Objective

The objective of this lab is to understand

• the Event-Query API Links to an external site.
• the use of entity variables.

Reading and Reference

Read the following:

• Pico State Lesson Links to an external site.
• Persistent Compute Objects
• Pico APIs: Events and Queries Links to an external site.

The following video may be helpful:

Pico State and Array and Map Operators Video Links to an external site.

You may find the following resources useful:

• Mutating Persistent Variables Links to an external site.
• KRL Manual 
• Developer Tips for the Pico Engine 

Prerequisites

You should have completed:

• Lab 2: Rules, Events, and Intermediaries

Implementation Notes

• The best way to use the KRL documentation is to search since you may not always appreciate how it's organized (or isn't).
• In this lab you will implement another ruleset for your pico that complements or adds functionality to the ruleset you built for Lab 3: Rules, Events, and Intermediaries. Note that this second ruleset depends on explicit events that the rules in the first ruleset raise. This is an important property of picos. 

Do This

Do the following:

1. Write a new ruleset called temperature_store to serve as a temperature module. This ruleset should have three rules:

   1. A rule named collect_temperatures that looks for wovyn:new_temperature_reading events and stores the temperature and timestamp event attributes in an entity variable. The entity variable should contain all the temperatures that have been processed. Hint: look at the resources above for persistent variable help.
   2. A rule named collect_threshold_violations that looks for wovyn:threshold_violation events and stores the violation temperature and a timestamp in a different entity variable that collects threshold violations.
   3. A rule named clear_temeratures that looks for a sensor:reading_reset event and resets both of the entity variables from the rules in (1) and (2).

2. Add three functions to the global block of the temperature_store ruleset. These functions should return an array of temperatures where each temperature is a timestamp and a temperature in degrees. 

   1. A function called temperatures that returns the contents of the temperature entity variable.

   2. A function called threshold_violations that returns the contents of the threshold violation entity variable.

   3. A function called inrange_temperatures that returns all the temperatures in the temperature entity variable that aren't in the threshold violation entity variable. (Note: I expect you to solve this without adding a rule that collects in-range temperatures)

3. Add a provides pragma to the meta block Links to an external site. of the temperature_store ruleset that lists the three functions. Also add a shares pragma to the meta block with the same list of functions.

4. Install your temperature_store ruleset in the same pico as your modified wovyn_base ruleset. Test your setup to ensure you are storing temperatures from the wovyn device. You can use curl, a program you write, or a browser app like the Advanced REST Client Links to an external site. to make calls to the functions. 

Deliverables 

Turn in the following:

1. URLs for your ruleset(s)
2. Short screencast (< 3 min with sound) showing your new functions and rules with a short deminstartion of the labs functionality.
3. Answers to the questions, including any posed in the preceding "do this" section.

Questions 

1. Explain how the rule collect_temperatures and the temperatures function work as an event-query API.

2. Explain your strategy for finding temperatures that are in range.

3. What happens if provides doesn't list the name of the temperatures function?

4. What happens if shares doesn't list it?