# Exercise: Create an API Endpoint
This exercise creates a tiny saga and exposes it as an API endpoint. You will run it through the **Train RPC** runner. The flow uses only three steps: `Start` → `Transform` → `Success`.
### Before you start
* You can access the **Devops** app and the **Saga** screen.
* The training deployment (including `train_rpc`) is installed and running.
* You know your API base URL: `https://[YOUR_ADMIN_API_DOMAIN]`.
### What you’ll build
* **Saga path**: `/HelloWorld`
* **Runner**: `train_rpc` (via **Allowed For: Train RPC**)
* **Response**: HTTP `200` with body `{message: "Hello World"}`
{% stepper %}
{% step %}
### Open the Saga screen
Open the [Saga](https://docs.rierino.com/devops/api-flows) screen from the [Devops](https://docs.rierino.com/devops) app.
Unless you changed routing, the UI is at `https://[YOUR_ADMIN_UI_DOMAIN]/app/devops/common/saga`.
Saga Screen
{% endstep %}
{% step %}
### Create a new saga
Click **CREATE NEW** in the left menu area. This clears the editor and opens a blank saga.
You’ll define the saga metadata first. Then you’ll design the step graph.
Create New Button
{% endstep %}
{% step %}
### Assign an ID
Click the pencil next to **ID:** and set a unique identifier.
For this exercise, use: `hello_world-0001`.
Saga ID Assignment
ID conventions (optional)
IDs can be auto-generated if an ID generator is configured. For core assets like sagas and queries, human-readable IDs make debugging and audits much easier.
{% hint style="info" %}
If you assign IDs manually, prefer alphanumerics plus `-` and `_`. For high-volume assets, consider a 4‑digit partition suffix (like `-0001`). For low-volume assets, you can skip partitioning.
{% endhint %}
{% endstep %}
{% step %}
### Define the saga (name, path, allowed runner)
Click the **Definition** icon (circled edit icon) to open saga metadata.
Definition Button
Fill in these fields:
Hello World Saga
* **Saga Name:** `Hello World`
* **Saga Domain (optional):** `util`
* **Status:** `ACTIVE`
* **Saga Description (optional):** `Returns "Hello World"`
* **Saga Path:** `/HelloWorld`
* **Version:** `0`
* **Allowed For:** `Train RPC`
* **Auto Fail:** `true`
This means requests to `/HelloWorld` routed through `train_rpc` will execute this saga.
{% hint style="info" %}
**Allowed For** limits which runner can execute the saga. Leaving it empty allows any runner to execute it.
{% endhint %}
{% endstep %}
{% step %}
### Add steps (Start → Transform → Success)
Drag a **Start** step from the stencil.
Saga Stencil
Drag a **Transform** step and a **Success** step.
Your canvas should look like this:
Saga Steps
Connect `Start` → `Transform`.
Saga Link
Connect `Transform` → `Success`.
Your graph should look like this:
Saga Links
{% endstep %}
{% step %}
### Configure the Transform step
The flow runs now, but it does not yet return `"Hello World"`. You’ll add a static value to the payload.
Select the [Transform](https://docs.rierino.com/devops/api-flows/configuring-saga-steps/transform-step) step. Click its pencil icon to edit.
Transform Step Icons
Set these values:
Transform Definition Screen
* **Step Name:** `Say Hello World`
* **Step Description (optional):** `This step adds a static "Hello World" message.`
* **Transform Class:** `Add Value to Payload`
* **Transform Parameters:**
* **Key:** `value`
* **Value:** `{message: "Hello World"}`
Click **Apply** to save the step configuration.
{% hint style="info" %}
**Why does Transform have a “Class”?**
Instead of adding a separate stencil shape for every transformation type, the step stays the same. You switch behavior by selecting a different **Class** (or **Action**) in the step config.
This makes it safer to evolve flows over time. You can change behavior without rebuilding the graph and breaking tracing continuity.
{% endhint %}
The step label should update on the canvas:
Defined Transform Step
{% endstep %}
{% step %}
### Save the saga
Click **SAVE** in the top-right corner.
Save Button
Wait for the runner to pick up changes. In most training setups this is quick. If your environment has a reload interval, give it \~10–30 seconds.
You should see a confirmation notification.
{% endstep %}
{% step %}
### Test the endpoint
Call the API endpoint from your REST client:
`https://[YOUR_ADMIN_API_DOMAIN]/api/request/train_rpc/HelloWorld`
Expected result:
* HTTP `200 OK`
* Body: `{message: "Hello World"}`
{% hint style="info" %}
Remember the saga path is case-sensitive. Use `/HelloWorld` exactly as configured.
{% endhint %}
{% endstep %}
{% endstepper %}
### Troubleshooting
* **404 Not Found**: confirm **Saga Path** and that you saved the saga. Also confirm it’s allowed for **Train RPC**.
* **403/401**: gateway auth is blocking the request. Check your token/session.
* **Saga does not trigger**: confirm the request is routed through `train_rpc` (URL contains `/train_rpc/`).
