feedback migrations additive first

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    Deploy zero-downtime database migrations using the expand-contract pattern for safe production schema changes.

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    Example session with this skill installed

    I need to rename the 'username' column to 'handle' in our production 'profiles' table. It has 5 million rows. Give me the migration plan.

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    • Compiled the feedback migrations additive

    I have analyzed the request to rename username to handle on the profiles table. This is a breaking change that requires a 4-step expand-contract cycle to avoid production downtime.

    Database Migration Plan: Rename username to handle

    StepActionTypeNotes
    1Add handle column as NULLAdditiveSafe for existing code.
    2Backfill handle from usernameDataUse chunked updates for 5M rows.
    3Deploy code: Dual-write + Fallback-readCodeWrite to both; read handle (fallback to username).
    4Tighten handle and drop usernameContractRun only after code is fully deployed and verified.

    Migration 1: Add New Column

    -- 20240520_add_handle_to_profiles.sql
-- Step 1: Add the new column as nullable.
ALTER TABLE profiles ADD COLUMN handle TEXT NULL;

    Backfill Script (Application Level)

    -- Run in batches of 10,000 to avoid long-held locks
UPDATE profiles 
SET handle = username 
WHERE handle IS NULL 
AND id BETWEEN $start AND $end;

    Migration 2: Tighten and Drop

    -- 20240522_tighten_handle_in_profiles.sql
-- Step 2: Only run after code writes to both columns and backfill is 100% verified.
ALTER TABLE profiles ALTER COLUMN handle SET NOT NULL;

-- 20240523_drop_username_from_profiles.sql
-- Step 3: Only run after code no longer references the 'username' column.
ALTER TABLE profiles DROP COLUMN username;

    Next steps

    • Deploy the additive migration and verify the new column exists.
    • Run the chunked backfill script during off-peak hours.
    • Update the User model to handle dual-writes to both fields.

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    About this skill

    Database schema changes often cause production downtime due to table locks or application crashes during rolling deploys. A single PR that adds a NOT NULL column or renames a field frequently breaks the running version of the app. This skill enforces the expand-contract pattern to ensure zero-downtime migrations.

    What it does

    • Enforces expand-contract by splitting breaking changes into safe, multi-stage deployments.
    • Validates nullability to ensure new columns are added as NULL first to prevent write failures.
    • Audit index creation for large tables, ensuring Postgres CONCURRENTLY usage to avoid ACCESS EXCLUSIVE locks.
    • Plans backfills by designing idempotent, chunked data updates for multi-million row tables.
    • Refines rename strategies by transitioning through dual-writing and fallback reading phases.

    How it works

    1. Classify the change based on the proposed SQL to determine if it requires 1, 2, or 3+ migration files.
    2. Draft the additive migration ensuring it is safe to deploy while the old application code is still running.
    3. Design the backfill and contraction steps, including the necessary application code logic for dual-writing or defensive reading.

    Frameworks & tools

    This skill works with any SQL-based migration system including Prisma, Alembic, Django Migrations, Sequelize, and raw SQL scripts. It includes specific optimizations for Postgres production environments.

    Why this beats prompting it yourself

    Generic AI prompts often suggest one-shot migrations that cause locks or 500 errors during deploys. This skill encodes strict industry-standard safety rules that prioritize uptime over brevity, catching "single-migration smells" before they reach CI.

    Use cases

    • Adding a NOT NULL constraint to a table with existing production data.
    • Renaming a high-traffic column without breaking the current app version.
    • Converting a column data type on a table with millions of rows.
    • Adding unique or secondary indexes to hot tables without blocking writes.

    Known limitations

    Does not provide performance tuning for specific database engine internals beyond locking behavior. Focused on the migration lifecycle rather than query optimization.

    How to use

    Skills on Agensi run inside your AI agent. Copy the prompt, paste it into your agent, and it connects to Agensi and installs this skill for you. If a payment is needed, your agent shows it to you first.

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