# Rule Engine
### What is the Rule Engine?
Rule Engine is an easy to use framework for building event-based workflows. There are 3 main components:
* **Message** - any incoming event. It can be an incoming data from devices, device life-cycle event, REST API event, RPC request, etc.
* **Rule Node** - a function that is executed on an incoming message. There are many different Node types that can filter, transform or execute some action on incoming Message.
* **Rule Chain** - nodes are connected with each other with relations, so the outbound message from rule node is sent to next connected rule nodes.
### Typical Use Cases
Tesenso IoT Cloud Rule Engine is a highly customizable framework for complex event processing. Here are some common use cases that one can configure via Tesenso IoT Cloud Rule Chains:
* Data validation and modification for incoming telemetry or attributes before saving to the database.
* Copy telemetry or attributes from devices to related assets so you can aggregate telemetry. For example data from multiple devices can be aggregated in related Asset.
* Create/Update/Clear alarms based on defined conditions.
* Trigger actions based on device life-cycle events. For example, create alerts if Device is Online/Offline.
* Load additional data required for processing. For example, load temperature threshold value for a device that is defined in Device’s Customer or Tenant attribute.
* Trigger REST API calls to external systems.
* Send emails when complex event occurs and use attributes of other entities inside Email Template.
* Take into account User preferences during event processing.
* Make RPC calls based on defined condition.
* Integrate with external pipelines like Kafka, Spark, AWS services, etc.
### Hello-World Example
Let’s assume your device is using DHT22 sensor to collect and push temperature to the Tesenso IoT Cloud. DHT22 sensor can measure temperature from -40°C to +80°C.
In this tutorial we will configure Tesenso IoT Cloud Rule Engine to store all temperature within -40 to 80°C range and log all other readings to the system log.
**Adding temperature validation node**
In Tesenso IoT Cloud UI go to **Rule Chains** section and open **Root Rule Chain**.
Drag and Drop [**Script Filter**](https://docs.tesenso.com/tesenso-iot-cloud/rule-engine/rule-engine/filter-rule-nodes/script-filter) rule node to the chain. Node configuration window will be opened. We will use this script for data validation:
```
return typeof msg.temperature === 'undefined' || (msg.temperature >= -40 && msg.temperature <= 80);
```
If temperature property not defined or temperature is valid - script will return **True**, otherwise it will return **False**. If script returns **True** incoming message will be routed to the next nodes that are connected with **True** relation.
Now we want that all **telemetry requests** pass through this validation script. We need to remove the existing **Post Telemetry** relation between **Message Type Switch** node and **Save Telemetry** node:
And connect [**Message Type Switch**](https://docs.tesenso.com/tesenso-iot-cloud/rule-engine/rule-engine/filter-rule-nodes/message-type-switch) node with [**Script Filter**](https://docs.tesenso.com/tesenso-iot-cloud/rule-engine/rule-engine/filter-rule-nodes/script-filter) node using **Post Telemetry** relation:
Next, we need to connect **Script Filter** node with **Save Telemetry** node using **True** relation. So all valid telemetry will be saved:
Also, we will connect **Script Filter** node with **Log Other** node using **False** relation. So that all not valid telemetry will be logged in the system log:
Press Save button to apply changes.
**Validate results**
For validating results we will need to create Device and submit telemetry to the Tesenso IoT Cloud. So go to **Devices** section and create new Device:
For posting device telemetry we will use Rest API. To do this this we will need to copy device access token from the device **Thermostat Home**.
Lets post temperature = 99. We will see that telemetry **was not** added in Device **Latest Telemetry** section:
```
curl -v -X POST -d '{"temperature":99}' http://tesenso.io:8080/api/v1/$ACCESS_TOKEN/telemetry --header "Content-Type:application/json"
```
**\*you need to replace $ACCESS\_TOKEN with actual device token**
Lets post temperature = 24. We will see that telemetry was saved successfully.
```
curl -v -X POST -d '{"temperature":24}' http://tesenso.io:8080/api/v1/$ACCESS_TOKEN/telemetry --header "Content-Type:application/json"
```
