Hello Holo Tutorial

Time & Level

Time: ~2 hours | Level: Beginner

Let’s begin with the classic Hello World Holo tutorial! You will see it’s super easy to create a distributed application with Holochain.

What will you learn

You’ll learn how to create a Holochain zome with a callable function, compile it into a DNA, and run it in the Holochain conductor. Once it’s running, you will learn how to call a zome function using curl.

Why it matters

This tutorial helps you get orientated to the basics of a hApp. These are the fundamental parts on which you will be building later, so it’s important to have a clear and solid understanding.

Setup

  1. Complete the installation guide. It will give you an app development, environment including the Holochain developer tool hc.
  2. Open up a terminal (if you’re using Windows 10, make sure you open the Ubuntu terminal, not the command prompt or Powershell).
  3. Enter the development environment. You’ll remember this command from the installation tutorial:
    nix-shell https://holochain.love
    

Nix Shell

You will see commands marked Run in nix-shell https://holochain.love throughout these tutorials. You should keep the nix-shell open and run these commands in it—don’t reopen nix-shell for every command unless the tutorial tells you to.

Initializing your new app

Pick a new home in which all your future Holochain applications will live. Something like ~/holochain/.

Then, create a coreconcepts folder for this tutorial series:

cd ~/holochain
mkdir coreconcepts
cd coreconcepts

It’s time to put the Holochain command line tool (hc) to work and make your app.

Initialize a new app and enter the app directory:

Run in nix-shell https://holochain.love

hc init cc_tuts
cd cc_tuts

Compile

Run hc and holochain from root directory.

All hc and holochain commands should be run from the project root (e.g., cc_tuts/), except of course hc init, because the root doesn’t exist at this point.

It’s always good to frequently compile your app to catch any mistakes early on.

Give it a go by asking hc to package your app:

Run in nix-shell https://holochain.love

hc package

‘Packaging your app’ means you are compiling the code into a DNA file and getting it ready to be run.

You should see a successful compilation like this:

Created DNA package file at "/Users/username/holochain/testing_tuts/hello_holo/dist/hello_holo.dna.json"
DNA hash: QmY7rhg4sf6xqQMRL1u1CnXVgmamTfxC59c9RaoFqM2eRs

Generate a zome

Your app doesn’t really do much right now because it needs a zome. A zome is Holochain’s way of organizing code into nice units that perform a certain task (like saying, “Hello”).

Generate a zome called hello inside the zome’s folder:

Run in nix-shell https://holochain.love

hc generate zomes/hello rust-proc

Compile

Run in nix-shell https://holochain.love

hc package

Zomes can take a little while to compile the first time. Compiling will be much faster the next time you do it. Feel free to move on with the tutorial while your app compiles.

If all went well you should see:

> cargo build --release --target=wasm32-unknown-unknown --target-dir=target
   Compiling hello v0.1.0 (/Users/username/holochain/coreconcepts/hello_hollo/zomes/hello/code)
    Finished release [optimized] target(s) in 11.95s
> cargo build --release --target=wasm32-unknown-unknown --target-dir=target
    Finished release [optimized] target(s) in 0.50s
Created DNA package file at "/Users/username/holochain/coreconcepts/hello_hollo/dist/hello_hollo.dna.json"
DNA hash: QmdNyxke1Z9Kunws4WUXHnt4cdKQnPogC7YPpfQx67fo1z

Folder layout

Look at the folder layout

Folder Layout

Open the lib.rs file

The zome is a Rust project and makes use of macros to keep you from having to write a lot of boilerplate code. The main file you will be editing is: hello_hollo/zomes/code/src/lib.rs.

Let’s have a look at the generated code—–open up the lib.rs file in an editor.

The following lines import the Holochain HDK. You are telling Rust, “Hey, I need things from all these crates in order to do my job.”

#![feature(proc_macro_hygiene)]

use hdk::prelude::*;
use hdk_proc_macros::zome;

There are a few sections of generated code that are not useful for this tutorial.

Remove the following piece of code:

-#[derive(Serialize, Deserialize, Debug, DefaultJson, Clone)]
-pub struct MyEntry {
-    content: String,
-}

The my_zome module is where all your zome code lives. #[zome] is a procedural macro that says that the following module defines all the things that Holochain should know about this zome. It saves you writing lots of code.

Change it to hello_zome for this tutorial series:

#[zome]
-mod my_zome {
+mod hello_zome {

The init function is run when a user starts the app for the first time. Every zome defines this function so it can do some initial setup tasks, but in this zome it doesn’t do anything.

    #[init]
    fn init() {

Return success with the empty value (). In Rust, () is called the unit type and is similar, though not identical, to a void type in other languages.

        Ok(())
    }

This required function is run at application start too, once by the new user and once by the existing peers. It checks that the user is allowed to join the network. In this case, it gives everyone a free pass.

    #[validate_agent]
    pub fn validate_agent(validation_data: EntryValidationData<AgentId>) {
        Ok(())
    }

Remove the following template code:

-      #[entry_def]
-      fn my_entry_def() -> ValidatingEntryType {
-         entry!(
-             name: "my_entry",
-             description: "this is a same entry defintion",
-             sharing: Sharing::Public,
-             validation_package: || {
-                 hdk::ValidationPackageDefinition::Entry
-             },
-             validation: | _validation_data: hdk::EntryValidationData<MyEntry>| {
-                 Ok(())
-             }
-         )
-     }
-
-     #[zome_fn("hc_public")]
-     fn create_my_entry(entry: MyEntry) -> ZomeApiResult<Address> {
-         let entry = Entry::App("my_entry".into(), entry.into());
-         let address = hdk::commit_entry(&entry)?;
-         Ok(address)
-     }
-
-     #[zome_fn("hc_public")]
-     fn get_my_entry(address: Address) -> ZomeApiResult<Option<Entry>> {
-         hdk::get_entry(&address)
-     }

A note about return values

You’ll often see Rust functions returning some sort of Result value. This is a special Rust type that can either be Ok(some_value) to show that the function succeeded or Err(some_error) to report an error. Required Holochain functions, like init and validators, are expected to return a special result type called ZomeApiResult, which shuttles data back and forth between your app and the conductor. It’s a useful structure, so it makes sense to use it in the API functions you write as well.

Add a function to say “Hello” :)

Now, tell the zome to return Hello Holo from a public function.

Locate the validate_agent function:

    pub fn validate_agent(validation_data: EntryValidationData<AgentId>) {
      Ok(())
    }

You’re going to put your public zome function after it.

The hc_public procedural macro will turn the function directly below it into a public function that GUIs, other zomes, and DNAs can call. It takes note of the function’s name, the parameters it accepts, and the type of value it returns, so Holochain can call it properly.

Add the hc_public macro:

    #[zome_fn("hc_public")]

The function hello_holo takes no arguments and returns a Holochain result type. We’re also telling Holochain that if the result is Ok, it will contain a string.

Start the function:

    pub fn hello_holo() -> ZomeApiResult<String> {

Return an Ok result that contains our greeting. into() is a bit of Rust oddness that just means “turn this slice into a String:”

        Ok("Hello Holo".into())
    }

Compile

Check your code
#![feature(proc_macro_hygiene)]

use hdk::prelude::*;
use hdk_proc_macros::zome;

// see https://developer.holochain.org/api/0.0.49-alpha1/hdk/ for info on using the hdk library

// This is a sample zome that defines an entry type "MyEntry" that can be committed to the
// agent's chain via the exposed function create_my_entry


#[zome]
mod hello_zome {

    #[init]
    fn init() {
        Ok(())
    }

    #[validate_agent]
    pub fn validate_agent(validation_data: EntryValidationData<AgentId>) {
        Ok(())
    }

    #[zome_fn("hc_public")]
    pub fn hello_holo() -> ZomeApiResult<String> {
        Ok("Hello Holo".into())
    }
}

If you find errors, remember to fix them before moving on. You can always get help on the Holochainforum.

Run in nix-shell https://holochain.love

hc package

Talk to your app through HTTP

To interact with your application, you can run it in HTTP mode.

Run in nix-shell https://holochain.love

hc run -i http

You can send a POST message to your app using curl, a little command for making HTTP requests. It’s included in the Holochain dev environment, so if you don’t have it installed in your machine, you can open a new terminal window and enter the nix-shell again:

nix-shell https://holochain.love

Enter the following request, which calls the hello_holo function and returns the result:

Run in nix-shell https://holochain.love

curl -X POST -H "Content-Type: application/json" -d '{"id": "0", "jsonrpc": "2.0", "method": "call", "params": {"instance_id": "test-instance", "zome": "hello", "function": "hello_holo", "args": {} }}' http://127.0.0.1:8888

And you should get back your string from the hello_holo function:

{"jsonrpc":"2.0","result":"{\"Ok\":\"Hello Holo\"}","id":"0"}

Congratulations—you have created your first distributed Holochain application! 🚀

Solution

You can check the full solution to this tutorial on here.

Key takeaways

  • A zome is compiled down to WebAssembly and run by the Holochain conductor.
  • You can call public zome functions through the conductor using HTTP.

Learn more

Was this helpful?