Defining Golem Components in Go

Creating a project

Golem's command line interface provides a set of predefined, Golem-specific examples to choose from as a starting point.

To see the available examples for Go, run:

$ golem-cli list-examples --language go

Then to create a new project based the default go example, run:

$ golem-cli new --language go --package-name 'golem:demo' go-example

The command will create a new Golem project in the go-example directory, and print short, language-specific instructions on how to build the project.

Specification-first approach

Golem and the Go toolchain currently requires defining the component's interface using the WebAssembly Interface Type (WIT) format. See the official documentation of this format (opens in a new tab) for reference.

Each new project generated with golem-cli contains a wit directory with at least one .wit file defining a world. This world can contain exports (exported functions and interfaces) and these exports will be the compiled Golem component's public API.

The first time a component is compiled (see the Building Components page for details), a couple of files get generated in a subdirectory named as your component. This go package contains the definitions of all the data types and interfaces defined in the WIT file(s).

To implement the specification written in WIT, the Go code must implement some of these generated interfaces and set them explicitly in the init function.

Exporting top-level functions

WIT allows exporting one or more top-level functions in the world section, for example:

package golem:demo;
 
world go-example {
    export hello-world: func() -> string;
}

To implement this function in Go, the following steps must be taken:

• make sure the generated golem/demo/go_example package is imported
• define an empty struct representing our component
• implement the generated GoExample interface for this struct
• call the SetGoExample function to install the implementation

Let's see in code:

package main
 
// make sure the generated package is imported
import (
	"golem/demo/go_example"
)
 
// define an empty struct representing our component
type GoExampleImpl struct {
}
 
// implement the generated `Guest` trait for this struct
func (e *GoExampleImpl) HelloWorld() string {
	return "Hello, World!"
}
 
// install the implementation
func init() {
	go_example.SetGoExample(&GoExampleImpl{})
}
 

Exporting interfaces

WIT supports defining and exporting whole interfaces, coupling together multiple functions and possibly custom data types.

Take the following example:

package golem:demo;
 
interface api {
  add: func(value: u64);
  get: func() -> u64;
}
 
world example {
  export api;
}

This is similar to having the two exported functions directly exported from the world section, but there is a corresponding Go interface generated that needs to be separately implemented and installed:

package main
 
import (
	"golem/demo/go_example"
)
 
type GoExampleImpl struct {
	// state can be stored in the implementation struct
	total uint64
}
 
// state can also be stored in global variables too
// var total uint64
 
func (e *GoExampleImpl) Add(value uint64) {
	e.total += value
	// or with global state:
	// total += value
}
 
func (e *GoExampleImpl) Get() uint64 {
	return e.total
	// or with global state:
	// return total
}
 
func init() {
	go_example.SetExportsGolemDemoApi(&GoExampleImpl{})
}

Exporting resources

The WIT format supports defining and exporting resources - entities defined by their constructor function and the available methods on them.

Golem supports exporting these resources as part of the worker's API.

The following example modifies the previously seen counter example to define it as a resource, getting the counter's name as a constructor parameter:

package golem:demo;
 
interface api {
  resource counter {
    constructor(name: string);
    add: func(value: u64);
    get: func() -> u64;
  }
}
 
world example {
  export api;
}

Resources can have multiple instances within a worker. Their constructor returns a handle which is then used to call the methods on the resource. Learn more about how resources can be implicitly created and invoked through Golem's APIs in the Invocations page.

To implement the above defined WIT resource in Go a few new steps must be taken:

• define a struct representing the resource - it can contain data!
• implement the interface generated as the resource's interface for this struct
• implement the constructor on the component struct

Let's see in code:

package main
 
import (
	"golem/demo/go_example"
)
 
// struct representing the component
type GoExampleImpl struct {
}
 
// struct representing an instance of the resource
type Counter struct {
	name    string
	current uint64
}
 
func (e *GoExampleImpl) ConstructorCounter(name string) go_example.ExportsGolemDemoApiCounter {
	return &Counter{
		name:    name,
		current: 0,
	}
}
 
func (e *Counter) MethodCounterAdd(value uint64) {
	e.current += value
}
 
func (e *Counter) MethodCounterGet() uint64 {
	return e.current
}
 
func init() {
	go_example.SetExportsGolemDemoApi(&GoExampleImpl{})
}

Data types defined in WIT

The WIT specifications contains some primitive and higher level data types and also allows defining custom data types which can be used as function parameters and return values on the exported functions, interfaces and resources.

The following table shows an example of each WIT data type and its corresponding Go type:

WIT records

The following WIT record type:

package golem:demo;
 
interface api {
    record user {
        id: u64,
        name: string,
    }
}

Will generate the following Go struct:

type ExportsGolemDemoApiUser struct {
	Id uint64,
	Name string
}

WIT variants

The following WIT variant type:

package golem:demo;
 
interface api {
    variant color {
        red,
        green,
        blue,
        rgb(u32)
    }
}

Will generate the following Go enum:

type ExportsGolemDemoApiColorKind int
 
const (
ExportsGolemDemoApiColorKindRed ExportsGolemDemoApiColorKind = iota
ExportsGolemDemoApiColorKindGreen
ExportsGolemDemoApiColorKindBlue
ExportsGolemDemoApiColorKindRgb
)
 
type ExportsGolemDemoApiColor struct {
  kind ExportsGolemDemoApiColorKind
  val any
}

as well as a set of helper functions.

WIT enums

The following WIT enum type:

package golem:demo;
 
interface api {
    enum color {
        red,
        green,
        blue
    }
}

Will generate the following Go enum:

type ExportsGolemDemoApiColorKind int
 
const (
ExportsGolemDemoApiColorKindRed ExportsGolemDemoApiColorKind = iota
ExportsGolemDemoApiColorKindGreen
ExportsGolemDemoApiColorKindBlue
)
 
type ExportsGolemDemoApiColor struct {
  kind ExportsGolemDemoApiColorKind
}

and a set of helper functions.

WIT flags

The following WIT flags type:

package golem:demo;
 
interface api {
    flags access {
        read,
        write,
        lst
    }
}

Will generate the following Go bitflags:

type ExportsGolemDemoApiAccess uint64
const (
ExportsGolemDemoApiAccess_Read ExportsGolemDemoApiAccess = 1 << iota
ExportsGolemDemoApiAccess_Write
ExportsGolemDemoApiAccess_Lst
)

Worker configuration

It is often required to pass configuration values to workers when they are started.

In general Golem supports three different ways of doing this:

1. Defining a list of string arguments passed to the worker, available as command line arguments
2. Defining a list of key-value pairs passed to the worker, available as environment variables.
3. Using resource constructors to pass configuration values to the worker.

Command line arguments and environment variables

First, make sure the Golem Go SDK is set up for the project.

Then you can use the golem_go_bindings package to initialize the standard library's environment at start of the component's methods:

package main
 
import (
	"fmt"
	"os"
 
	// import the std helper
	"github.com/golemcloud/golem-go/std"
 
	"golem/demo/go_example"
)
 
type GoExampleImpl struct {
}
 
func (e *GoExampleImpl) HelloWorld() string {
    // initialize the std os package from the golem environment
	// this initialization has to be called at the start of the implementation methods
	std.Init(std.Packages{Os: true})
 
    // access command line arguments and environment variables through the standard packages
	fmt.Printf("args: %+v", os.Args)
	fmt.Printf("env: %+v", os.Environ())
 
	return "Hello, World!"
}
 
func init() {
	go_example.SetGoExample(&GoExampleImpl{})
}

Environment variables can be specified when a worker is explicitly created, but there are some environment variables that are always set by Golem:

• GOLEM_WORKER_NAME - the name of the worker
• GOLEM_COMPONENT_ID - the ID of the worker's component
• GOLEM_COMPONENT_VERSION - the version of the component used for this worker

In addition to these, when using Worker to Worker communication, workers created by remote calls inherit the environment variables of the caller.

This feature makes environment variables a good fit for passing configuration such as hostnames, ports, or access tokens to trees of workers.

Resource constructors

As explained earlier, Golem workers can export resources and these resources can have constructor parameters.

Although resources can be used in many ways, one pattern for Golem is only create a single instance of the exported resource in each worker, and use it to pass configuration values to the worker. This is supported by Golem's worker invocation syntax directly, allowing to implicitly create workers and the corresponding resource by a single invocation as described on the Invocations page.

Managing state

Golem workers are stateful. There are three major techniques to store and manipulate state in a Golem worker implemented in Go:

1. Using a global variable
2. Using fields in the implementation struct
3. Using resources and storing state in the resource's struct

All techniques were demonstrated above in the code examples.