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containerd is an industry-standard container runtime with an emphasis on simplicity, robustness and portability. It is available as a daemon for Linux and Windows, which can manage the complete container lifecycle of its host system: image transfer and storage, container execution and supervision, low-level storage and network attachments, etc.

containerd is designed to be embedded into a larger system, rather than being used directly by developers or end-users.

![architecture](design/architecture.png)

## Getting Started

See our documentation on [containerd.io](containerd.io):
* [for ops and admins](docs/ops.md)
* [namespaces](docs/namespaces.md)
* [client options](docs/client-opts.md)

See how to build containerd from source at [BUILDING](BUILDING.md).

If you are interested in trying out containerd see our example at [Getting Started](docs/getting-started.md).


## Runtime Requirements

Runtime requirements for containerd are very minimal. Most interactions with
the Linux and Windows container feature sets are handled via [runc](https://github.com/opencontainers/runc) and/or
OS-specific libraries (e.g. [hcsshim](https://github.com/Microsoft/hcsshim) for Microsoft). The current required version of `runc` is always listed in [RUNC.md](/RUNC.md).

There are specific features
used by containerd core code and snapshotters that will require a minimum kernel
version on Linux. With the understood caveat of distro kernel versioning, a
reasonable starting point for Linux is a minimum 4.x kernel version.

The overlay filesystem snapshotter, used by default, uses features that were
finalized in the 4.x kernel series. If you choose to use btrfs, there may
be more flexibility in kernel version (minimum recommended is 3.18), but will
require the btrfs kernel module and btrfs tools to be installed on your Linux
distribution.

To use Linux checkpoint and restore features, you will need `criu` installed on
your system. See more details in [Checkpoint and Restore](#checkpoint-and-restore).

Build requirements for developers are listed in [BUILDING](BUILDING.md).

## Features

### Client

containerd offers a full client package to help you integrate containerd into your platform.

```go

import (
  "github.com/containerd/containerd"
  "github.com/containerd/containerd/cio"
)


func main() {
	client, err := containerd.New("/run/containerd/containerd.sock")
	defer client.Close()
}

```

### Namespaces

Namespaces allow multiple consumers to use the same containerd without conflicting with each other.  It has the benefit of sharing content but still having separation with containers and images.

To set a namespace for requests to the API:

```go
context = context.Background()
// create a context for docker
docker = namespaces.WithNamespace(context, "docker")

containerd, err := client.NewContainer(docker, "id")
```

To set a default namespace on the client:

```go
client, err := containerd.New(address, containerd.WithDefaultNamespace("docker"))
```

### Distribution

```go
// pull an image
image, err := client.Pull(context, "docker.io/library/redis:latest")

// push an image
err := client.Push(context, "docker.io/library/redis:latest", image.Target())
```

### Containers

In containerd, a container is a metadata object.  Resources such as an OCI runtime specification, image, root filesystem, and other metadata can be attached to a container.

```go
redis, err := client.NewContainer(context, "redis-master")
defer redis.Delete(context)
```

### OCI Runtime Specification

containerd fully supports the OCI runtime specification for running containers.  We have built in functions to help you generate runtime specifications based on images as well as custom parameters.

You can specify options when creating a container about how to modify the specification.

```go
redis, err := client.NewContainer(context, "redis-master", containerd.WithNewSpec(oci.WithImageConfig(image)))
```

### Root Filesystems

containerd allows you to use overlay or snapshot filesystems with your containers.  It comes with builtin support for overlayfs and btrfs.

```go
// pull an image and unpack it into the configured snapshotter
image, err := client.Pull(context, "docker.io/library/redis:latest", containerd.WithPullUnpack)

// allocate a new RW root filesystem for a container based on the image
redis, err := client.NewContainer(context, "redis-master",
	containerd.WithNewSnapshot("redis-rootfs", image),
	containerd.WithNewSpec(oci.WithImageConfig(image)),
)

// use a readonly filesystem with multiple containers
for i := 0; i < 10; i++ {
	id := fmt.Sprintf("id-%s", i)
	container, err := client.NewContainer(ctx, id,
		containerd.WithNewSnapshotView(id, image),
		containerd.WithNewSpec(oci.WithImageConfig(image)),
	)
}
```

### Tasks

Taking a container object and turning it into a runnable process on a system is done by creating a new `Task` from the container.  A task represents the runnable object within containerd.

```go
// create a new task
task, err := redis.NewTask(context, cio.Stdio)
defer task.Delete(context)

// the task is now running and has a pid that can be use to setup networking
// or other runtime settings outside of containerd
pid := task.Pid()

// start the redis-server process inside the container
err := task.Start(context)

// wait for the task to exit and get the exit status
status, err := task.Wait(context)
```

### Checkpoint and Restore

If you have [criu](https://criu.org/Main_Page) installed on your machine you can checkpoint and restore containers and their tasks.  This allow you to clone and/or live migrate containers to other machines.

```go
// checkpoint the task then push it to a registry
checkpoint, err := task.Checkpoint(context, containerd.WithExit)

err := client.Push(context, "myregistry/checkpoints/redis:master", checkpoint)

// on a new machine pull the checkpoint and restore the redis container
image, err := client.Pull(context, "myregistry/checkpoints/redis:master")

checkpoint := image.Target()

redis, err = client.NewContainer(context, "redis-master", containerd.WithCheckpoint(checkpoint, "redis-rootfs"))
defer container.Delete(context)

task, err = redis.NewTask(context, cio.Stdio, containerd.WithTaskCheckpoint(checkpoint))
defer task.Delete(context)

err := task.Start(context)
```

### Releases and API Stability

Please see [RELEASES.md](RELEASES.md) for details on versioning and stability
of containerd components.

### Development reports.

Weekly summary on the progress and what is being worked on.
https://github.com/containerd/containerd/tree/master/reports

### Communication

For async communication and long running discussions please use issues and pull requests on the github repo.
This will be the best place to discuss design and implementation.

For sync communication we have a community slack with a #containerd channel that everyone is welcome to join and chat about development.

**Slack:** https://dockr.ly/community

### Reporting security issues

__If you are reporting a security issue, please reach out discreetly at security@containerd.io__.

## Licenses

The containerd codebase is released under the [Apache 2.0 license](LICENSE.code).
The README.md file, and files in the "docs" folder are licensed under the
Creative Commons Attribution 4.0 International License under the terms and
conditions set forth in the file "[LICENSE.docs](LICENSE.docs)". You may obtain a duplicate
copy of the same license, titled CC-BY-4.0, at http://creativecommons.org/licenses/by/4.0/.
