This guide will help you get started with contributing to the Opik backend.

:::callout{intent="tip"}
Before you start, please review our general [Contribution Overview](https://www.comet.com/contributing/overview) and the [Contributor
License Agreement (CLA)](https://github.com/comet-ml/opik/blob/main/CLA.md).
:::

## Project Structure

The Opik backend is a Java application (source in `apps/opik-backend`) that forms the core of the Opik platform. It handles data ingestion, storage, API requests, and more.

## Setting Up Your Development Environment

We provide multiple ways to develop the backend. Choose the approach that best fits your workflow:

::::::tabs
:::::tab{title="Local Process Mode (Recommended)"}
**Best for rapid development**

This mode runs the backend as a local process while infrastructure and other services run in Docker:

::::tabs
:::tab{title="Linux/Mac"}
```bash
# From repository root - restart everything
scripts/dev-runner.sh --be-only-restart

# Or just start (faster if already built)
scripts/dev-runner.sh --be-only-start
```
:::

:::tab{title="Windows"}
```powershell
# From repository root - restart everything
scripts\dev-runner.ps1 --be-only-restart

# Or just start (faster if already built)
scripts\dev-runner.ps1 --be-only-start
```
:::
::::

The backend API will be accessible at `http://localhost:8080`.

**Benefits:**

- Fast rebuilds and restarts
- Easy debugging
- Faster code changes without Docker container rebuilds

**Prerequisites:**

- Java Development Kit (JDK) 25
- Apache Maven 3.8+
:::::

:::::tab{title="Docker Mode"}
**Best for testing the complete system end to end**

This mode runs everything in Docker containers:

::::tabs
:::tab{title="Linux/Mac"}
```bash
# From repository root
./opik.sh --build

# Or start without rebuilding
./opik.sh
```
:::

:::tab{title="Windows"}
```powershell
# From repository root
.\opik.ps1 --build

# Or start without rebuilding
.\opik.ps1
```
:::
::::

The backend API will be accessible at `http://localhost:8080`.

**Benefits:**

- Closest to production environment
- No local Java/Maven installation needed
- Consistent environment across team

**Prerequisites:**

- Docker and Docker Compose
:::::

:::::tab{title="Manual Setup"}
**Best for understanding the build process**

Set up each component manually:

::::tabs
:::tab{title="Linux/Mac"}
1. **Start infrastructure services:** The backend relies on Clickhouse, MySQL, and Redis etc.
   ```bash
   ./opik.sh --infra --port-mapping
   ```

2. **Build the backend:**
   ```bash
   cd apps/opik-backend
   mvn clean install
   ```

3. **Run database migrations:**
   ```bash
   # MySQL migrations
   java -jar target/opik-backend-*.jar db migrate config.yml

   # ClickHouse migrations
   java -jar target/opik-backend-*.jar dbAnalytics migrate config.yml
   ```

4. **Start the backend:**
   ```bash
   java -jar target/opik-backend-*.jar server config.yml
   ```
:::

:::tab{title="Windows"}
1. **Start infrastructure services:** The backend relies on Clickhouse, MySQL, and Redis etc.
   ```powershell
   .\opik.ps1 --infra --port-mapping
   ```

2. **Build the backend:**
   ```powershell
   cd apps\opik-backend
   mvn clean install
   ```

3. **Run database migrations:**
   ```powershell
   # MySQL migrations
   java -jar target\opik-backend-*.jar db migrate config.yml

   # ClickHouse migrations
   java -jar target\opik-backend-*.jar dbAnalytics migrate config.yml
   ```

4. **Start the backend:**
   ```powershell
   java -jar target\opik-backend-*.jar server config.yml
   ```
:::
::::

The backend API will be accessible at `http://localhost:8080`.

**Prerequisites:**

- Java Development Kit (JDK) 25
- Apache Maven 3.8+
- Docker and Docker Compose (for infrastructure)
:::::
::::::

For comprehensive documentation on all development modes, troubleshooting, and advanced workflows, see our [Local Development Guide](https://www.comet.com/contributing/guides/local-development).

### 4. Code Formatting

We use Spotless for code formatting. Before submitting a PR, please ensure your code is formatted correctly:

::::::tabs
:::::tab{title="Using dev-runner (Recommended)"}
::::tabs
:::tab{title="Linux/Mac"}
```bash
# From repository root
scripts/dev-runner.sh --lint-be
```
:::

:::tab{title="Windows"}
```powershell
# From repository root
scripts\dev-runner.ps1 --lint-be
```
:::
::::
:::::

:::tab{title="Manual Maven"}
```bash
cd apps/opik-backend
mvn spotless:apply
```
:::
::::::

Our CI (Continuous Integration) will check formatting using `mvn spotless:check` and fail the build if it's not correct.

### 5. Running Tests

Ensure your changes pass all backend tests:

```bash
cd apps/opik-backend
mvn test
```

Tests leverage the `testcontainers` library to run integration tests against real instances of external services (Clickhouse, MySQL, etc.). Ports for these services are randomly assigned by the library during tests to avoid conflicts.

### 6. Submitting a Pull Request

After implementing your changes, ensuring tests pass, and code is formatted, commit your work and open a Pull Request against the `main` branch of the `comet-ml/opik` repository.

## Advanced Backend Topics

::::::::accordion-group
:::accordion{title="Health Check"}
To check the health of your locally running backend application, you can access the health check endpoint in your browser or via `curl` at `http://localhost:8080/healthcheck`.
:::

:::::::accordion{title="Database Migrations (Liquibase)"}
Opik uses [Liquibase](https://www.liquibase.com/) for managing database schema changes (DDL migrations) for both MySQL and ClickHouse.

- **Location**: Migrations are located at `apps/opik-backend/src/main/resources/liquibase/{{DB}}/migrations` (where `{{DB}}` is `db` for MySQL or `dbAnalytics` for ClickHouse).
- **Automation**: Execution is typically automated via the `apps/opik-backend/run_db_migrations.sh` script, Docker images, and Helm charts in deployed environments.

**Running Migrations in Local Development:**

::::::tabs
:::::tab{title="Using dev-runner (Recommended)"}
::::tabs
:::tab{title="Linux/Mac"}
```bash
# From repository root
scripts/dev-runner.sh --migrate
```
:::

:::tab{title="Windows"}
```powershell
# From repository root
scripts\dev-runner.ps1 --migrate
```
:::
::::

This command will:

- Start infrastructure services if needed
- Build the backend if no JAR file exists
- Run both MySQL and ClickHouse migrations automatically
:::::

:::tab{title="Manual Execution"}
To run DDL migrations manually (replace `{project.pom.version}` and `{database}` as needed):

- **Check pending migrations:** `java -jar target/opik-backend-{project.pom.version}.jar {database} status config.yml`
- **Run migrations:** `java -jar target/opik-backend-{project.pom.version}.jar {database} migrate config.yml`
- **Create schema tag:** `java -jar target/opik-backend-{project.pom.version}.jar {database} tag config.yml {tag_name}`
- **Rollback migrations:**
  - `java -jar target/opik-backend-{project.pom.version}.jar {database} rollback config.yml --count 1`
  - OR `java -jar target/opik-backend-{project.pom.version}.jar {database} rollback config.yml --tag {tag_name}`

Where `{database}` is either `db` (for MySQL) or `dbAnalytics` (for ClickHouse).
:::
::::::

**Requirements for DDL Migrations:**

- Must be backward compatible (new fields optional/defaulted, column removal in stages, no renaming of active tables/columns).
- Must be independent of application code changes.
- Must not cause downtime.
- Must have a unique name.
- Must contain a rollback statement (or use `empty` if Liquibase cannot auto-generate one). Refer to [Evolutionary Database Design](https://martinfowler.com/articles/evodb.html) and [Liquibase Rollback Docs](https://docs.liquibase.com/secure/user-guide-5-0/what-is-a-rollback).
- For more complex migration, apply the transition phase. Refer to [Evolutionary Database Design](https://martinfowler.com/articles/evodb.html)
:::::::

:::accordion{title="Data Migrations (DML)"}
DML (Data Manipulation Language) migrations are for changes to data itself, not the schema.

- **Execution**: These are not run automatically. They must be run manually by a system admin using a database client.
- **Documentation**: DML migrations are documented in the [self-host changelog](/guides/overview-self-host-changelog) and the scripts are placed at
  `apps/opik-backend/data-migrations` along with detailed instructions.
- **Requirements for DML Migrations**:
  - Must be backward compatible (no data deletion unless 100% safe, allow rollback, no performance degradation).
  - Must include detailed execution instructions.
  - Must be batched appropriately to avoid disrupting operations.

* Must not cause downtime.
* Must have a unique name.
* Must contain a rollback statement.
:::

:::accordion{title="Accessing ClickHouse Directly"}
You can query the ClickHouse REST endpoint directly. For example, to get the version:

```bash
echo 'SELECT version()' | curl -H 'X-ClickHouse-User: opik' -H 'X-ClickHouse-Key: opik' 'http://localhost:8123/' -d @-
```

Sample output: `23.8.15.35`
:::
::::::::

## Related pages

- [Documentation](./contributing-guides-documentation.md)
- [Local Development Setup](./contributing-guides-local-development.md)
- [Python SDK](./contributing-guides-python-sdk.md)
- [TypeScript SDK](./contributing-guides-typescript-sdk.md)
- [Opik Optimizer](./contributing-guides-agent-optimizer-sdk.md)
- [Frontend](./contributing-guides-frontend.md)

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