Skip to main content

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, use the auth-created
# "public" schema as the search path.
--dev-url "docker://postgres/15/dev?search_path=public"

# When working on multiple database schemas.
--dev-url "docker://postgres/15/dev"

The docker driver also supports the postgis and pgvector images in its URL format:

--dev-url "docker://postgis/latest/dev"
--dev-url "docker://pgvector/pg17/dev"

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

# When working on a single database schema.
docker+postgres://org/image/dev?search_path=public
docker+postgres://ghcr.io/namespace/image:tag/dev?search_path=public
# For local/official images, leave host empty or use "_".
docker+postgres://_/local/dev?search_path=public
docker+postgres://_/official:latest/dev?search_path=public

# When working on multiple database schemas.
docker+postgres://org/image/dev
# Default database is "postgres".
docker+postgres://org/image:tag

Baseline Schema​

Atlas Pro Feature

The docker and the dev blocks are available for Atlas Pro users. To use these features, run:

atlas login

In some cases, you may need to configure a dev-database with a predefined schema state to ensure that any Atlas computation using the dev-database starts from this state. For example, this might be necessary if your schemas or migrations depend on objects not managed by you, such as PostgreSQL extensions or external functions used in your schema.

To configure a dev-database with a "baseline schema," you can use either the docker or dev blocks. Below are examples of both options.

Docker with baseline schema​

The docker block allows you to run an ephemeral Docker container with the relevant image and set up a baseline schema state to apply after the container is created.

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")
}

When needed, the build block can be used to build the docker image with custom configurations:

docker "postgres" "dev" {
  image = "postgres:16-aws"
  // The built image will be tagged as "postgres:16-aws".
  build {
    context    = "path/to/context"
    dockerfile = "Dockerfile"
    target     = "build-target"
    args       = {
      key = "value"
    }
  }
}

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.

Dev-database with baseline schema​

Sometimes, you might want to use a running database as a dev-database, like AWS RDS or another managed service, but still need to set up a baseline state that your schema or migrations depend on. In such cases, you can use the dev block to configure Atlas to connect to an empty database provided by the url argument, apply the baseline schema state to it, and use it for further computations. On exit, Atlas restores the database to its original state.

dev "postgres" "rds" {
  url      = var.dev_url
  baseline = file("baseline.sql")
}

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

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.