Database Migrations: A Detailed Overview
1. Purpose of Database Migrations
The primary goal of database migrations is to synchronize the database schema with the application's data model. This is essential when:
Adding new features to an application that require changes to the database.
Modifying existing database structures to improve performance or adapt to new requirements.
Fixing issues or bugs in the schema.
Keeping multiple environments (e.g., development, staging, production) consistent.
2. Components of a Database Migration
A typical migration involves:
Schema Changes: Modifying tables, columns, indexes, or constraints. For example:
Adding new tables or columns.
Renaming or removing existing tables or columns.
Changing data types of columns.
Adding or modifying primary keys, foreign keys, or indexes.
Data Transformations: Moving or transforming existing data to fit the new schema. For example:
Populating new columns with default or calculated values.
Restructuring data to match new relationships.
Rollback Mechanism: Providing a way to undo changes in case of errors or unexpected issues.
3. How Database Migrations Work
a. Migration Files
Migrations are typically written as scripts or classes that describe the changes to the database. These files:
Define the schema changes or data transformations (e.g., using SQL or migration frameworks).
Track the order of migrations to ensure they are applied sequentially.
b. Version Control
Migration frameworks often use a versioning system (e.g., timestamps or sequential numbers) to track which migrations have been applied. This prevents duplicate executions and maintains consistency across environments.
c. Execution
Migrations are executed using a migration tool or framework. The tool:
Reads the migration file.
Applies the changes to the database.
Updates a record (e.g., in a special
migrationstable) to mark the migration as applied.
d. Rollback
If a migration introduces errors, the rollback script can revert the database to its previous state.
How Zilean Handles Database Migrations
Zilean migrations automatically run when the application starts. This is done by checking the database for the latest migration, and then running any migrations that have not been applied.
Some migrations take longer to run than others, but the overall idea here is you will not have to worry about running migrations manually. This is all handled by Zilean.
Database Index management is also performed by Zilean on startup. This is done by checking the database for any missing indexes, and then creating them if they do not exist, Ensuring that the database is optimized for the application.