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Backfill runbook

Skill zakelfassi/skills-driven-development/examples/data-pipeline/skills/backfill-runbook

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Install
npx -y skills add zakelfassi/skills-driven-development --skill backfill-runbook

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Safely backfill historical data for a pipeline stage — scope the date range, run a dry-run, execute in chunks to avoid overwhelming the database, and verify row counts after each chunk. Use when historical data is missing, when a pipeline stage is added retroactively, or when asked to "backfill the {stage} table from {start_date}".

SKILL.md

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Backfill Runbook

Safely reprocess historical data for a pipeline stage without locking tables or losing existing rows.

Inputs

  • Stage name (e.g., customer_ltv)
  • Start date (YYYY-MM-DD)
  • End date (YYYY-MM-DD, defaults to yesterday)
  • Chunk size (number of days per chunk, defaults to 7)
  • Dry run? (boolean, defaults to true — always start with dry-run)

Steps

  1. Scope the backfill Estimate the volume:

    select count(*), min(event_date), max(event_date)
    from raw.{source_table}
    where event_date between '{start_date}' and '{end_date}';
    

    Multiply by your stage's typical row-expansion ratio. If the result is >10M rows, reduce the chunk size to 3 days.

  2. Dry-run the first chunk

    python pipelines/transforms/{stage_name}/transform.py \
      --start {start_date} \
      --end {start_date + 6 days} \
      --dry-run
    

    The dry-run prints the SQL or DataFrame operations without writing output. Review for correctness before proceeding.

  3. Back up the target table (if it exists)

    create table {stage_name}_backup_{today} as
    select * from {layer}.{stage_name};
    

    Keep the backup for 7 days.

  4. Execute in chunks

    python scripts/backfill.py \
      --stage {stage_name} \
      --start {start_date} \
      --end {end_date} \
      --chunk-days {chunk_size} \
      --pause-seconds 5
    

    The backfill script:

    • Processes one chunk at a time
    • Pauses {pause_seconds} between chunks (reduces lock contention)
    • Logs progress: [chunk {n}/{total}] {start}–{end}: {rows_written} rows
    • Writes a checkpoint file (backfill-{stage}-{run_id}.checkpoint) so it can resume after a failure
  5. Verify each chunk (or verify the full range after completion)

    -- Row count by date
    select event_date, count(*) as rows
    from {layer}.{stage_name}
    where event_date between '{start_date}' and '{end_date}'
    group by event_date
    order by event_date;
    

    Compare against the raw source counts from step 1. Flag any date with zero rows.

  6. Run the data quality gate (invoke data-quality-gate skill) Check null rates, ranges, and referential integrity on the backfilled range.

  7. Drop the backup (after 7 days, if no issues)

    drop table {stage_name}_backup_{today};
    

Conventions

  • Always start with --dry-run; never skip this step
  • Chunk size ≤ 7 days for tables with >1M rows/day; ≤ 30 days for smaller tables
  • Backfills run during off-peak hours (after 22:00 UTC on weekdays, any time on weekends)
  • The backfill script uses INSERT OVERWRITE (or REPLACE INTO) semantics — idempotent per chunk
  • Checkpoint files are committed to backfills/ so the team can see what has been run

Edge Cases

  • Pipeline failure mid-chunk: Resume from the last checkpoint: python scripts/backfill.py --resume backfill-{stage}-{run_id}.checkpoint.
  • Source data changed retroactively (late-arriving events): Add --allow-late-data flag; document in the backfill log that the range was reprocessed for late-arrival reasons.
  • Backfill conflicts with a running daily load: Check the scheduler; delay the backfill or pause the daily DAG temporarily. Never run both simultaneously on the same partition.
  • Very long date range (>1 year): Split into multiple backfill jobs of ≤90 days each; run sequentially and verify between jobs.

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