What are Django database migrations and how do you safely run them in a zero-downtime deployment? Django डेटाबेस माइग्रेशन्स क्या हैं और ज़ीरो-डाउनटाइम डिप्लॉयमेंट में इन्हें सुरक्षित रूप से कैसे चलाएँ?

Migrations are Django's version-controlled way of evolving the database schema to match model changes over time, generated with makemigrations and applied with migrate; each migration file is a Python module describing schema operations that Django can apply forward or, in many cases, roll back.

In a zero-downtime deployment with multiple running instances, migrations must be backward-compatible during the rollout window: old code and new code briefly run side-by-side against the same database, so a migration that drops a column the old code still reads will break the old instances before they're replaced. The safe pattern is splitting risky changes into multiple deploys — add the new column as nullable first, deploy code that writes to both old and new fields, backfill data, then only remove the old field in a later deploy once no running code depends on it.

python manage.py makemigrations
python manage.py migrate --plan