← Containers / containers

Getting started

In this guide, you will deploy a Worker that can make requests to one or more Containers in response to end-user requests. In this example, each container runs a small webserver written in Go.

This example Worker should give you a sense for simple Container use, and provide a starting point for more complex use cases.

In this guide, we will build and push a container image alongside your Worker code. By default, this process uses Docker ↗ to do so.

You must have Docker running locally when you run wrangler deploy. For most people, the best way to install Docker is to follow the docs for installing Docker Desktop ↗. Other tools like Colima ↗ may also work.

You can check that Docker is running properly by running the docker info command in your terminal. If Docker is running, the command will succeed. If Docker is not running, the docker info command will hang or return an error including the message "Cannot connect to the Docker daemon".

Run the following command to create and deploy a new Worker with a container, from the starter template:

npm create cloudflare@latest -- --template=cloudflare/templates/containers-template

When you want to deploy a code change to either the Worker or Container code, you can run the following command using Wrangler CLI:

npx wrangler deploy

On deploy, Wrangler uploads your Worker, builds and pushes the container image with Docker, and updates container instances on Cloudflare's network. The first build and push usually take the longest. Later deploys reuse cached image layers ↗.

After deploying, list containers in your account and their status:

npx wrangler containers list

List images in the Cloudflare Registry:

npx wrangler containers images list

Open the URL for your Worker. It should look like https://hello-containers.<YOUR_WORKERS_SUBDOMAIN>.workers.dev.

• Requests to /container/1 or /container/2 route to specific containers. Each path after /container/ maps to a unique container.
• Requests to /lb load-balance across three containers chosen at random.

Read the response body to confirm which instance handled the request. If the Worker responds but container routes still error, wait for provisioning, then check Containers ↗ logs in the dashboard.

Now that you've deployed your first container, let's explain what is happening in your Worker's code, in your configuration file, in your container's code, and how requests are routed.

Your Wrangler configuration file defines the configuration for both your Worker and your container:

{
	"containers": [
		{
			"max_instances": 10,
			"class_name": "MyContainer",
			"image": "./Dockerfile",
		},
	],
	"durable_objects": {
		"bindings": [
			{
				"name": "MY_CONTAINER",
				"class_name": "MyContainer",
			},
		],
	},
	"migrations": [
		{
			"tag": "v1",
			"new_sqlite_classes": ["MyContainer"],
		},
	],
}

[[containers]]
max_instances = 10
class_name = "MyContainer"
image = "./Dockerfile"

[[durable_objects.bindings]]
name = "MY_CONTAINER"
class_name = "MyContainer"

[[migrations]]
tag = "v1"
new_sqlite_classes = [ "MyContainer" ]

Important points about this config:

• image points to a Dockerfile, to a directory containing a Dockerfile, or to a fully qualified image reference such as registry.cloudflare.com/<YOUR_ACCOUNT_ID>/<IMAGE>:<TAG>.
• class_name must be a Durable Object class name.
• max_instances declares the maximum number of simultaneously running container instances that will run.
• The Durable Object must use new_sqlite_classes not new_classes.

Your container image must be able to run on the linux/amd64 architecture, but aside from that, has few limitations.

In the example you just deployed, it is a simple Golang server that responds to requests on port 8080 using the MESSAGE environment variable that will be set in the Worker and an auto-generated environment variable CLOUDFLARE_DEPLOYMENT_ID.

func handler(w http.ResponseWriter, r *http.Request) {
	message := os.Getenv("MESSAGE")
	instanceId := os.Getenv("CLOUDFLARE_DEPLOYMENT_ID")

	fmt.Fprintf(w, "Hi, I'm a container and this is my message: %s, and my instance ID is: %s", message, instanceId)
}

First note MyContainer which extends the Container ↗ class:

export class MyContainer extends Container {
  defaultPort = 8080;
  sleepAfter = '10s';
  envVars = {
    MESSAGE: 'I was passed in via the container class!',
  };

  override onStart() {
    console.log('Container successfully started');
  }

  override onStop() {
    console.log('Container successfully shut down');
  }

  override onError(error: unknown) {
    console.log('Container error:', error);
  }
}

This defines basic configuration for the container:

• defaultPort sets the port that the fetch and containerFetch methods will use to communicate with the container. It also blocks requests until the container is listening on this port.
• sleepAfter sets the timeout for the container to sleep after it has been idle for a certain amount of time.
• envVars sets environment variables that will be passed to the container when it starts.
• onStart, onStop, and onError are hooks that run when the container starts, stops, or errors, respectively.

The Container class itself extends DurableObject, so your subclass has access to the full Durable Object API. The Durable Object handles routing, lifecycle, and persistent state, while the container process runs your image inside a Linux VM. This means you can use this.ctx.storage to persist data that survives container restarts and resides close to the container itself.

Refer to the Container class reference and the low-level Durable Object container API for more details.

When a request enters Cloudflare, your Worker's fetch handler is invoked. This is the code that handles the incoming request. The fetch handler in the example code, launches containers in two ways, on different routes:

• Making requests to /container/ passes requests to a new container for each path. This is done by spinning up a new Container instance. You may note that the first request to a new path takes longer than subsequent requests, this is because a new container is booting.

  if (pathname.startsWith("/container")) {
  	const container = env.MY_CONTAINER.getByName(pathname);
  	return await container.fetch(request);
  }

• Making requests to /lb will load balance requests across several containers. This uses a simple getRandom helper method, which picks an ID at random from a set number (in this case 3), then routes to that Container instance. You can replace this with any routing or load balancing logic you choose to implement:

  if (pathname.startsWith("/lb")) {
  	const container = await getRandom(env.MY_CONTAINER, 3);
  	return await container.fetch(request);
  }

This allows for multiple ways of using Containers:

• If you simply want to send requests to many stateless and interchangeable containers, you should load balance.
• If you have stateful services or need individually addressable containers, you should request specific Container instances.
• If you are running short-lived jobs, want fine-grained control over the container lifecycle, want to parameterize container entrypoint or env vars, or want to chain together multiple container calls, you should request specific Container instances.

The Containers Dashboard ↗ shows you helpful information about your Containers, including:

• Status and Health
• Metrics
• Logs

After launching your Worker, go to the Containers Dashboard by selecting Workers & Pages > Containers in the dashboard sidebar.

To do more:

• Modify the image by changing the Dockerfile and running wrangler deploy
• Refer to Deploy Containers for Workers Builds and rollout behavior
• Browse examples for more patterns
• Check the Frequently Asked Questions for platform behavior and limitations