are per-column, not global
• Referential checks are warn severity by default unless the FK is critical for downstream joins

Edge Cases

• Threshold is too tight and causes false positives: Increase the threshold and document the change in git with a comment; don't just bump it silently.
• Data arrives in batches (some partitions are empty): Add a min_rows_per_partition check rather than a total row-count check.
• Reference table is unavailable (API down): Wrap the referential check in a try/except; emit a warn-level metric and continue rather than failing the pipeline.
• Check runs too slowly on large datasets: Sample 10% of rows for range/null checks; document the sampling in the check's docstring.