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ยท 4 min read
Rotem Tamir

With the release of v0.5.0, we are happy to announce a very significant milestone for the project. While this version includes some cool features (such as multi-file schemas) and a swath of incremental improvements and bugfixes, there is one feature that we're particularly excited about and want to share with you in this post.

As most outages happen directly as a result of a change to a system, Atlas provides users with the means to verify the safety of planned changes before they happen. The sqlcheck package provides interfaces for analyzing the contents of SQL files to generate insights on the safety of many kinds of changes to database schemas. With this package, developers may define an Analyzer that can be used to diagnose the impact of SQL statements on the target database.

This functionality is exposed to CLI users via the migrate lint subcommand. By utilizing the sqlcheck package, Atlas can now check your migration directory for common problems and issues.

atlas migrate lint in actionโ€‹

Recall that Atlas uses a dev database to plan and simulate schema changes. Let's start by spinning up a container that will serve as our dev database:

docker run --name atlas-db-dev -d -p 3307:3306 -e MYSQL_ROOT_PASSWORD=pass  mysql

Next let's create schema.hcl, the HCL file which will contain the desired state of our database:

schema.hcl
schema "example" {
}
table "users" {
schema = schema.example
column "id" {
type = int
}
column "name" {
type = varchar(255)
}
primary_key {
columns = [
column.id
]
}
}

To simplify the commands we need to type in this demo, let's create an Atlas project file to define a local environment.

atlas.hcl
env "local" {
src = "./schema.hcl"
url = "mysql://root:pass@localhost:3306"
dev = "mysql://root:pass@localhost:3307"
}

Next, let's plan the initial migration that creates the users table:

atlas migrate diff --env local

Observe that the migrations/ directory was created with an .sql file and a file named atlas.sum:

โ”œโ”€โ”€ atlas.hcl
โ”œโ”€โ”€ migrations
โ”‚ โ”œโ”€โ”€ 20220714090139.sql
โ”‚ โ””โ”€โ”€ atlas.sum
โ””โ”€โ”€ schema.hcl

This is the contents of our new migration script:

-- add new schema named "example"
CREATE DATABASE `example`;
-- create "users" table
CREATE TABLE `example`.`users` (`id` int NOT NULL, `name` varchar(255) NOT NULL, PRIMARY KEY (`id`)) CHARSET utf8mb4 COLLATE utf8mb4_0900_ai_ci;

Next, let's make a destructive change to the schema. Destructive changes are changes to a database schema that result in loss of data, such as dropping a column or table. Let's remove the name name column from our desired schema:

schema.hcl
schema "example" {
}
table "users" {
schema = schema.example
column "id" {
type = int
}
// Notice the "name" column is missing.
primary_key {
columns = [
column.id
]
}
}

Now, let's plan a migration to this new schema:

atlas migrate diff --env local

Observe the new migration which Atlas planned for us:

-- modify "users" table
ALTER TABLE `example`.`users` DROP COLUMN `name`;

Finally, let's use atlas migrate lint to analyze this change and verify it's safety:

atlas migrate lint --env local --latest 1

Destructive changes detected in file 20220714090811.sql:

L2: Dropping non-virtual column "name"

When we run the lint command, we need to instruct Atlas on how to decide what set of migration files to analyze. Currently, two modes are supported.

  • --git-base <branchName>: which selects the diff between the provided branch and the current one as the changeset.
  • --latest <n> which selects the latest n migration files as the changeset.

As expected, Atlas analyzed this change and detected a destructive change to our database schema. In addition, Atlas users can analyze the migration directory to automatically detect:

  • Data-dependent changes
  • Migration Directory integrity
  • Backward-incompatible changes (coming soon)
  • Drift between the desired and the migration directory (coming soon)
  • .. and more

Wrapping upโ€‹

We started Atlas more than a year ago because we felt that the industry deserves a better way to manage databases. A huge amount of progress has been made as part of the DevOps movement on the fronts of managing compute, networking and configuration. So much, in fact, that it always baffled us to see that the database, the single most critical component of any software system, did not receive this level of treatment.

Until today, the task of verifying the safety of migration scripts was reserved to humans (preferably SQL savvy, and highly experienced). We believe that with this milestone we are beginning to pave a road to a reality where teams can move as quickly and safely with their databases as they can with their code.

Have questions? Feedback? Find our team on our Discord server.