diff options
Diffstat (limited to 'doc/development/database/multiple_databases.md')
| -rw-r--r-- | doc/development/database/multiple_databases.md | 106 |
1 files changed, 97 insertions, 9 deletions
diff --git a/doc/development/database/multiple_databases.md b/doc/development/database/multiple_databases.md index a17ad798305..1338e83070f 100644 --- a/doc/development/database/multiple_databases.md +++ b/doc/development/database/multiple_databases.md @@ -15,10 +15,46 @@ To scale GitLab, the we are database for CI/CD tables was [introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64289) in GitLab 14.1. This feature is still under development, and is not ready for production use. +### Development setup + By default, GitLab is configured to use only one main database. To opt-in to use a main database, and CI database, modify the `config/database.yml` file to have a `main` and a `ci` database -configurations. For example, given a `config/database.yml` like below: +configurations. + +You can set this up using [GDK](#gdk-configuration) or by +[manually configuring `config/database.yml`](#manually-set-up-the-cicd-database). + +#### GDK configuration + +If you are using GDK, you can follow the following steps: + +1. On the GDK root directory, run: + + ```shell + gdk config set gitlab.rails.multiple_databases true + ``` + +1. Open your `gdk.yml`, and confirm that it has the following lines: + + ```yaml + gitlab: + rails: + multiple_databases: true + ``` + +1. Reconfigure GDK: + + ```shell + gdk reconfigure + ``` + +1. [Create the new CI/CD database](#create-the-new-database). + +#### Manually set up the CI/CD database + +You can manually edit `config/database.yml` to split the databases. +To do so, consider a `config/database.yml` file like the example below: ```yaml development: @@ -44,7 +80,7 @@ test: &test statement_timeout: 120s ``` -Edit the `config/database.yml` to look like this: +Edit it to split the databases into `main` and `ci`: ```yaml development: @@ -88,6 +124,25 @@ test: &test statement_timeout: 120s ``` +Next, [create the new CI/CD database](#create-the-new-database). + +#### Create the new database + +After configuring GitLab for the two databases, create the new CI/CD database: + +1. Create the new `ci:` database, load the DB schema into the `ci:` database, + and run any pending migrations: + + ```shell + bundle exec rails db:create db:schema:load:ci db:migrate + ``` + +1. Restart GDK: + + ```shell + gdk restart + ``` + <!-- NOTE: The `validate_cross_joins!` method in `spec/support/database/prevent_cross_joins.rb` references the following heading in the code, so if you make a change to this heading, make sure to update @@ -557,11 +612,31 @@ Don't hesitate to reach out to the [sharding group](https://about.gitlab.com/handbook/engineering/development/enablement/sharding/) for advice. +##### Avoid `dependent: :nullify` and `dependent: :destroy` across databases + +There may be cases where we want to use `dependent: :nullify` or `dependent: :destroy` +across databases. This is technically possible, but it's problematic because +these hooks run in the context of an outer transaction from the call to +`#destroy`, which creates a cross-database transaction and we are trying to +avoid that. Cross-database transactions caused this way could lead to confusing +outcomes when we switch to decomposed, because now you have some queries +happening outside the transaction and they may be partially applied while the +outer transaction fails, which could lead to surprising bugs. + +If you need to do some cleanup after a `destroy` you will need to choose +from some of the options above. If all you need to do is cleanup the child +records themselves from PostgreSQL then you could consider using ["loose foreign +keys"](loose_foreign_keys.md). + ## `config/database.yml` -GitLab will support running multiple databases in the future, for example to [separate tables for the continuous integration features](https://gitlab.com/groups/gitlab-org/-/epics/6167) from the main database. In order to prepare for this change, we [validate the structure of the configuration](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67877) in `database.yml` to ensure that only known databases are used. +GitLab is adding support to run multiple databases, for example to +[separate tables for the continuous integration features](https://gitlab.com/groups/gitlab-org/-/epics/6167) +from the main database. In order to prepare for this change, we +[validate the structure of the configuration](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67877) +in `database.yml` to ensure that only known databases are used. -Previously, the `config/database.yml` would look like this: +Previously, the `config/database.yml` looked like this: ```yaml production: @@ -571,15 +646,16 @@ production: ... ``` -With the support for many databases the support for this -syntax is deprecated and will be removed in [15.0](https://gitlab.com/gitlab-org/gitlab/-/issues/338182). +With the support for many databases this +syntax is [deprecated](https://gitlab.com/gitlab-org/gitlab/-/issues/338182) +and will be removed in [15.0](https://gitlab.com/gitlab-org/gitlab/-/issues/338182). The new `config/database.yml` needs to include a database name to define a database configuration. Only `main:` and `ci:` database -names are supported today. The `main:` needs to always be a first +names are supported. The `main:` database must always be a first entry in a hash. This change applies to decomposed and non-decomposed -change. If an invalidate or deprecated syntax is used the error -or warning will be printed during application start. +change. If an invalid or deprecated syntax is used the error +or warning is printed during application start. ```yaml # Non-decomposed database @@ -603,3 +679,15 @@ production: database: gitlabhq_production_ci ... ``` + +## Foreign keys that cross databases + +There are many places where we use foreign keys that reference across the two +databases. This is not possible to do with two separate PostgreSQL +databases, so we need to replicate the behavior we get from PostgreSQL in a +performant way. We can't, and shouldn't, try to replicate the data guarantees +given by PostgreSQL which prevent creating invalid references, but we still need a +way to replace cascading deletes so we don't end up with orphaned data +or records that point to nowhere, which might lead to bugs. As such we created +["loose foreign keys"](loose_foreign_keys.md) which is an asynchronous +process of cleaning up orphaned records. |