>Digital-flame

✅ Short Answer

Flyway lets you undo a migration using special undo scripts — but only if you’re using Flyway Teams Edition.

In open-source Flyway, undo must be done manually (e.g., with a rollback script or by reverting migrations in Git and resetting the DB).

🔍 Flyway Teams Edition: Undo Scripts

To enable automatic rollbacks, you write U__ undo scripts that correspond to versioned migrations.

🛠️ Example

If you have this migration:

V2__add_users_table.sql

You can write this undo file:

U2__drop_users_table.sql

Content:

DROP TABLE users;

Then you can call:

flyway undo

✅ Flyway looks up the most recently applied versioned migration (V2)
✅ Finds the corresponding U2 script
✅ Executes it and removes V2 from flyway_schema_history

⚠️ Notes on Undo Scripts

• Undo is not repeatable — it’s only for the latest applied migration
• Only one version can be undone per command
• Undo does not apply to repeatable migrations (R__)

🧱 Open Source Workarounds

If you’re using Flyway Community Edition (open-source):

1. Create a manual rollback script (e.g., rollback.sql)
2. Run it manually if something goes wrong
3. Or, if you’re in dev: reset the DB + clean + re-migrate:

flyway clean
flyway migrate

⚠️ flyway clean deletes all schema objects — only use in dev/test environments.

📌 Key Takeaways

✅ In Flyway Teams, use U__ scripts + flyway undo to rollback versioned migrations
❌ In Community Edition, there is no automatic undo — handle rollbacks manually
✅ Best practice: write custom rollback scripts or ensure your migrations are idempotent and safe

A Flyway baseline tells Flyway:

“Start tracking from this version — assume all migrations before this are already applied.”

It lets you start using Flyway with an existing database without re-running old scripts.

🔍 Detailed Explanation

Imagine you have a production database with many tables, but you’re just now introducing Flyway.

If you add versioned migration files like:

V1__init_schema.sql
V2__add_users.sql

Flyway will try to apply them from scratch — and crash, because the tables already exist.

✅ Solution: Use a baseline

🛠️ How to Baseline a Database

You run:

flyway baseline

Or in Java:

flyway.baseline();

Or configure it via properties:

flyway.baselineVersion=1
flyway.baselineDescription=Baseline existing schema
flyway.baselineOnMigrate=true

This creates the flyway_schema_history table with a starting version (e.g. 1), without running any scripts.

💡 When to Use a Baseline

🧾 What Gets Recorded?

In flyway_schema_history:

Now Flyway knows to start from V2 and ignore earlier scripts.

⚠️ Important Notes

• You can only baseline before any migrations are applied
• If the history table exists, baseline() will fail unless it was empty
• Useful with baselineOnMigrate=true for auto-baselining on first migration

📌 Key Takeaways

✅ flyway baseline sets the starting point for tracking in a live schema
✅ Prevents Flyway from reapplying already existing changes
✅ Ideal for introducing Flyway into production projects
✅ Sets up versioning safely without touching existing data

✅ Short Answer

Yes, you can write migrations in Java by implementing the JavaMigration interface or extending BaseJavaMigration.
This allows you to write complex, logic-driven database changes in code, including loops, conditional logic, or calling other APIs.

🔍 How to Write a Java-Based Migration

✅ Step 1: Create a class like this:

import org.flywaydb.core.api.migration.BaseJavaMigration;
import org.flywaydb.core.api.migration.Context;

public class V3__Add_default_users extends BaseJavaMigration {
    @Override
    public void migrate(Context context) throws Exception {
        try (var statement = context.getConnection().createStatement()) {
            statement.execute("INSERT INTO users (name, email) VALUES ('admin', 'admin@example.com')");
        }
    }
}

✅ Step 2: Place the file in:

src/main/java/db/migration/

📦 By default, Flyway scans this package:

db.migration

🧠 Java Migration Naming Convention

Same naming rules apply as for SQL files.

✅ When to Use Java Migrations

⚠️ Limitations

• No support for undo migrations in Java
• You still need to track the version and description manually in the class name
• Slightly more setup effort than SQL

📌 Key Takeaways

✅ Flyway supports Java-based migrations for advanced, logic-heavy use cases
✅ Use BaseJavaMigration to create versioned migrations in Java
✅ Name your classes like V2__Description.java and place them under db.migration
✅ Ideal when SQL is not expressive or safe enough for your migration logic

**flyway.migrate()** is the core method in Flyway used to apply pending migrations to your database.

It scans the migration files, checks which ones haven’t been applied (based on flyway_schema_history), and executes them in order.

🧠 When Would You Use It?

• In Java code (e.g. Spring, CLI tools, or tests) to trigger migrations programmatically
• As an alternative to the CLI command:

flyway migrate

🛠️ Example (Java API)

Flyway flyway = Flyway.configure()
    .dataSource("jdbc:postgresql://localhost:5432/mydb", "user", "password")
    .load();

flyway.migrate();

✅ This connects to the database
✅ Looks for migration files in the default location (db/migration)
✅ Applies any unapplied V__ or R__ migration files
✅ Logs them in flyway_schema_history

🔍 What Does It Actually Do?

1. Scans migration files (in classpath or specified location)
2. Checks what’s already applied (via flyway_schema_history)
3. Validates checksums and version order
4. Executes unapplied versioned and repeatable migrations
5. Updates the tracking table with new entries

🚫 It Will Not:

• Re-run already executed versioned migrations
• Run undo scripts (U__) — those require explicit commands
• Automatically repair broken checksums — for that, use flyway.repair()

✅ When to Use flyway.migrate() (Instead of CLI)

• Inside a Spring Boot app’s startup logic
• In JUnit tests that need fresh schema setup
• In custom admin tools or bootstrap scripts

📌 Key Takeaways

✅ flyway.migrate() is the Java method to apply migrations programmatically
✅ It works just like the flyway migrate CLI command
✅ Applies pending versioned and repeatable migrations, logs them, and ensures safe, idempotent schema changes

If you rename a Flyway migration file that was already executed, Flyway will treat it as a new migration and might:

• ❌ Try to re-run it (if version number changes)
• ❌ Throw a checksum error (if only filename changes)
• ✅ Require repair to fix inconsistencies

🔎 What Happens Depends On the Change

1. 🔁 Renaming a Versioned Migration (e.g., V1__init.sql → V1__initial_setup.sql)

• ✅ Same version (V1), new description
• ❌ Flyway compares checksums → checksum mismatch
• 🔒 Migration is not re-run, but Flyway throws a validation error

To fix:

flyway repair

This tells Flyway: “Yes, I deliberately changed the file name or contents — trust me.”

2. 🆕 Changing the Version (V1 → V2)

E.g., V1__init.sql → V2__init.sql

• ✅ Flyway sees this as a new migration
• ❌ It runs it again, even if the contents are identical to V1
• ⛔ Can result in duplicate changes, errors, or schema conflicts

3. 🔁 Renaming a Repeatable Migration (R__...)

• The description part (after R__) is the unique identifier
• Renaming it → Flyway thinks it’s a new repeatable
• ✅ Re-runs the new one
• ✅ Deletes the old one from tracking

📋 Summary Table

✅ Best Practices

• 🔒 Never rename versioned migration files after they are applied
• ✅ If you must rename → follow with flyway repair to fix metadata
• 🔁 Repeatable migrations can be renamed, but do it thoughtfully
• 🧪 Always test migrations in a staging DB before production

📌 Key Takeaways

✅ Renaming a Flyway migration can break integrity checks or cause re-execution
✅ Versioned migrations are sensitive to filenames and checksums
✅ Use flyway repair when you intentionally rename or edit an applied migration
❌ Avoid renaming files casually in production environments

• Versioned migrations run once, in a strict order, and never again.
• Repeatable migrations can run multiple times — they’re re-applied whenever the file changes.

They serve different purposes in managing your database schema.

🔍 Detailed Comparison

🧪 Example — Versioned Migration

V1__create_users_table.sql

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name VARCHAR(100)
);

✅ Runs once, logged in flyway_schema_history, never runs again unless manually repaired or rolled back.

🧪 Example — Repeatable Migration

R__create_views.sql

CREATE OR REPLACE VIEW active_users AS
SELECT * FROM users WHERE active = true;

✅ Will re-run any time this file changes (checksum is re-calculated on each migrate)

🔄 When Do Repeatable Migrations Run?

1. After all versioned migrations
2. If:
   • The file content changed (checksum mismatch)
   • It has never run before

📌 Key Takeaways

✅ Use versioned migrations for all structural schema changes (tables, columns, constraints)
✅ Use repeatable migrations for redefinable objects (views, stored procedures, triggers, seed data)
✅ Repeatables help you keep non-critical DDL up-to-date across environments

The flyway_schema_history table is a metadata table automatically created by Flyway in your database.
It records which migrations have been applied, in what order, by whom, and when — ensuring each migration is executed exactly once.

🔎 Detailed Explanation

When you run flyway migrate, Flyway:

1. Checks if flyway_schema_history exists — if not, it creates it
2. Scans your migration files (e.g., V1__init.sql, V2__add_table.sql)
3. Compares them against what’s already been recorded
4. Applies only the new (unapplied) migrations
5. Logs each successfully applied migration as a new row

📋 Key Columns in flyway_schema_history

🧠 Why It Matters

• ✅ Prevents Flyway from re-applying the same migrations
• ✅ Detects checksum mismatches if a migration file is modified
• ✅ Ensures your environments stay in sync
• ✅ Enables repeatable migrations and undo tracking

🔄 Example Entry

📌 Key Takeaways

✅ flyway_schema_history is Flyway’s way of tracking migration progress
✅ Ensures that every migration is applied exactly once and in order
✅ Enables rollback, validation, and schema safety across environments

Flyway uses a structured naming convention for migration files:

V<Version>__<Description>.sql

✅ The V stands for versioned migration
✅ A double underscore __ separates the version from the description
✅ The description is free-form (no spaces) and helps identify the purpose
✅ The file must end in .sql (or .java for Java-based migrations)

🔎 Detailed Breakdown

🔸 Example:

V1__create_users_table.sql
V2__add_email_column.sql
V3__create_orders_table.sql

V1, V2, V3 → version numbers (can be integers or dotted like 1.1, 2.5.3)

__ (double underscore) → required separator

create_users_table → optional human-readable description (use snake_case)

🧪 Special File Types

⚠️ Important Notes

• Versions must increase monotonically (V1, V2, V3, …)
• Descriptions must not contain spaces
• Flyway sorts migrations based on version number
• You cannot have two migrations with the same version

✅ File System Example

/resources/db/migration/
├── V1__init_schema.sql
├── V2__add_users_table.sql
├── V3__add_roles_table.sql
├── R__refresh_views.sql

📌 Key Takeaways

✅ The standard Flyway naming pattern is V<version>__<description>.sql
✅ Use U<version>__... for undo and R__... for repeatable migrations
✅ Naming is how Flyway orders, tracks, and applies migrations safely

Flyway is a lightweight, Java-based database migration tool that helps you version, manage, and automate schema changes using plain SQL or Java migration scripts.

It’s simple, fast, and widely used in microservices, CI/CD pipelines, and Spring Boot projects.

🔎 Detailed Explanation

Flyway works by:

1. Keeping a migration history table (flyway_schema_history) in your database
2. Applying migration files in order, based on version numbers in filenames (e.g. V1__init.sql)
3. Ensuring each migration is only applied once per environment
4. Supporting both forward migrations and undo scripts

🛠️ Typical Workflow

1. You create a migration file:

V1__create_users_table.sql

Flyway runs it and logs it in flyway_schema_history

You create the next version:

V2__add_email_to_users.sql

Flyway detects the new file and applies it

💡 File Naming Convention

✅ Features

• Supports SQL-based and Java-based migrations
• Works with almost any RDBMS (PostgreSQL, MySQL, Oracle, etc.)
• Easy to plug into Spring Boot
• Supports baseline, repair, clean, and undo
• Integrates well with CI/CD tools like Jenkins, GitLab, etc.

📌 Key Takeaways

✅ Flyway is a simple, reliable tool for versioning your database schema
✅ Uses a filename-based versioning system and a tracking table
✅ Works well with SQL or Java, and is especially popular in Spring Boot ecosystems
✅ Keeps your environments in sync and repeatable

✅ Short Answer

The logicalFilePath is an identifier for a changelog file that Liquibase uses internally to track changesets — instead of the physical file path.

It helps ensure consistent tracking of changesets even if the file is moved, renamed, or used in a different environment or CI/CD path.

🔎 Detailed Explanation

By default, Liquibase identifies a changeset using:

ID + AUTHOR + FILENAME

• ID — unique within the file
• AUTHOR — who created it
• FILENAME — path to the changelog file (e.g., src/main/resources/db/changelog.xml)

If the file is renamed or moved, that FILENAME changes — and Liquibase thinks it’s a different changeset, which can cause duplicate execution or checksum errors ❌.

🎯 Enter logicalFilePath

You can override the filename tracking with a stable, logical path:

<databaseChangeLog logicalFilePath="changelog/main.xml"
                   xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
                   ...>

Even if the physical file moves to a different folder, the logicalFilePath stays the same, so Liquibase knows it’s the same changeset.

🧠 Use Cases

✅ With logicalFilePath, changesets are tracked like this:

ID + AUTHOR + LOGICAL_FILE_PATH

Instead of:

ID + AUTHOR + PHYSICAL_FILE_PATH

📌 Key Takeaways

✅ logicalFilePath gives a stable identity to changelogs across environments
✅ Prevents duplicate execution or checksum errors if a file is renamed or moved
✅ Essential for collaborative projects, CI/CD, or multi-repo setups