Strategies to handle Tenant upgrades and new Schema changes

Handling database upgrades and schema changes in a multi-tenant system is critical to ensure minimal downtime, consistent data integrity, and support for different versions of the schema across tenants. Here's a structured approach to manage this process:

1. Database Versioning

Each schema should have a version number to keep track of changes. Use a version control system for database schemas, just like for application code. Tools like Flyway or Liquibase can be used to track and apply schema changes consistently across tenants.

Versioning Strategy:

• Schema Version Table: Create a table in each tenant's database or schema that tracks the current schema version.

  sql
  CREATE TABLE SchemaVersion (    VersionNumber INT,    AppliedAt DATETIME  );  

• Baseline Version: Assign a baseline version when onboarding a tenant, so each tenant's schema can be upgraded from its specific starting point.

2. Schema Change Tools

Use automated database migration tools to handle schema changes, allowing you to version, apply, and roll back changes safely.

• Flyway: A popular choice for managing SQL-based schema migrations. It applies schema updates based on versioned migration scripts.
  • Each migration is written as a SQL or Java file (e.g., V1.1__Add_Column.sql).
  • Flyway automatically checks the database schema version and applies the necessary migrations sequentially.
• Liquibase: Another option that offers more control over schema updates, including rollback scripts. Liquibase can manage schema changes declaratively through XML, YAML, JSON, or SQL files.

3. Upgrade Process for Multi-Tenant Systems

Approach 1: Rolling Schema Updates

This approach allows tenants to remain operational during schema upgrades. Each tenant's schema is updated independently, minimizing downtime and allowing for gradual upgrades.

Steps:

1. Prepare Backward-Compatible Changes:

   • When adding new columns or tables, ensure that they do not affect the current functionality.
   • Ensure new features are toggled off (using feature flags) until all tenants are upgraded.

2. Apply Schema Migrations Tenant-by-Tenant:

   • Use the schema version table to check the current version for each tenant.
   • Apply the migration scripts based on the tenant's current version. The migration tool (e.g., Flyway) will ensure each tenant's schema is updated sequentially.
   • You can run migrations for each tenant separately to avoid locking issues or heavy load on the database.

   Example Flyway Command:

   bash
   flyway -url=jdbc:sqlserver://yourserver;databaseName=tenant_db -schemas=tenant1_schema -user=user -password=password migrate  

3. Test and Verify:

   • After upgrading the schema for each tenant, verify that the schema matches the expected version.
   • Perform validation using database health checks or by testing key queries to ensure data integrity.

4. Feature Enablement:

   • Once all tenants have been upgraded to the required version, enable the new features via feature flags.
   • Ensure backward compatibility by maintaining support for both old and new versions until all tenants have migrated.

Approach 2: Blue-Green Deployment for Databases

For large schema changes that might affect all tenants simultaneously, a blue-green deployment strategy can be applied to the database:

• Blue Environment: The existing schema that all tenants are using.
• Green Environment: The updated schema version. New code is deployed against this version.

Steps:

1. Prepare Green Schema: Set up the new schema (Green) in a parallel environment. Apply the necessary schema changes without affecting the live environment.

2. Dual Writes: Implement dual-write logic in your application, where data is written to both the old and new schemas during the transition period. This allows the system to maintain consistency while tenants transition to the new schema.

3. Switch Traffic: Once the new schema is verified, redirect tenants to the green environment with the updated schema.

4. Roll Back: If issues are found in the Green environment, you can immediately roll back to the Blue environment.

Approach 3: Tenant-by-Tenant Phased Upgrades

If you have many tenants, you can phase upgrades over time by upgrading a small batch of tenants in each phase. This allows you to test the schema changes on a subset of tenants before rolling them out more broadly.

Steps:

1. Select a Batch of Tenants: Begin with a few non-critical or test tenants.
2. Apply Schema Changes: Use Flyway/Liquibase to apply schema updates only to the selected batch.
3. Monitor and Test: Closely monitor application performance and test the system for that batch.
4. Roll Out to More Tenants: Once verified, repeat the process for other tenant groups until all are updated.

4. Backward Compatibility and Zero-Downtime Deployments

Always ensure that database schema changes are backward-compatible with the current version of your application to avoid breaking tenants that have not yet been upgraded.

Best Practices:

• Additive Changes First: Start by adding new columns or tables without modifying or removing existing structures. Existing queries and services should continue to work without disruption.

  • Example: When adding a new column, set default values or allow nulls to avoid breaking old code.

  sql
  ALTER TABLE Orders ADD COLUMN NewStatus NVARCHAR(100) DEFAULT 'Pending';  

• Deprecation Process: Only remove old fields or tables after all tenants have migrated and the system is no longer using those columns.

• Data Migration: If data needs to be transformed or moved as part of the schema change, use background jobs to migrate data progressively, reducing downtime.

  • Example: When splitting a column into two, add the new columns first, copy the data over, then gradually switch to using the new columns in code.

5. Handling Schema Conflicts and Rollbacks

In a multi-tenant system, it's crucial to be prepared for potential schema conflicts and the need for rollbacks.

Schema Conflict Handling:

• Tenant-Specific Logic: Sometimes, one tenant may require a schema change that conflicts with another tenant's schema version. Ensure that schema upgrades are tenant-specific when necessary, and that one tenant's schema change doesn't affect others.

• Test on Staging Environments: Before applying schema updates, test the changes on a staging environment with data similar to each tenant's schema and version. This ensures that you catch schema conflicts early.

Rollback Strategy:

• Database Snapshots: Take snapshots or backups before applying schema changes. In case of failure, you can restore the schema to its previous version.

• Migration Rollback Scripts: For every migration, write a corresponding rollback script that can revert the schema to its previous state. Liquibase has built-in rollback capabilities, while Flyway requires writing explicit rollback scripts.

6. Database Upgrade Automation Using CI/CD

Automating the schema upgrade process using CI/CD pipelines ensures consistency, minimizes errors, and reduces downtime.

• Azure DevOps Pipelines: Integrate schema migration tools (Flyway, Liquibase) into your CI/CD pipelines. This ensures that every database change is automatically applied as part of the deployment.

• Automated Testing: Include database migration tests in your pipeline to ensure that the schema changes work as expected before being deployed to production.

Example YAML for an Azure DevOps pipeline integrating Flyway:

yaml
trigger:    branches:      include:        - main    jobs:  - job: DatabaseMigrations    steps:      - task: UseFlyway@2        inputs:          flywayCommand: 'migrate'          databaseUrl: '$(DB_URL)'          username: '$(DB_USER)'          password: '$(DB_PASSWORD)'  

Conclusion

Handling database upgrades and schema changes in a multi-tenant system involves careful planning to ensure backward compatibility, automation through migration tools like Flyway or Liquibase, and the use of strategies like rolling updates or blue-green deployments. By using these techniques, you can ensure minimal disruption to tenants while continuously evolving your system's database structure.