328 lines
12 KiB
Markdown
328 lines
12 KiB
Markdown
# Background Migrations
|
|
|
|
Background migrations can be used to perform data migrations that would
|
|
otherwise take a very long time (hours, days, years, etc) to complete. For
|
|
example, you can use background migrations to migrate data so that instead of
|
|
storing data in a single JSON column the data is stored in a separate table.
|
|
|
|
If the database cluster is considered to be in an unhealthy state, background
|
|
migrations automatically reschedule themselves for a later point in time.
|
|
|
|
## When To Use Background Migrations
|
|
|
|
In the vast majority of cases you will want to use a regular Rails migration
|
|
instead. Background migrations should be used when migrating _data_ in
|
|
tables that have so many rows this process would take hours when performed in a
|
|
regular Rails migration.
|
|
|
|
Background migrations _may_ also be used when executing numerous single-row queries
|
|
for every item on a large dataset. Typically, for single-record patterns, runtime is
|
|
largely dependent on the size of the dataset, hence it should be split accordingly
|
|
and put into background migrations.
|
|
|
|
Background migrations _may not_ be used to perform schema migrations, they
|
|
should only be used for data migrations.
|
|
|
|
Some examples where background migrations can be useful:
|
|
|
|
- Migrating events from one table to multiple separate tables.
|
|
- Populating one column based on JSON stored in another column.
|
|
- Migrating data that depends on the output of external services (e.g. an API).
|
|
|
|
> **Note:**
|
|
> If the background migration is part of an important upgrade, make sure it's announced
|
|
> in the release post. Discuss with your Project Manager if you're not sure the migration falls
|
|
> into this category.
|
|
|
|
## Isolation
|
|
|
|
Background migrations must be isolated and can not use application code (e.g.
|
|
models defined in `app/models`). Since these migrations can take a long time to
|
|
run it's possible for new versions to be deployed while they are still running.
|
|
|
|
It's also possible for different migrations to be executed at the same time.
|
|
This means that different background migrations should not migrate data in a
|
|
way that would cause conflicts.
|
|
|
|
## Idempotence
|
|
|
|
Background migrations are executed in a context of a Sidekiq process.
|
|
Usual Sidekiq rules apply, especially the rule that jobs should be small
|
|
and idempotent.
|
|
|
|
See [Sidekiq best practices guidelines](https://github.com/mperham/sidekiq/wiki/Best-Practices)
|
|
for more details.
|
|
|
|
Make sure that in case that your migration job is going to be retried data
|
|
integrity is guaranteed.
|
|
|
|
## How It Works
|
|
|
|
Background migrations are simple classes that define a `perform` method. A
|
|
Sidekiq worker will then execute such a class, passing any arguments to it. All
|
|
migration classes must be defined in the namespace
|
|
`Gitlab::BackgroundMigration`, the files should be placed in the directory
|
|
`lib/gitlab/background_migration/`.
|
|
|
|
## Scheduling
|
|
|
|
Scheduling a background migration should be done in a post-deployment migration.
|
|
To do so, simply use the following code while
|
|
replacing the class name and arguments with whatever values are necessary for
|
|
your migration:
|
|
|
|
```ruby
|
|
BackgroundMigrationWorker.perform_async('BackgroundMigrationClassName', [arg1, arg2, ...])
|
|
```
|
|
|
|
Usually it's better to enqueue jobs in bulk, for this you can use
|
|
`BackgroundMigrationWorker.bulk_perform_async`:
|
|
|
|
```ruby
|
|
BackgroundMigrationWorker.bulk_perform_async(
|
|
[['BackgroundMigrationClassName', [1]],
|
|
['BackgroundMigrationClassName', [2]]]
|
|
)
|
|
```
|
|
|
|
You'll also need to make sure that newly created data is either migrated, or
|
|
saved in both the old and new version upon creation. For complex and time
|
|
consuming migrations it's best to schedule a background job using an
|
|
`after_create` hook so this doesn't affect response timings. The same applies to
|
|
updates. Removals in turn can be handled by simply defining foreign keys with
|
|
cascading deletes.
|
|
|
|
If you would like to schedule jobs in bulk with a delay, you can use
|
|
`BackgroundMigrationWorker.bulk_perform_in`:
|
|
|
|
```ruby
|
|
jobs = [['BackgroundMigrationClassName', [1]],
|
|
['BackgroundMigrationClassName', [2]]]
|
|
|
|
BackgroundMigrationWorker.bulk_perform_in(5.minutes, jobs)
|
|
```
|
|
|
|
### Rescheduling background migrations
|
|
|
|
If one of the background migrations contains a bug that is fixed in a patch
|
|
release, the background migration needs to be rescheduled so the migration would
|
|
be repeated on systems that already performed the initial migration.
|
|
|
|
When you reschedule the background migration, make sure to turn the original
|
|
scheduling into a no-op by clearing up the `#up` and `#down` methods of the
|
|
migration performing the scheduling. Otherwise the background migration would be
|
|
scheduled multiple times on systems that are upgrading multiple patch releases at
|
|
once.
|
|
|
|
## Cleaning Up
|
|
|
|
>**Note:**
|
|
Cleaning up any remaining background migrations _must_ be done in either a major
|
|
or minor release, you _must not_ do this in a patch release.
|
|
|
|
Because background migrations can take a long time you can't immediately clean
|
|
things up after scheduling them. For example, you can't drop a column that's
|
|
used in the migration process as this would cause jobs to fail. This means that
|
|
you'll need to add a separate _post deployment_ migration in a future release
|
|
that finishes any remaining jobs before cleaning things up (e.g. removing a
|
|
column).
|
|
|
|
As an example, say you want to migrate the data from column `foo` (containing a
|
|
big JSON blob) to column `bar` (containing a string). The process for this would
|
|
roughly be as follows:
|
|
|
|
1. Release A:
|
|
1. Create a migration class that perform the migration for a row with a given ID.
|
|
1. Deploy the code for this release, this should include some code that will
|
|
schedule jobs for newly created data (e.g. using an `after_create` hook).
|
|
1. Schedule jobs for all existing rows in a post-deployment migration. It's
|
|
possible some newly created rows may be scheduled twice so your migration
|
|
should take care of this.
|
|
1. Release B:
|
|
1. Deploy code so that the application starts using the new column and stops
|
|
scheduling jobs for newly created data.
|
|
1. In a post-deployment migration you'll need to ensure no jobs remain.
|
|
1. Use `Gitlab::BackgroundMigration.steal` to process any remaining
|
|
jobs in Sidekiq.
|
|
1. Reschedule the migration to be run directly (i.e. not through Sidekiq)
|
|
on any rows that weren't migrated by Sidekiq. This can happen if, for
|
|
instance, Sidekiq received a SIGKILL, or if a particular batch failed
|
|
enough times to be marked as dead.
|
|
1. Remove the old column.
|
|
|
|
This may also require a bump to the [import/export version][import-export], if
|
|
importing a project from a prior version of GitLab requires the data to be in
|
|
the new format.
|
|
|
|
## Example
|
|
|
|
To explain all this, let's use the following example: the table `services` has a
|
|
field called `properties` which is stored in JSON. For all rows you want to
|
|
extract the `url` key from this JSON object and store it in the `services.url`
|
|
column. There are millions of services and parsing JSON is slow, thus you can't
|
|
do this in a regular migration.
|
|
|
|
To do this using a background migration we'll start with defining our migration
|
|
class:
|
|
|
|
```ruby
|
|
class Gitlab::BackgroundMigration::ExtractServicesUrl
|
|
class Service < ActiveRecord::Base
|
|
self.table_name = 'services'
|
|
end
|
|
|
|
def perform(service_id)
|
|
# A row may be removed between scheduling and starting of a job, thus we
|
|
# need to make sure the data is still present before doing any work.
|
|
service = Service.select(:properties).find_by(id: service_id)
|
|
|
|
return unless service
|
|
|
|
begin
|
|
json = JSON.load(service.properties)
|
|
rescue JSON::ParserError
|
|
# If the JSON is invalid we don't want to keep the job around forever,
|
|
# instead we'll just leave the "url" field to whatever the default value
|
|
# is.
|
|
return
|
|
end
|
|
|
|
service.update(url: json['url']) if json['url']
|
|
end
|
|
end
|
|
```
|
|
|
|
Next we'll need to adjust our code so we schedule the above migration for newly
|
|
created and updated services. We can do this using something along the lines of
|
|
the following:
|
|
|
|
```ruby
|
|
class Service < ActiveRecord::Base
|
|
after_commit :schedule_service_migration, on: :update
|
|
after_commit :schedule_service_migration, on: :create
|
|
|
|
def schedule_service_migration
|
|
BackgroundMigrationWorker.perform_async('ExtractServicesUrl', [id])
|
|
end
|
|
end
|
|
```
|
|
|
|
We're using `after_commit` here to ensure the Sidekiq job is not scheduled
|
|
before the transaction completes as doing so can lead to race conditions where
|
|
the changes are not yet visible to the worker.
|
|
|
|
Next we'll need a post-deployment migration that schedules the migration for
|
|
existing data. Since we're dealing with a lot of rows we'll schedule jobs in
|
|
batches instead of doing this one by one:
|
|
|
|
```ruby
|
|
class ScheduleExtractServicesUrl < ActiveRecord::Migration[4.2]
|
|
disable_ddl_transaction!
|
|
|
|
class Service < ActiveRecord::Base
|
|
self.table_name = 'services'
|
|
end
|
|
|
|
def up
|
|
Service.select(:id).in_batches do |relation|
|
|
jobs = relation.pluck(:id).map do |id|
|
|
['ExtractServicesUrl', [id]]
|
|
end
|
|
|
|
BackgroundMigrationWorker.bulk_perform_async(jobs)
|
|
end
|
|
end
|
|
|
|
def down
|
|
end
|
|
end
|
|
```
|
|
|
|
Once deployed our application will continue using the data as before but at the
|
|
same time will ensure that both existing and new data is migrated.
|
|
|
|
In the next release we can remove the `after_commit` hooks and related code. We
|
|
will also need to add a post-deployment migration that consumes any remaining
|
|
jobs and manually run on any un-migrated rows. Such a migration would look like
|
|
this:
|
|
|
|
```ruby
|
|
class ConsumeRemainingExtractServicesUrlJobs < ActiveRecord::Migration[4.2]
|
|
disable_ddl_transaction!
|
|
|
|
class Service < ActiveRecord::Base
|
|
include ::EachBatch
|
|
|
|
self.table_name = 'services'
|
|
end
|
|
|
|
def up
|
|
# This must be included
|
|
Gitlab::BackgroundMigration.steal('ExtractServicesUrl')
|
|
|
|
# This should be included, but can be skipped - see below
|
|
Service.where(url: nil).each_batch(of: 50) do |batch|
|
|
range = batch.pluck('MIN(id)', 'MAX(id)').first
|
|
|
|
Gitlab::BackgroundMigration::ExtractServicesUrl.new.perform(*range)
|
|
end
|
|
end
|
|
|
|
def down
|
|
end
|
|
end
|
|
```
|
|
|
|
The final step runs for any un-migrated rows after all of the jobs have been
|
|
processed. This is in case a Sidekiq process running the background migrations
|
|
received SIGKILL, leading to the jobs being lost. (See
|
|
[more reliable Sidekiq queue][reliable-sidekiq] for more information.)
|
|
|
|
If the application does not depend on the data being 100% migrated (for
|
|
instance, the data is advisory, and not mission-critical), then this final step
|
|
can be skipped.
|
|
|
|
This migration will then process any jobs for the ExtractServicesUrl migration
|
|
and continue once all jobs have been processed. Once done you can safely remove
|
|
the `services.properties` column.
|
|
|
|
## Testing
|
|
|
|
It is required to write tests for:
|
|
|
|
- The background migrations' scheduling migration.
|
|
- The background migration itself.
|
|
- A cleanup migration.
|
|
|
|
You can use the `:migration` RSpec tag when testing the migrations.
|
|
See the
|
|
[Testing Rails migrations](testing_guide/testing_migrations_guide.md#testing-a-non-activerecordmigration-class)
|
|
style guide.
|
|
|
|
When you do that, keep in mind that `before` and `after` RSpec hooks are going
|
|
to migrate you database down and up, which can result in other background
|
|
migrations being called. That means that using `spy` test doubles with
|
|
`have_received` is encouraged, instead of using regular test doubles, because
|
|
your expectations defined in a `it` block can conflict with what is being
|
|
called in RSpec hooks. See [issue #35351][issue-rspec-hooks]
|
|
for more details.
|
|
|
|
## Best practices
|
|
|
|
1. Make sure to know how much data you're dealing with.
|
|
1. Make sure that background migration jobs are idempotent.
|
|
1. Make sure that tests you write are not false positives.
|
|
1. Make sure that if the data being migrated is critical and cannot be lost, the
|
|
clean-up migration also checks the final state of the data before completing.
|
|
1. Make sure to know how much time it'll take to run all scheduled migrations.
|
|
1. When migrating many columns, make sure it won't generate too many
|
|
dead tuples in the process (you may need to directly query the number of dead tuples
|
|
and adjust the scheduling according to this piece of data).
|
|
1. Make sure to discuss the numbers with a database specialist, the migration may add
|
|
more pressure on DB than you expect (measure on staging,
|
|
or ask someone to measure on production).
|
|
|
|
[migrations-readme]: https://gitlab.com/gitlab-org/gitlab/blob/master/spec/migrations/README.md
|
|
[issue-rspec-hooks]: https://gitlab.com/gitlab-org/gitlab-foss/issues/35351
|
|
[reliable-sidekiq]: https://gitlab.com/gitlab-org/gitlab-foss/issues/36791
|
|
[import-export]: ../user/project/settings/import_export.md
|