Step 3 - Adding variables and methods

Now let's start adding some code to our backend!

We will use Motoko programming language. You can get a quick overview of Motoko at the Motoko Language Tour.

We will edit file src/poll_backend/main.mo.

Creating an actor

Backend logic on the Internet Computer is implemented as canisters. In Motoko, canisters are represented as "Actors".

note

Actor is a computer science concept of a component that:

• Includes both code and data
• Executes everything in a single thread, so that the developer doesn't have to deal with concurrency problems
• Communicates with external world by sending and receiving messages :::

To create an actor, delete everything from main.mo file and add the following code:

actor {

}

As you can see this actor is empty - it doesn't receive or send any messages. Also it doesn't define any data structures. We will change in the next section.

Add the question data structure

For our poll app, we want to the poll to have the main question. For example, "What is the best blockchain?"

To add the question, include the following code inside the actor:

    var question: Text = "enter your question";

The new statement does the following:

• Creates a new actor's variable question
• Declares the type of this variable Text, which is the standard type for strings in Motoko
• Sets the default value for the variable to the value "enter your question"

Adding access methods

To do any useful work, actors needs to be able to communicate with the external world. Actors communicate using " messages" that are represented as functions.

For our poll actor, we want to be able to:

• Get the current main question
• Set the current main question

So, we will add these two methods to our actor:

public query func getQuestion() : async Text { 
    question 
};

public func setQuestion(q: Text) { 
    question := q 
};

The first method getQuestion takes the value of the question variable and return it to the caller. Some important observations:

• All methods that return values on the Internet Computer have to be declared async to allow asynchronous execution.
• Because the getQuestion method doesn't change any data, it can be declared as query to improve performance.

The second method setQuestion receives the value q and assigns it to the variable question. Since setQuestion changes the actor's data, it is NOT declared as query that makes it an "update method".

Queries vs Updates

Updates are executed on all machines of the subnet and the result will have to pass through a cryptographic consensus algorithm. Queries are executed only on one node of a subnet. Because the consensus is required to change data, it's not possible to change data in queries.

	Queries	Updates
Resource consumption	Low	High
Response times	Fast (300ms-900ms)	Slow (3s-10s)
Cost	Free	Cost cycles
Datat change	Not allowed to change data	Allowed to change data

:::

Full code

After all the changes the code should look like this:

actor {
    var question: Text = "enter your question";

    public query func getQuestion() : async Text { 
      question 
    };
    
    public func setQuestion(q: Text) { 
      question := q 
    };
}

Summary

During this step we learned:

• Internet Computer backend code consists of actors
• Actors are components that interact with the external world via messages
• Actors can have data as variables
• Query methods are fast and free, but they can't change the data
• Update methods are slower, but they can change the data

In the next article we will see how to deploy the app locally and play with it using Candid UI.