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Write a Template from Scratch

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This guided tour introduces you to the different parts of a Coder template by showing you how to create a template from scratch.

You'll write a simple template that provisions a workspace as a Docker container with Ubuntu.

Before you start

To follow this guide, you'll need:

A computer or cloud computing instance with both Docker and Coder installed on it.

When setting up your computer or computing instance, make sure to install Docker first, then Coder. Otherwise, you'll need to add the coder user to the docker group.

The URL for your Coder instance. If you're running Coder locally, the default URL is http://127.0.0.1:3000.
A text editor. For this tour, we use GNU nano.

Haven't written Terraform before? Check out Hashicorp's Getting Started Guides.

What's in a template

The main part of a Coder template is a Terraform tf file. A Coder template often has other files to configure the other resources that the template needs. In this tour you'll also create a Dockerfile.

Coder can provision all Terraform modules, resources, and properties. The Coder server essentially runs a terraform apply every time a workspace is created, started, or stopped.

Here's a simplified diagram that shows the main parts of the template we'll create.

Template architecture

1. Create template files

On your local computer, create a directory for your template and create the Dockerfile.

mkdir template-tour
cd template-tour
mkdir build
nano build/Dockerfile

You'll enter a simple Dockerfile that starts with the official Ubuntu image. In the editor, enter and save the following text in Dockerfile then exit the editor:

FROM ubuntu

RUN apt-get update \
	&& apt-get install -y \
	sudo \
	curl \
	&& rm -rf /var/lib/apt/lists/*

ARG USER=coder
RUN useradd --groups sudo --no-create-home --shell /bin/bash ${USER} \
	&& echo "${USER} ALL=(ALL) NOPASSWD:ALL" >/etc/sudoers.d/${USER} \
	&& chmod 0440 /etc/sudoers.d/${USER}
USER ${USER}
WORKDIR /home/${USER}

Notice how Dockerfile adds a few things to the parent ubuntu image, which your template needs later:

It installs the sudo and curl packages.
It adds a coder user, including a home directory.

2. Set up template providers

Now you can edit the Terraform file, which provisions the workspace's resources.

nano main.tf

We'll start by setting up our providers. At a minimum, we need the coder provider. For this template, we also need the docker provider:

terraform {
  required_providers {
    coder = {
      source  = "coder/coder"
      version = "~> 0.8.3"
    }
    docker = {
      source  = "kreuzwerker/docker"
      version = "~> 3.0.1"
    }
  }
}

provider "coder" {
}

provider "docker" {
}

locals {
  username = data.coder_workspace.me.owner
}

data "coder_provisioner" "me" {
}

data "coder_workspace" "me" {
}

Notice that the provider blocks for coder and docker are empty. In a more practical template, you would add arguments to these blocks to configure the providers, if needed.

The coder_workspace data source provides details about the state of a workspace, such as its name, owner, and so on. The data source also lets us know when a workspace is being started or stopped. We'll take advantage of this information in later steps to do these things:

Set some environment variables based on the workspace owner.
Manage ephemeral and persistent storage.

3. coder_agent

All templates need to create and run a Coder agent. This lets developers connect to their workspaces. The coder_agent resource runs inside the compute aspect of your workspace, typically a VM or container. In our case, it will run in Docker.

You do not need to have any open ports on the compute aspect, but the agent needs curl access to the Coder server. Remember that we installed curl in Dockerfile, earlier.

This snippet creates the agent:

resource "coder_agent" "main" {
  arch                   = data.coder_provisioner.me.arch
  os                     = "linux"
  startup_script_timeout = 180
  startup_script         = <<-EOT
    set -e

    # install and start code-server
    curl -fsSL https://code-server.dev/install.sh | sh -s -- --method=standalone --prefix=/tmp/code-server --version 4.11.0
    /tmp/code-server/bin/code-server --auth none --port 13337 >/tmp/code-server.log 2>&1 &
  EOT

  env = {
    GIT_AUTHOR_NAME = "${data.coder_workspace.me.owner}"
    GIT_COMMITTER_NAME = "${data.coder_workspace.me.owner}"
    GIT_AUTHOR_EMAIL = "${data.coder_workspace.me.owner_email}"
    GIT_COMMITTER_EMAIL = "${data.coder_workspace.me.owner_email}"
  }

  metadata {
    display_name = "CPU Usage"
    key          = "0_cpu_usage"
    script       = "coder stat cpu"
    interval     = 10
    timeout      = 1
  }

  metadata {
    display_name = "RAM Usage"
    key          = "1_ram_usage"
    script       = "coder stat mem"
    interval     = 10
    timeout      = 1
  }
}

Because Docker is running locally in the Coder server, there is no need to authenticate coder_agent. But if your coder_agent were running on a remote host, your template would need authentication credentials.

This template's agent also runs a startup script, sets environment variables, and provides metadata.

The startup script installs code-server, a browser-based VS Code app that runs in the workspace. We'll give users access to code-server through coder_app, later.

The env block sets environments variables for the workspace. We use the data source from coder_workspace to set the environment variables based on the workspace's owner. This way, the owner can make git commits immediately without any manual configuration.

Your template can use metadata to show information to the workspace owner. Coder displays this metadata in the Coder dashboard. Our template has metadata blocks for CPU and RAM usage.

4. coder_app

A coder_app resource lets a developer use an app from the workspace's Coder dashboard.

Apps in a Coder workspace

This is commonly used for web IDEs such as code-server, RStudio, and JupyterLab.

To install and code-server in the workspace, remember that we installed it in the startup_script argument in coder_agent. We make it available from a workspace with a coder_app resource. See web IDEs for more examples.

resource "coder_app" "code-server" {
  agent_id     = coder_agent.main.id
  slug         = "code-server"
  display_name = "code-server"
  url          = "http://localhost:13337/?folder=/home/${local.username}"
  icon         = "/icon/code.svg"
  subdomain    = false
  share        = "owner"

  healthcheck {
    url       = "http://localhost:13337/healthz"
    interval  = 5
    threshold = 6
  }
}

You can also use a coder_app resource to link to external apps, such as links to wikis or cloud consoles.

resource "coder_app" "coder-server-doc" {
  agent_id     = coder_agent.main.id
  icon         = "/emojis/1f4dd.png"
  slug         = "getting-started"
  url          = "https://coder.com/docs/code-server"
  external     = true
}

5. Persistent and ephemeral resources

Managing the lifecycle of template resources is important. We want to make sure that workspaces use computing, storage, and other services efficiently.

We want our workspace's home directory to persist after the workspace is stopped so that a developer can continue their work when they start the workspace again.

We do this in 2 parts:

Our docker_volume resource uses the lifecycle block with the ignore_changes = all argument to prevent accidental deletions.
To prevent Terraform from destroying persistent Docker volumes in case of a workspace name change, we use an immutable parameter, like data.coder_workspace.me.id.

You'll see later that we make sure that our Docker container is ephemeral with the Terraform count meta-argument.

resource "docker_volume" "home_volume" {
  name = "coder-${data.coder_workspace.me.id}-home"
  # Protect the volume from being deleted due to changes in attributes.
  lifecycle {
    ignore_changes = all
  }
}

For details, see Resource persistence.

6. Set up the Docker container

To set up our Docker container, our template has a docker_image resource that uses build/Dockerfile, which we created earlier.

resource "docker_image" "main" {
  name = "coder-${data.coder_workspace.me.id}"
  build {
    context = "./build"
    build_args = {
      USER = local.username
    }
  }
  triggers = {
    dir_sha1 = sha1(join("", [for f in fileset(path.module, "build/*") : filesha1(f)]))
  }
}

Our docker_container resource uses coder_workspace start_count to start and stop the Docker container:

resource "docker_container" "workspace" {
  count = data.coder_workspace.me.start_count
  image = docker_image.main.name
  # Uses lower() to avoid Docker restriction on container names.
  name = "coder-${data.coder_workspace.me.owner}-${lower(data.coder_workspace.me.name)}"
  # Hostname makes the shell more user friendly: coder@my-workspace:~$
  hostname = data.coder_workspace.me.name
  # Use the docker gateway if the access URL is 127.0.0.1
  entrypoint = ["sh", "-c", replace(coder_agent.main.init_script, "/localhost|127\\.0\\.0\\.1/", "host.docker.internal")]
  env = [
    "CODER_AGENT_TOKEN=${coder_agent.main.token}",
  ]
  host {
    host = "host.docker.internal"
    ip   = "host-gateway"
  }
  volumes {
    container_path = "/home/${local.username}"
    volume_name    = docker_volume.home_volume.name
    read_only      = false
  }
}

7. Create the template in Coder

Save main.tf and exit the editor.

Now that we've created the files for our template, we can add them to our Coder deployment.

We can do this with the Coder CLI or the Coder dashboard. For this tour, we'll use the Coder CLI.

First, you'll need to log in to your Coder deployment from the CLI. This is where you need the URL for your deployment:

$ coder login https://coder.example.com
Your browser has been opened to visit:

        https://coder.example.com/cli-auth

> Paste your token here:

In your web browser, enter your credentials:

Logging in to your Coder deployment

Copy the session token into the clipboard:

Logging in to your Coder deployment

And paste it into the CLI:

> Welcome to Coder, marc! You're authenticated.
$

Now you can add your template files to your Coder deployment:

$ pwd
/home/marc/template-tour
$ coder templates create
> Upload "."? (yes/no) yes

The Coder CLI tool gives progress information then prompts you to confirm:

> Confirm create? (yes/no) yes

The template-tour template has been created! Developers can provision a workspace with this template using:

   coder create --template="template-tour" [workspace name]

In your web browser, log in to your Coder dashboard, select Templates. Your template is ready to use for new workspaces.

Your new template, ready to use