# Atlas — Declarative Database Schema Management

> Atlas is a database schema management tool that uses a declarative approach to define and apply schema changes. It supports PostgreSQL, MySQL, SQLite, SQL Server, and ClickHouse with automatic migration planning and drift detection.

## Install

Save the content below to `.claude/skills/` or append to your `CLAUDE.md`:

# Atlas — Declarative Database Schema Management

## Quick Use
```bash
# Install Atlas CLI
curl -sSf https://atlasgo.sh | sh

# Inspect an existing database schema
atlas schema inspect -u "postgres://localhost:5432/mydb?sslmode=disable"

# Define desired schema in HCL
cat > schema.hcl <<EOF
table "users" {
  schema = schema.public
  column "id" { type = int }
  column "name" { type = varchar(255) }
  primary_key { columns = [column.id] }
}
EOF

# Apply the schema (Atlas plans the migration automatically)
atlas schema apply -u "postgres://localhost:5432/mydb?sslmode=disable" --to file://schema.hcl
```

## Introduction
Atlas takes a declarative approach to database migrations: you define the desired schema state and Atlas computes the SQL statements needed to reach it. This eliminates hand-written migration files for many workflows and catches schema drift between environments automatically. It also supports a versioned migration workflow for teams that prefer explicit migration files.

## What Atlas Does
- Computes migration plans by diffing the current database state against a desired schema definition
- Supports declarative schemas in HCL, SQL, or ORM definitions (GORM, Ent, Sequelize, Django, and more)
- Detects schema drift between environments and reports differences
- Generates versioned migration files for traditional sequential migration workflows
- Lints migration files to catch destructive changes before they reach production

## Architecture Overview
Atlas works by connecting to a database, reading its current schema into an in-memory graph, and comparing it against a target state. The target can be an HCL file, a SQL schema dump, another live database, or an ORM schema. The diff engine produces a set of planned changes which Atlas can apply directly or output as migration files. Versioned migrations are stored alongside a checksum file for integrity verification.

## Self-Hosting & Configuration
- Single binary available for Linux, macOS, and Windows with no runtime dependencies
- Configure database URLs via CLI flags, environment variables, or an atlas.hcl project file
- Use dev-url to spin up a temporary database for safe migration planning and linting
- Integrate with CI/CD via GitHub Actions or the atlas migrate lint command
- Atlas Cloud (optional) provides a hosted migration directory and approval workflow

## Key Features
- Automatic migration planning from declarative schema definitions
- Schema drift detection comparing live databases against expected state
- Migration linting that warns about destructive operations like column drops
- ORM integration reads schemas from GORM, Ent, Django, SQLAlchemy, and more
- Checksum-based migration integrity verification similar to Go module sums

## Comparison with Similar Tools
- **Flyway** — Versioned migration files only; no declarative schema or automatic plan generation
- **Liquibase** — XML/YAML changesets with rollback support; more verbose and Java-dependent
- **golang-migrate** — Lightweight CLI for SQL migrations; no schema inspection or drift detection
- **Alembic** — Python and SQLAlchemy specific; Atlas is language-agnostic with broader ORM support
- **Prisma Migrate** — TypeScript ORM migrations; Atlas covers more databases and ORMs

## FAQ
**Q: Do I have to use the declarative workflow?**
A: No. Atlas supports both declarative (schema apply) and versioned (migrate) workflows. You can use either or combine them.

**Q: Which ORMs does Atlas integrate with?**
A: Atlas can read schemas from GORM, Ent, Django, SQLAlchemy, Sequelize, TypeORM, Hibernate, and more through its provider system.

**Q: Is Atlas Cloud required?**
A: No. Atlas CLI is fully functional without Atlas Cloud. The cloud service adds collaboration features like migration approval and schema visualization.

**Q: How does it handle data migrations?**
A: Atlas focuses on schema (DDL) changes. For data migrations, you can add custom SQL statements to versioned migration files.

## Sources
- https://github.com/ariga/atlas
- https://atlasgo.io/docs

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Source: https://tokrepo.com/en/workflows/atlas-declarative-database-schema-management-201b0f1d
Author: AI Open Source