Quickstart: Create a Go or Rust function in Azure using Visual Studio Code

Article
07/26/2024

In this article, you use Visual Studio Code to create a custom handler function that responds to HTTP requests. After testing the code locally, you deploy it to the serverless environment of Azure Functions.

Custom handlers can be used to create functions in any language or runtime by running an HTTP server process. This article supports both Go and Rust.

Completing this quickstart incurs a small cost of a few USD cents or less in your Azure account.

Configure your environment

Before you get started, make sure you have the following requirements in place:

Go
Rust

An Azure account with an active subscription. Create an account.
Visual Studio Code on one of the supported platforms.
The Azure Functions extension for Visual Studio Code.
Go, latest version recommended. Use the go version command to check your version.

Install or update Core Tools

The Azure Functions extension for Visual Studio Code integrates with Azure Functions Core Tools so that you can run and debug your functions locally in Visual Studio Code using the Azure Functions runtime. Before getting stated, it's a good idea to install Core Tools locally or update an existing installation to use the latest version.

In Visual Studio Code, select F1 to open the command palette, and then search for and run the command Azure Functions: Install or Update Core Tools.

This command starts a package-based installation of the latest version of Core Tools.

Create your local project

In this section, you use Visual Studio Code to create a local Azure Functions custom handlers project. Later in this article, you'll publish your function code to Azure.

In Visual Studio Code, press F1 to open the command palette and search for and run the command Azure Functions: Create New Project....
Choose the directory location for your project workspace and choose Select. You should either create a new folder or choose an empty folder for the project workspace. Don't choose a project folder that is already part of a workspace.

Provide the following information at the prompts:

Prompt	Selection
Select a language for your function project	Choose `Custom Handler`.
Select a template for your project's first function	Choose `HTTP trigger`.
Provide a function name	Type `HttpExample`.
Authorization level	Choose `Anonymous`, which enables anyone to call your function endpoint. For more information, see Authorization level.
Select how you would like to open your project	Choose `Open in current window`.

Using this information, Visual Studio Code generates an Azure Functions project with an HTTP trigger. You can view the local project files in the Explorer.

Create and build your function

The function.json file in the HttpExample folder declares an HTTP trigger function. You complete the function by adding a handler and compiling it into an executable.

Go
Rust

Press Ctrl + N (Cmd + N on macOS) to create a new file. Save it as handler.go in the function app root (in the same folder as host.json).

In handler.go, add the following code and save the file. This is your Go custom handler.

package main

import (
    "fmt"
    "log"
    "net/http"
    "os"
)

func helloHandler(w http.ResponseWriter, r *http.Request) {
    message := "This HTTP triggered function executed successfully. Pass a name in the query string for a personalized response.\n"
    name := r.URL.Query().Get("name")
    if name != "" {
        message = fmt.Sprintf("Hello, %s. This HTTP triggered function executed successfully.\n", name)
    }
    fmt.Fprint(w, message)
}

func main() {
    listenAddr := ":8080"
    if val, ok := os.LookupEnv("FUNCTIONS_CUSTOMHANDLER_PORT"); ok {
        listenAddr = ":" + val
    }
    http.HandleFunc("/api/HttpExample", helloHandler)
    log.Printf("About to listen on %s. Go to https://127.0.0.1%s/", listenAddr, listenAddr)
    log.Fatal(http.ListenAndServe(listenAddr, nil))
}

Press Ctrl + Shift + ` or select New Terminal from the Terminal menu to open a new integrated terminal in VS Code.
Compile your custom handler using the following command. An executable file named handler (handler.exe on Windows) is output in the function app root folder.
```
go build handler.go
```

Press Ctrl + Shift + ` or select New Terminal from the Terminal menu to open a new integrated terminal in VS Code.
In the function app root (the same folder as host.json), initialize a Rust project named handler.
```
cargo init --name handler
```
In Cargo.toml, add the following dependencies necessary to complete this quickstart. The example uses the warp web server framework.
```
[dependencies]
warp = "0.3"
tokio = { version = "1", features = ["rt", "macros", "rt-multi-thread"] }
```

In src/main.rs, add the following code and save the file. This is your Rust custom handler.

use std::collections::HashMap;
use std::env;
use std::net::Ipv4Addr;
use warp::{http::Response, Filter};

#[tokio::main]
async fn main() {
    let example1 = warp::get()
        .and(warp::path("api"))
        .and(warp::path("HttpExample"))
        .and(warp::query::<HashMap<String, String>>())
        .map(|p: HashMap<String, String>| match p.get("name") {
            Some(name) => Response::builder().body(format!("Hello, {}. This HTTP triggered function executed successfully.", name)),
            None => Response::builder().body(String::from("This HTTP triggered function executed successfully. Pass a name in the query string for a personalized response.")),
        });

    let port_key = "FUNCTIONS_CUSTOMHANDLER_PORT";
    let port: u16 = match env::var(port_key) {
        Ok(val) => val.parse().expect("Custom Handler port is not a number!"),
        Err(_) => 3000,
    };

    warp::serve(example1).run((Ipv4Addr::LOCALHOST, port)).await
}

Compile a binary for your custom handler. An executable file named handler (handler.exe on Windows) is output in the function app root folder.
```
cargo build --release
cp target/release/handler .
```

Configure your function app

The function host needs to be configured to run your custom handler binary when it starts.

Open host.json.
In the customHandler.description section, set the value of defaultExecutablePath to handler (on Windows, set it to handler.exe).
In the customHandler section, add a property named enableForwardingHttpRequest and set its value to true. For functions consisting of only an HTTP trigger, this setting simplifies programming by allow you to work with a typical HTTP request instead of the custom handler request payload.

Confirm the customHandler section looks like this example. Save the file.

"customHandler": {
  "description": {
    "defaultExecutablePath": "handler",
    "workingDirectory": "",
    "arguments": []
  },
  "enableForwardingHttpRequest": true
}

The function app is configured to start your custom handler executable.

Run the function locally

You can run this project on your local development computer before you publish to Azure.

In the integrated terminal, start the function app using Azure Functions Core Tools.
```
func start
```
With Core Tools running, navigate to the following URL to execute a GET request, which includes ?name=Functions query string.

http://localhost:7071/api/HttpExample?name=Functions
A response is returned, which looks like the following in a browser:
Information about the request is shown in Terminal panel.
Press Ctrl + C to stop Core Tools.

After you've verified that the function runs correctly on your local computer, it's time to use Visual Studio Code to publish the project directly to Azure.

Before you can publish your app, you must sign in to Azure.

If you aren't already signed in, choose the Azure icon in the Activity bar. Then in the Resources area, choose Sign in to Azure....

If you're already signed in and can see your existing subscriptions, go to the next section. If you don't yet have an Azure account, choose Create an Azure Account.... Students can choose Create an Azure for Students Account....
When prompted in the browser, choose your Azure account and sign in using your Azure account credentials. If you create a new account, you can sign in after your account is created.
After you've successfully signed in, you can close the new browser window. The subscriptions that belong to your Azure account are displayed in the sidebar.

Compile the custom handler for Azure

In this section, you publish your project to Azure in a function app running Linux. In most cases, you must recompile your binary and adjust your configuration to match the target platform before publishing it to Azure.

Go
Rust

In the integrated terminal, compile the handler to Linux/x64. A binary named handler is created in the function app root.
- macOS
- Linux
- Windows
```
GOOS=linux GOARCH=amd64 go build handler.go
```
```
GOOS=linux GOARCH=amd64 go build handler.go
```
```
set GOOS=linux
set GOARCH=amd64
go build handler.go
```
Change the defaultExecutablePath in host.json from handler.exe to handler. This instructs the function app to run the Linux binary.

Create a file at .cargo/config. Add the following contents and save the file.
```
[target.x86_64-unknown-linux-musl]
linker = "rust-lld"
```

In the integrated terminal, compile the handler to Linux/x64. A binary named handler is created. Copy it to the function app root.

rustup target add x86_64-unknown-linux-musl
cargo build --release --target=x86_64-unknown-linux-musl
cp target/x86_64-unknown-linux-musl/release/handler .

If you are using Windows, change the defaultExecutablePath in host.json from handler.exe to handler. This instructs the function app to run the Linux binary.
Add the following line to the .funcignore file:
```
target
```
This prevents publishing the contents of the target folder.

Create the function app in Azure

In this section, you create a function app and related resources in your Azure subscription.

Choose the Azure icon in the Activity bar. Then in the Resources area, select the + icon and choose the Create Function App in Azure option.

Provide the following information at the prompts:

Prompt	Selection
Select subscription	Choose the subscription to use. You won't see this when you have only one subscription visible under Resources.
Enter a globally unique name for the function app	Type a name that is valid in a URL path. The name you type is validated to make sure that it's unique in Azure Functions.
Select a runtime stack	Choose Custom Handler.
Select a location for new resources	For better performance, choose a region near you.

The extension shows the status of individual resources as they are being created in Azure in the Azure: Activity Log panel.

Log of Azure resource creation

When the creation is complete, the following Azure resources are created in your subscription. The resources are named based on your function app name:
- A resource group, which is a logical container for related resources.
- A standard Azure Storage account, which maintains state and other information about your projects.
- A function app, which provides the environment for executing your function code. A function app lets you group functions as a logical unit for easier management, deployment, and sharing of resources within the same hosting plan.
- An App Service plan, which defines the underlying host for your function app.
- An Application Insights instance connected to the function app, which tracks usage of your functions in the app.
A notification is displayed after your function app is created and the deployment package is applied.

Tip

By default, the Azure resources required by your function app are created based on the function app name you provide. By default, they're also created in the same new resource group with the function app. If you want to either customize the names of these resources or reuse existing resources, you need to publish the project with advanced create options instead.

Deploy the project to Azure

Important

Deploying to an existing function app always overwrites the contents of that app in Azure.

Choose the Azure icon in the Activity bar, then in the Workspace area, select your project folder and select the Deploy... button.
Select Deploy to Function App..., choose the function app you just created, and select Deploy.
After deployment completes, select View Output to view the creation and deployment results, including the Azure resources that you created. If you miss the notification, select the bell icon in the lower right corner to see it again.

Run the function in Azure

Back in the Resources area in the side bar, expand your subscription, your new function app, and Functions. Right-click (Windows) or Ctrl - click (macOS) the HttpExample function and choose Execute Function Now....
In Enter request body you see the request message body value of { "name": "Azure" }. Press Enter to send this request message to your function.
When the function executes in Azure and returns a response, a notification is raised in Visual Studio Code.

Clean up resources

When you continue to the next step and add an Azure Storage queue binding to your function, you'll need to keep all your resources in place to build on what you've already done.

Otherwise, you can use the following steps to delete the function app and its related resources to avoid incurring any further costs.

In Visual Studio Code, press F1 to open the command palette. In the command palette, search for and select Azure: Open in portal.
Choose your function app and press Enter. The function app page opens in the Azure portal.
In the Overview tab, select the named link next to Resource group.
On the Resource group page, review the list of included resources, and verify that they're the ones you want to delete.
Select Delete resource group, and follow the instructions.

Deletion may take a couple of minutes. When it's done, a notification appears for a few seconds. You can also select the bell icon at the top of the page to view the notification.

For more information about Functions costs, see Estimating Consumption plan costs.

Next steps

Learn about Azure Functions custom handlers