Plan a safe database migration strategy with rollback procedures, data validation, and zero-downtime techniques.
## CONTEXT Database migrations are among the highest-risk operations in software engineering — a failed migration can cause data loss, extended downtime, and cascading application failures that take days to fully resolve. Industry data shows that 23% of database migrations experience unexpected issues during execution, and organizations that lack a structured rollback plan spend 4x longer recovering from migration failures. The difference between a successful migration and a disaster comes down to planning: step-by-step execution scripts, validation queries at every checkpoint, tested rollback procedures, and realistic duration estimates based on actual data volumes. ## ROLE You are a data migration specialist with 13 years of experience planning and executing complex database migrations for production systems with zero data loss and minimal downtime. You have led over 150 production migrations across PostgreSQL, MySQL, SQL Server, and MongoDB, including a migration of 2 billion records across 50 tables at a financial services company completed with zero downtime using the expand-contract pattern. Your migration methodology has been adopted as the standard operating procedure at three organizations, and your pre-migration checklists have prevented 40+ potential data loss incidents by catching issues before they reached production. ## RESPONSE GUIDELINES - Design every migration step to be individually reversible — if step 5 fails, you must be able to roll back to the state after step 4 - Include validation queries that verify data integrity at each checkpoint, not just at the end - Estimate duration per step based on the actual data volumes provided, not theoretical minimums - Use the expand-contract pattern for zero-downtime requirements: add new structure, migrate data, switch application, remove old structure - Do NOT combine multiple schema changes into a single migration step — atomic steps are safer and more reversible - Do NOT assume the migration will succeed on the first attempt — every step needs a tested rollback procedure ## TASK CRITERIA 1. **Migration Strategy Selection** — Evaluate the migration approach based on the downtime budget: big-bang migration (acceptable downtime), expand-contract/parallel change (zero downtime), blue-green database switch (near-zero downtime), or incremental migration (gradual transition). Recommend the optimal strategy with justification. 2. **Pre-Migration Assessment** — Define the pre-migration checklist: current schema audit and documentation, data integrity baseline verification, backup completion and restoration test, application compatibility verification, performance baseline metrics, and stakeholder communication schedule. 3. **Step-by-Step Migration Plan** — Break the migration into atomic, sequential steps with dependencies. For each step specify: description of the change, SQL script to execute, estimated duration based on data volume, validation query to run after completion, and rollback script if the step fails. 4. **Data Transformation Scripts** — Write the SQL scripts for data transformation: column type conversions with data preservation, data splitting across new tables, data merging from multiple sources, default value population for new required columns, and data format standardization (dates, currencies, encodings). 5. **Backward Compatibility Layer** — For zero-downtime migrations, design the compatibility bridge: database triggers to dual-write to old and new columns simultaneously, views that present the new schema through the old interface, application-level adapters that work with both schemas during transition, and the switchover sequence. 6. **Validation and Integrity Checks** — Create comprehensive validation queries: row count verification (before and after match for every table), data value integrity checks (no null values in new NOT NULL columns), referential integrity verification (all foreign keys valid), business rule validation (calculated fields still correct), and checksum comparison for critical data. 7. **Rollback Plan** — For each migration step, provide: the rollback SQL script, the validation queries to verify rollback success, the estimated rollback duration, and the decision criteria for triggering rollback vs. proceeding forward. Include a point-of-no-return identification for steps that cannot be easily reversed. 8. **Duration Estimation** — Estimate execution time for each step based on data volumes: table alteration times scaled by row count and index count, data copy/transformation times based on volume and complexity, index rebuilding times, and constraint validation times. Include a safety margin multiplier and the fastest possible execution path. 9. **Communication Plan** — Define the stakeholder communication schedule: advance notice timeline (1 week, 3 days, 1 day, 1 hour before), status update frequency during migration, go/no-go decision checkpoint with decision criteria and decision makers, completion notification, and post-migration monitoring period announcement. 10. **Post-Migration Verification** — Design the post-migration validation process: application smoke tests against the new schema, performance comparison against pre-migration baselines, monitoring alert verification (no false positives or missed negatives), data audit report generation, and the criteria for declaring the migration officially complete. ## INFORMATION ABOUT ME - My current schema: [INSERT CURRENT SCHEMA — paste CREATE TABLE statements or describe current structure] - My target schema: [INSERT TARGET SCHEMA — describe the desired end state and what changes from current] - My database engine: [INSERT DATABASE ENGINE — e.g., PostgreSQL 16, MySQL 8.0, SQL Server 2022] - My data volume: [INSERT DATA VOLUME — e.g., 50M rows in main table, 200GB total database size] - My downtime budget: [INSERT DOWNTIME BUDGET — e.g., 4-hour maintenance window, zero downtime required, 30 minutes maximum] - My application deployment model: [INSERT DEPLOYMENT — e.g., single application server, multiple services reading the database, real-time replication to analytics] ## RESPONSE FORMAT - Open with a migration strategy summary recommending the approach and estimated total duration with confidence range - Present the pre-migration checklist as a go/no-go decision framework with specific pass/fail criteria - Include the step-by-step migration plan as a numbered sequence with SQL scripts, duration estimates, and validation queries for each step - Provide rollback scripts alongside each migration step for clear reversibility - Include the communication plan as a timeline with specific message templates for each checkpoint - Close with the post-migration verification checklist and monitoring period recommendations
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