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Dev Database

Introduction

Atlas uses the concept of "Dev Database" to provide extra safety and correctness to the migration process. a development database (a twin environment) to validate schemas, simulate migrations and calculate the state of the migration directory by replaying the historical changes. Let's go over a few examples to explain the benefits of using a dev/twin database. For a one-time use Atlas can spin up an ephemeral local docker container for you with a special docker driver.

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.

Validation

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

test.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" -f test.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" -f test.hcl
--dev-url="mysql://root:pass@:3308/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.

test.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" -f test.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" -f test.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" -f test.hcl
--dev-url="mysql://root:pass@:3307/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.