Docs: Add database architecture docs (#19800)

contribute/architecture/README.md

@@ -6,4 +6,5 @@ Learn more about the backend architecture:
 
 - Part 1: [Services](services.md) 
 - Part 2: [Communication](communication.md)
+- Part 3: [Database](database.md)
 

contribute/architecture/database.md

+# Database
+
+Grafana uses a database to persist settings between restarts. In fact, if you don't specify one, Grafana creates a [SQLite3](https://www.sqlite.org/) database file on your local disk. This guide explains how to store and retrieve data from the database.
+
+Grafana supports the [following databases](https://grafana.com/docs/installation/requirements/#database):
+
+- [MySQL](https://www.mysql.com/)
+- [PostgreSQL](https://www.postgresql.org/)
+- [SQLite3](https://www.sqlite.org/)
+
+Grafana uses the [XORM](https://xorm.io) framework for persisting objects to the database. For more information on how to use XORM, refer to the [documentation](http://gobook.io/read/github.com/go-xorm/manual-en-US/).
+
+[Services](services.md) don't use XORM directly. Instead, services use the _SQL store_, a special type of service that provides an abstraction for the database layer. There are two ways of using the `sqlstore`: using `sqlstore` handlers, and using the `SqlStore` instance.
+
+## `sqlstore` handlers
+
+> **Deprecated:** We are deprecating `sqlstore` handlers in favor of using the `SqlStore` object directly in each service. Since most services still use the `sqlstore` handlers, we still want to explain how they work.
+
+The `sqlstore` package allows you to register [command handlers](communication.md#handle-commands) that either store, or retrieve objects from the database. `sqlstore` handlers are similar to services:
+
+- [Services](services.md) are command handlers that _contain business logic_.
+- `sqlstore` handlers are command handlers that _access the database_.
+
+### Register a `sqlstore` handler
+
+> **Deprecated:** Refer to the [deprecation note for `sqlstore` handlers](#sqlstore-handlers).
+
+To register a handler:
+
+- Create a new file `myrepo.go` in the `sqlstore` package.
+- Create a [command handler](communication.md#handle-commands).
+- Register the handler in the `init` function:
+
+```go
+func init() {
+    bus.AddHandler("sql", DeleteDashboard)
+}
+
+func DeleteDashboard(cmd *models.DeleteDashboardCommand) error {
+    return inTransaction(func(sess *DBSession) error {
+        _, err := sess.Exec("DELETE FROM dashboards WHERE dashboard_id=?", cmd.DashboardID)
+        return err
+    })
+}
+```
+
+Here, `inTransaction` is a helper function in the `sqlstore` package that provides a session, that lets you execute SQL statements.
+
+## `SqlStore`
+
+As opposed to a `sqlstore` handler, the `SqlStore` is a service itself. The `SqlStore` has the same responsibility however: to store and retrieve objects, to and from the database.
+
+To use the `SqlStore`, inject the `SQLStore` in your service struct:
+
+```go
+type MyService struct {
+    SQLStore *sqlstore.SqlStore `inject:""`
+}
+```
+
+You can now make SQL queries in any of your [command handlers](communication.md#handle-commands) or [event listeners](communication.md#subscribe-to-an-event):
+
+```go
+func (s *MyService) DeleteDashboard(cmd *models.DeleteDashboardCommand) error {
+    if err := s.SQLStore.WithDbSession(ctx, func(sess *sqlstore.DBSession) error {
+        _, err := sess.Exec("DELETE FROM dashboards WHERE dashboard_id=?", cmd.DashboardID)
+        return err
+    })
+}
+```
+
+For transactions, use the `WithTransactionalDbSession` method instead.
+
+## Migrations
+
+As Grafana evolves, it becomes necessary to create _schema migrations_ for one or more database tables.
+
+To see all the types of migrations you can add, refer to [migrations.go](/pkg/services/sqlstore/migrator/migrations.go).
+
+Before you add a migration, make sure that you:
+
+- Never change a migration that has been committed and pushed to master.
+- Always add new migrations, to change or undo previous migrations.
+
+Add a migration using one of the following methods:
+
+- Add migrations in the `migrations` package.
+- Implement the `DatabaseMigrator` for the service.
+
+**Important:** If there are previous migrations for a service, use that method. By adding migrations using both methods, you risk running migrations in the wrong order.
+
+### Add migrations in `migrations` package
+
+Most services have their migrations located in the [migrations](/pkg/services/sqlstore/migrations/migrations.go) package.
+
+To add a migration:
+
+- Open the [migrations.go](/pkg/services/sqlstore/migrations/migrations.go) file.
+- In the `AddMigrations` function, find the `addXxxMigration` function for the service you want to create a migration for.
+- At the end of the `addXxxMigration` function, register your migration:
+
+[Example](https://github.com/grafana/grafana/blob/00d0640b6e778ddaca021670fe851fe00982acf2/pkg/services/sqlstore/migrations/migrations.go#L55-L70)
+
+### Implement `DatabaseMigrator`
+
+During initialization, SQL store queries the service registry, and runs migrations for every service that implements the [DatabaseMigrator](https://github.com/grafana/grafana/blob/44c2007498c76c2dbb48e8366b4af410f1ee1b98/pkg/registry/registry.go#L101-L106) interface.
+
+To add a migration:
+
+- If needed, add the `AddMigration(mg *migrator.Migrator)` method to the service.
+- At the end of the `AddMigration` method, register your migration:
+
+```go
+func (s *MyService) AddMigration(mg *migrator.Migrator) {
+    // ...
+
+    mg.AddMigration("Add column age", NewAddColumnMigration(table, &Column{
+        Name:     "age",
+        Type:     migrator.DB_BigInt,
+        Nullable: true,
+    }))
+}
+```