[PM-11873] Add Database Testing Instructions

docs/contributing/testing/database/index.md

@@ -0,0 +1,92 @@
+# Database Integration Testing
+
+Since Bitwarden has multiple database options (4), testing them all automatically is incredibly
+important so that we don't have to have a full QA process for each database type. This is where the
+`Infrastructure.IntegrationTest` project comes in, it allows setting up tests similarly to how the
+databases are consumed in the application. Through their common interface, generally an
+`I*Repository`. These tests are automatically ran on all supported databases in our testing
+pipeline.
+
+## Creating a new test
+
+To create a new database test, just add the `[DatabaseTheory]` and `[DatabaseData]` attributes to
+test. Now, in the parameters of the test you are able to "inject" any repository layer services
+directly into the test. The test will run for every database that is
+[configured in the current environment](#configure-the-tests). Since you inject the interface of the
+service, some runs of the test will use the Dapper based repository implementation targeting
+Microsoft SQL Server and others will use the Entity Framework Core based implementations, which we
+use for MySql, Postgres, and SQLite.
+
+The goal of database tests is to test the business logic that is encapsulated in a given method. If
+the stored procedure in SQL Server then calls another procedure to update the
+`User.AccountRevisionDate` then the same EF implementation should do that as well. Then by running
+the test against all variants, we are ensuring all the variants are feature equal. Locally, you may
+want to only the SQL Server tests along with one EF implementation, SQLite is often the easiest.
+This may work well for a very long time and save you some time overall but there are differences
+between the EF database providers such that you will one day get errors in the CI pipeline.
+
+## Configure the databases
+
+The databases are expected to have the latest migrations applied to them.
+
+## Configure the tests
+
+The tests are configured through
+[.NET Configuration](https://learn.microsoft.com/en-us/dotnet/core/extensions/configuration). In the
+order they are applied, user secrets, environment variables prefixed with `BW_TEST_`, and command
+line args.
+
+```C#
+public class Root
+{
+    public Database[] Databases { get; set; }
+}
+
+public class Database
+{
+    public SupportedDatabaseProviders Type { get; set; }
+    public string ConnectionString { get; set; } = default!;
+    public bool UseEf { get; set; }
+    public bool Enabled { get; set; } = true;
+}
+```
+
+The `Type` property is an enum with the supported values being `SqlServer`, `MySql`, `Postgres`, or
+`Sqlite`. The `UseEf` property is only utilized it the `Type` is set to `SqlServer`, by default
+`SqlServer` will be configured with the Dapper repositories but by setting `UseEf` to `true`, it
+will be configured with the Entity Framework Core repositories. `Enabled` allows you to easily
+disable one database but not delete the entry, it can be helpful if you are encountering a problem
+with just a single database type and want to run the tests just for it instead of for all of them.
+
+### Locally
+
+To set the tests up locally you may want to add the configuration to your `dev/secrets.json` file.
+An example entry you might add is:
+
+```json
+{
+  ...other config...
+  "databases": [
+    {
+      "type": "SqlServer",
+      "connectionString": "myConnectionString"
+    }
+  ]
+}
+```
+
+You can also configure the tests just like the pipeline, read more below.
+
+### Pipeline
+
+The database tests have been pre-configured to run on all supported databases in the
+[`test-database.yml`](https://github.com/bitwarden/server/blob/main/.github/workflows/test-database.yml)
+([permalink](https://github.com/bitwarden/server/blob/f7bc5dfb2ea31ca7b4c36238295cdcc4008ad958/.github/workflows/test-database.yml))
+file.
+
+The pipeline uses environment variables, an example entry you might add is:
+
+```bash
+BW_TEST_DATABASES__0__TYPE: SqlServer
+BW_TEST_DATABASES__0__CONNECTIONSTRING: myConnectionString
+```