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Of course, even with the most advanced external rules definition, there is always a chance that you will still need to adjust the code. Business rules engines simply reduce the risk of needing to change the code.
# **Demo Application**
This article features a demo application called "**RulesEspresso Demo**," available on **ODC Forge**.
* In the ODC Portal, navigate to **Forge - All Assets**.
* Search for "**RulesEspresso Demo**".
* Click on **Install**.
This demo relies on:
* **RulesEspresso** - This external logic library is the rules engine implementation you can use in your own applications.
# RulesEspresso Introduction
RulesEspresso is a **simple** implementation of a business rules engine. At a glance it allows you to
* Provide a flat property **JSON document** containing data as the **rules evaluation context**.
* Include an optional **JSON Schema** for data validation.
* Define one or more rules using **C# expressions** to evaluate data conditions, returning either true or false.
* Optional **result C# expressions** for each rule to add **calculated properties to the rules evaluation context** if the rule expression evaluates to true.
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At the time of writing, RulesEspresso only supports flat JSON documents, which means you cannot use nested objects or arrays.
**ruleDefinitions**
This parameter accepts a list of individual rules. Each rule consists of:
* **RuleName** - Any text. Each rule is evaluated separately, and you can use the rule name to identify it in the result of the Evaluate Rules action.
* **EvaluationExpression** - A boolean C# expression. This is where you specify a condition for the rule evaluation. Attributes of the dataObject are referred to by their names, and you can use C# methods within the conditions, such as DateTime.Now.
* **ResultExpression** - An optional C# expression that is evaluated if the EvaluationExpression is true. The ResultExpression can return a static or computed value, adding to the rules evaluation context, allowing you to use the value in subsequent rules.
* **ResultPropertyName** - Required if ResultExpression is set. This is the property name under which the result of ResultExpression should be added to the rules evaluation context.
**jsonSchema**
Takes an optional JSON schema to validate the dataObject. This validation is done only once, applying only to the input parameter and not to any ResultExpression values added during rules evaluation. The JSON schema is an excellent way to ensure that all necessary input parameters for rules evaluation are provided and have the correct values.
## Example
Let us do an example. We want to pass the total value of a purchase busket, along with a membership status and calculate a discount based on that:
* Bronze status members receive 2%
* Silver status members receive 5%
* Gold status members receive 10%
Our **dataObject** parameter looks like this
```json
{
"totalValue": 7450.00,
"memberStatus": "Silver"
}
```
The **ruleDefinitions**
Rule 1:
* **RuleName**: IsBronze
* **EvaluationExpression**: memberStatus == “Bronze”
* **ResultExpression**: totalValue \* 2 / 100
* **ResultPropertyName**: discount
Rule 2:
* **RuleName**: IsSilver
* **EvaluationExpression**: memberStatus == “Silver”
* **ResultExpression**: totalValue \* 5 / 100
* **ResultPropertyName**: discount
Rule 3:
* **RuleName**: IsGold
* **EvaluationExpression**: memberStatus == “Gold”
* **ResultExpression**: totalValue \* 10 / 100
* **ResultPropertyName**: discount
These rules evaluate the value of memberStatus and calculate the discount.
Finally, we are adding a **jsonSchema** to ensure that memberStatus is either Bronze, Silver, or Gold, and that totalValue is greater than 0.
```json
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"type": "object",
"properties": {
"totalValue": {
"type": "number",
"exclusiveMinimum": 0
},
"memberStatus": {
"type": "string",
"enum": ["Bronze", "Silver", "Gold"]
}
},
"required": ["totalValue", "memberStatus"],
"additionalProperties": false
}
```
You can try it out in the demo application and the Last Evaluation Result should look like this
![](https://cdn.hashnode.com/res/hashnode/image/upload/v1750141311713/22e26841-31d6-413d-a447-879ae416642e.png align="center")
Change the values and add some extra rule expressions.
## Result
After evaluation, the results are returned in the **ValidationResult** output parameter:
* **IsValid** - This boolean value is true if all rules evaluated to true.
* **Document** - Contains the rules evaluation context JSON document, including all added result properties.
* **RuleResults** - An array of individual rule results. There is one entry for each rule evaluated, with the RuleName and a boolean IsValid property indicating whether the rule evaluated to true. You can use these individual entries to selectively take action within an action flow based on the result of a rule.
# Summary
This article introduces the **RulesEspresso** external logic library for OutSystems Developer Cloud. **RulesEspresso** is a simple rules engine that lets you externalize business rules instead of hard-coding them into OutSystems applications. While it handles many use cases, for more complex scenarios, you might want to consider a Business Rules Management Solution. These solutions offer many extra features, including custom code integration and enterprise-level business rules evaluation, unlike **RulesEspresso**, which is designed for application-centric rules evaluation.
I hope you enjoyed it and would appreciate your feedback.
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