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As the application grows your database schema also will evolve. The problem is pretty common: when you add some feature that influences a database schema(adding, removing, or replacing some fields) you will have to update already existing documents to the latest schema version. Otherwise working with old documents will be impossible, because you will struggle with errors when you will rely on an updated field in your code. So you will run migrations where you will resolve code and schema mismatching. Migrator is a service that runs MongoDB migrations, handles versioning, and keeps logs for every migration. It performs changes/migrations to current database data, to match new schema or new requirements. Those changes are stored in the migrator/migrations folder.
Any changes to the project’s database are very sensitive and should be consistent. But also frequently migrations have operations that can take a lot of time to apply. It can create downtime for both database and API services. So to reduce these effects - Migrator is running as a separate service.

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How it works in the Ship

Every time Migrator is started, it is getting current successful migration version from the migrationVersion collection. And tries to apply every migration above this version in a sequence. Every migration from migrator/migrations will be called one by one, and every time it will be logged to the migrationLog collection either with:
  1. completed status with updating the current version
  2. failed status without updating the current version
Warning: Sequence of migrations will stop on failed migration and won’t apply versions above it.

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How to add a new migration.

  1. To add new migration - add new #.ts file inside еру migrator/migrations folder (with the name of the next version, that is higher than the current version)ю
  2. Create new Migration with the #.ts migration number and description.

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Example

We already have migration 1.ts, so let’s add another one. We have a collection of users, but we need to assign some of them special rights within our app. Let’s add a new boolean field isAdmin to the user schema.
user.schema.ts
const schema = z.object({
  _id: z.string(),
  createdOn: z.date().optional(),
  updatedOn: z.date().optional(),
  deletedOn: z.date().optional().nullable(),
  role: z.string(),
  isAdmin: z.boolean().default(false)
});
After adding this field to the schema, every new user will automatically have isAdmin upon creation with true or false. But old users don’t have the field isAdmin at all. So let’s add it.
migrator/migrations/2.ts
const migration = new Migration(2, 'Add isAdmin field to users');

migration.migrate = async () => {
  const userIds = await userService.distinct('_id', {
    role: 'admin',
  });

  const updateFn = (userId: string) => userService.atomic.updateOne(
    { _id: userId },
    { $set: { isAdmin: true } },
  );

  await promiseUtil.promiseLimit(userIds, 50, updateFn);
};

export default migration;
Warning: Don’t use methods that emit events during the operation. Instead, use atomic operations like atomic.updateOne or updateOne with publishEvents: false

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Promise Limit

Every migration should use promiseLimit to perform changes to the collections, to avoid insufficient resources to complete operations: 
promiseLimit(documents: unknown[], limit: number, operator: (doc: any) => any)

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How it deploys and runs

The main idea behind the ship’s Migrator is to run it before API and Scheduler deployment. Therefore, if any migration fails, then the API or Scheduler updates will not be applied. And they will always work with an up-to-date schema. More on Kubernetes and DO Apps deployment.

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How to re-run failed migration

Migrator always run migrations only above already applied ones. So to re-run failed one, you simply start to migrate the process again. For development: 
npm run migrate-dev
For production: 
npm run migrate

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How to check failed migration logs

First, you can check migrationLog and find your migration with the status failed. It contains the error and errorStack fields. For Kubernetes deployment, you can check the log inside its container by: 
kubectl get pods -A
which is used to check migrator_container_name and namespace, and then: 
kubectl log -f migrator_container_name -n namespace