# Track usage for billing

If your product offers a usage based billing component such as monthly active users then you will need a way to track usage and calculate the invoice total at the end of each period.

Bunny provides an API for tracking feature usage and will take care of the billing calculations for you.

#### Usage calculations

When a **usage based charge** is added to a **price list** in Bunny you will need to set the Usage Calculation Type to one of the following values. This will determine how the usage is calculated at billing time.

* **Sum** - Default. The billing process will take a sum of quantity values for all feature usage records created during the billing period.
* **Max** - The billing process will take the maximum quantity value from all feature usage records created during the billing period.
* **Last** - The billing process will take the quantity value of the last feature usage record created during the billing period.

{% hint style="warning" %}
*For example,* If you wanted to charge based on the number of SMS messages sent during a period then you use the **Sum** calculation and create a feature usage record for every SMS that is sent.\
\
Or, alternatively if you wanted to track monthly active users then you would need to keep note of each tenants billing day from Bunny. You would then do a calculation to find the total users for the period and send this in a single request to Bunny using the **Last** option.
{% endhint %}

### Prerequisites

Before you can start tracking feature usage from your SaaS application you need to complete the following steps on the Bunny side first.

1. [Create an API client & install a Bunny SDK](https://docs.bunny.com/developer/using-the-graphql-api/api)
2. Create a **Feature** in Bunny with **Is Unit** enabled. Then create a product plan and price list with a usage based charge using the feature unit.\
   \&#xNAN;*Take note of the **code** that you set for the feature as you will need that in the next step.*
3. Create a subscription based on the plan and price list that you setup in the previous step and take note of the Subscription ID number.

### Create a feature usage record

Feature usage is tracked via the [featureUsageCreate](https://docs.bunny.com/developer/using-the-graphql-api/mutations/featureusagecreate) mutation. If the **usageAt** attribute is not supplied then it will default to the current datetime.

{% tabs %}
{% tab title="GraphQL" %}
**FeatureUsageAttributes**

```json
{
  "attributes": {
    "quantity": 1,
    "usageAt": "2018/10/21 11:53:12 00",
    "subscriptionId": 123456,
    "featureCode": "users"
  }
}
```

**Mutation**

{% code overflow="wrap" %}

```graphql
mutation featureUsageCreate ($attributes: FeatureUsageAttributes!) {
  featureUsageCreate (attributes: $attributes) {
    errors
  }
}
```

{% endcode %}
{% endtab %}

{% tab title="Ruby" %}

```ruby
response = BunnyApp::FeatureUsage.create(
  quantity: 5, 
  feature_code: 'user', 
  tenant_code: 'ayz-123', 
  usage_at: '2022-03-10'
)
```

{% endtab %}

{% tab title="Node.js" %}

```javascript
let featureCode = 'sms'
let quantity = 25
let usageAt = '2018/10/21 11:53:12 00'
let subscriptionId = 123456

res = await bunny.featureUsageCreate(featureCode, quantity, subscriptionId)
```

{% endtab %}
{% endtabs %}

{% hint style="warning" %}
If a billing period has already passed and been processed then setting a historical date for **usageAt** will not be picked up in the next billing run.
{% endhint %}