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Credit systems let you track actions with different credit costs from a single balance pool. A credit system is made up of a list of features that can draw from it, and a credit cost per unit of usage for each feature.
Example
You have a Pro plan that gives users 100 basic messages per month, and 10 premium messages per month. These 2 balances are separate and independent of each other. To give your users more flexibility, you instead decide to use a credit system, where:
  • basic message: costs 1 credit per message
  • premium message: costs 10 credits per message
Instead of having 2 separate balances for each message type, your Pro plan can have 200 credits per month. Your users can use the credits in any combination of basic and premium messages they want.

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Creating a credit system

Make sure you have some metered features created before creating a credit system.
Define metered features, then create a credit_system feature with a creditSchema that maps each feature to a credit cost:
autumn.config.ts
import { feature, item, plan } from 'atmn';

export const basicMessage = feature({
  id: 'basic_message',
  name: 'Basic Message',
  type: 'metered',
  consumable: true,
});

export const premiumMessage = feature({
  id: 'premium_message',
  name: 'Premium Message',
  type: 'metered',
  consumable: true,
});

export const credits = feature({
  id: 'credits',
  name: 'Credits',
  type: 'credit_system',
  creditSchema: [
    { meteredFeatureId: basicMessage.id, creditCost: 1 },
    { meteredFeatureId: premiumMessage.id, creditCost: 10 },
  ],
});

export const pro = plan({
  id: 'pro',
  name: 'Pro',
  price: { amount: 20, interval: 'month' },
  items: [
    item({
      featureId: credits.id,
      included: 200,
      reset: { interval: 'month' },
    }),
  ],
});
Push changes with atmn push.
ExampleIf each premium_request is worth 3 credits, then using 6 premium requests will cost 18 credits.
Now you can add this credit system to a plan, such as granting 50 credits per month or charging $1 per credit.

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Tracking and limiting credit usage

When implementing a credit system into your application, you should interact with the underlying features — not the credit system itself. This means passing in the underlying feature_id when checking or tracking usage.

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Checking access

Before allowing a customer to use a feature, check if they have enough credits to do so. If each “premium request” is worth 3 credits, then this example will check if the customer has at least 18 credits remaining. 
import { Autumn } from "autumn-js";

const autumn = new Autumn({ secretKey: "am_sk_test_1234" });

const response = await autumn.customers.check({
  customerId: "user_123",
  featureId: "premium_request",
  requiredBalance: 6,
});

console.log(response.allowed);
In this case, we have a balance of 100 credits remaining, so we’re allowed to use our 6 “premium requests” feature.
If a feature is not defined in the credit system, it will return allowed: false

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Tracking usage

Since the customer has sufficient credits, you can let them use their 6 “premium requests”. Afterwards, you can track the usage to update their balance. This will decrement the customer’s balance by 18 credits (6 requests * 3 credits per request). 
import { Autumn } from "autumn-js";

const autumn = new Autumn({ secretKey: "am_sk_test_1234" });

await autumn.customers.track({
  customerId: "user_123",
  featureId: "premium_request",
  value: 6,
});
Since the customer started with a balance of 100 credits, and used 18 credits, their remaining balance is 82 credits.

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Stacking with direct balances

A feature can have both a direct balance and belong to a credit system. When this happens, the balances stack and direct balances are always consumed before credit system balances, regardless of interval.
Example
A customer’s plan grants 10 premium messages per month directly, plus 200 credits per month from a credit system (where each premium message costs 10 credits).

When the customer sends a premium message, Autumn deducts from the direct premium message balance first. Once those 10 direct messages are used up, subsequent premium messages draw from the credit pool instead.
The check endpoint accounts for both balances. If the customer has 5 direct premium messages remaining plus 100 credits (enough for 10 more premium messages), check will report that the customer is allowed.

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Monetary credits

You may want your credit system to represent a monetary value: eg, $10 of credits. To implement this, you can map each credit to a cent value (eg, 1 credit = 1 cent).
  1. When creating your credit system, define credit amounts in the per-cent cost
Eg: if each premium_request costs 3 cents, our credit cost should be 3.
  1. When adding the credits to a plan, set the granted amount of credits in cents
Eg, if customers get 5 USD credits for free, they should have an included usage of 500.
  1. When charging for the credits, set the cost of each credit to 1 cent
See the credits pricing guide for a more detailed example of setting up a monetary credits system

Credits Example