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Dev Database

Introduction​

Some commands require a URL pointing to a "Dev Database", typically a temporary and locally running database, which Atlas uses to process and validate users' schemas, migrations and more. This requirement is necessary as Atlas cannot replicate every database type 'X' in every version 'Y'.

To simplify the process of creating temporary databases for one-time use, Atlas can spin up an ephemeral local Docker container using the special docker driver, and clean it up at the end of the process. Here are a few examples of how to use the docker driver:

# When working on a single database schema.
--dev-url "docker://mysql/8/dev"

# When working on multiple database schemas.
--dev-url "docker://mysql/8"

To work with a custom Docker image, use one of the following formats:

# When working on a single database schema.
docker+mysql://org/image/dev
docker+mysql://user/image:tag/dev
# For local/official images, leave host empty or use "_".
docker+mysql:///local/dev
docker+mysql://_/mariadb:latest/dev

# When working on multiple database schemas.
docker+mysql://local
docker+mysql://org/image
docker+mysql://user/image:tag
docker+mysql://_/mariadb:latest

Baseline Schema​

LOGIN REQUIRED

The docker block is available for logged-in users only. To use this feature, run:

atlas login

In some cases, there is a need to configure a baseline schema for the dev database so that every computation using the dev-database starts from this baseline. For example, users' schemas or migrations rely on objects, extensions, or other schema resources that are not managed by the project.

To configure such a baseline, use the docker block with the relevant image and pass to it the script for creating the base schema for the project:

docker "postgres" "dev" {
  image  = "postgres:15"
  schema = "public"
  baseline = <<SQL
   CREATE SCHEMA "auth";
   CREATE EXTENSION IF NOT EXISTS "uuid-ossp" SCHEMA "auth";
   CREATE TABLE "auth"."users" ("id" uuid NOT NULL DEFAULT auth.uuid_generate_v4(), PRIMARY KEY ("id"));
  SQL
}

env "local" {
  src = "file://schema.pg.hcl"
  dev = docker.postgres.dev.url
}

When managing multiple schemas or there is a need to generate migrations with table qualifiers, the schema argument should not be set:

docker "postgres" "dev" {
  image  = "postgres:15"
  // Use the file() function to read
  // the contents of the baseline script.
  baseline = file("baseline.sql")
}

MySQL and MariaDB users can use the docker "mysql" / docker "mariadb" to configure such block.

The code for the above examples is available in the ariga/atlas-examples repository, within the migrate-diff-hcl and schema-apply-sql directories.

Validation​

The section below explains in more detail how Atlas uses the dev-database to process and validate users' schemas.

Suppose we want to the add the following CHECK constraint to the table below:

schema.hcl
table "t" {
  schema = schema.test
  column "c" {
    type = int
  }
  check "ck" {
    expr = "c <> d"
  }
}

After running schema apply, we get the following error because the CHECK constraint is invalid, as column d does not exist.

atlas schema apply \
  --url "mysql://root:pass@:3308/test" \
  --to "schema.hcl"
-- Planned Changes:
-- Modify "t" table
ALTER TABLE `test`.`t` ADD CONSTRAINT `ck` CHECK (c <> d), DROP COLUMN `c1`, ADD COLUMN `c` int NOT NULL
✔ Apply
Error: modify "t" table: Error 1054: Unknown column 'd' in 'check constraint ck expression'
exit status 1

Atlas cannot predict such errors without applying the schema file on the database, because some cases require parsing and compiling SQL expressions, traverse their AST and validate them. This is already implemented by the database engine.

Migration failures can leave the database in a broken state. Some databases, like MySQL, do not support transactional migrations due to implicit COMMIT. However, this can be avoided using the --dev-url option. Passing this to schema apply will first create and validate the desired state (the HCL schema file) on temporary named-databases (schemas), and only then continue to apply the changes if it passed successfully.

atlas schema apply \
  --url "mysql://root:pass@:3308/test" \
  --to "schema.hcl" \
  --dev-url "docker://mysql/8/test"
Error: create "t" table: Error 3820: Check constraint 'ck' refers to non-existing column 'd'.
exit status 1

Diffing​

Atlas adopts the declarative approach for maintaining the schemas desired state, but provides two ways to manage and apply changes on the database: schema apply and migrate diff. In both commands, Atlas compares the "current", and the "desired" states and suggests a migration plan to migrate the "current" state to the "desired" state. For example, the "current" state can be an inspected database or a migration directory, and the "desired" state can be an inspected database, or an HCL file.

Schemas that are written in HCL files are defined in natural form by humans. However, databases store schemas in normal form (also known as canonical form). Therefore, when Atlas compares two different forms it may suggest incorrect or unnecessary schema changes, and using the --dev-url option can solve this (see the above section for more in-depth example).

Let's see it in action, by adding an index-expression to our schema.

schema.hcl
table "t" {
  schema = schema.test
  column "c" {
    type = varchar(32)
  }
  index "i" {
    on {
      expr = "upper(concat('c', c))"
    }
  }
}
atlas schema apply \
  --url "mysql://root:pass@:3308/test" \
  --to "schema.hcl"
-- Planned Changes:
-- Modify "t" table
ALTER TABLE `test`.`t` ADD INDEX `i` ((upper(concat('c', c))))
✔ Apply

We added a new index-expression to our schema, but using schema inspect will show our index in its normal form.

atlas schema inspect --url "mysql://root:pass@:3308/test"
table "t" {
  schema = schema.test
  column "c" {
    null = false
    type = varchar(32)
  }
  index "i" {
    on {
      expr = "upper(concat(_utf8mb4'c',`c`))"
    }
  }
}

Therefore, running schema apply again will suggest unnecessary schema changes.

atlas schema apply \
  --url "mysql://root:pass@:3308/test" \
  --to "schema.hcl"
-- Planned Changes:
-- Modify "t" table
ALTER TABLE `test`.`t` DROP INDEX `i`
-- Modify "t" table
ALTER TABLE `test`.`t` ADD INDEX `i` ((upper(concat('c', c))))
✔ Abort

Similarly to the previous example, we will use the --dev-url option to solve this.

atlas schema apply \
  --url "mysql://root:pass@:3308/test" \
  --to "schema.hcl" \
  --dev-url "docker://mysql/8/test"
Schema is synced, no changes to be made

Hooray! Our desired schema is synced and no changes have to be made.

info

Atlas cleans up after itself! You can use the same instance of a "Dev Database" for multiple environments, as long as they are not accessed concurrently.