HCL schema definition (Atlas DSL)

Atlas accepts the desired schema as SQL (most common) or as HCL, Atlas's own schema DSL. Both are desired-state inputs to the same atlas schema apply and atlas migrate diff commands (Steps 3-4), passed via --to file://<schema-file>.

HCL example

The users table from Step 2's SQL example, expressed in HCL:

table "users" {
  schema = schema.public
  column "id" {
    null = false
    type = serial
  }
  column "email" {
    null = false
    type = varchar(255)
  }
  primary_key { columns = [column.id] }
  index "unique_email" { unique = true; columns = [column.email] }
}

HCL vs SQL - which to author

• SQL is the most common choice and the widest-supported: any feature the target DBMS accepts in DDL can be expressed.
• HCL is a single, engine-neutral DSL across all supported engines.
• Some DBMS-specific features (e.g., Oracle PL/SQL packages, PostgreSQL extension management) have limited HCL support - fall back to SQL schema for those (see atlas-caveats.md).

Either representation produces the same versioned migrations once diffed against the --dev-url scratch DB.