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Clear Containers Architecture

Overview

This is an architectural overview of Clear Containers, based on the 3.0 release.

The Clear Containers runtime (cc-runtime) is compatible with the OCI runtime specification and therefore works seamlessly with the Docker* Engine pluggable runtime architecture. It also supports the Kubernetes* Container Runtime Interface (CRI) through the CRI-O* implementation. In other words, you can transparently select between the default Docker and CRI-O runtime (runc) and cc-runtime.

Docker and Clear Containers

cc-runtime creates a QEMU*/KVM virtual machine for each container or pod the Docker engine or Kubernetes' kubelet creates.

The container process is then spawned by an agent running as a daemon inside the virtual machine. The agent opens two virtio serial interfaces (Control and I/O) in the guest, and QEMU exposes them as serial devices on the host. cc-runtime uses the control device for sending container management commands to the agent while the I/O serial device is used to pass I/O streams (stdout, stderr, stdin) between the guest and the Docker Engine.

For any given container, both the init process and all potentially executed commands within that container, together with their related I/O streams, need to go through two virtio serial interfaces exported by QEMU. The Clear Containers proxy (cc-proxy) multiplexes and demultiplexes those commands and streams for all container virtual machines. There is only one cc-proxy instance running per Clear Containers host.

On the host, each container process's removal is handled by a reaper in the higher layers of the container stack. In the case of Docker it is handled by containerd-shim. In the case of CRI-O it is handled by conmon. For clarity, for the remainder of this document the term "container process reaper" will be used to refer to either reaper. As Clear Containers processes run inside their own virtual machines, the container process reaper can not monitor, control or reap them. cc-runtime fixes that issue by creating an additional shim process (cc-shim) between the container process reaper and cc-proxy. A cc-shim instance will both forward signals and stdin streams to the container process on the guest and pass the container stdout and stderr streams back up the stack to CRI-O or Docker via the container process reaper. cc-runtime creates a cc-shim daemon for each container and for each OCI command received to run within an already running container (example, docker exec).

The container workload, that is, the actual OCI bundle rootfs, is exported from the host to the virtual machine. In the case where a block-based graph driver is configured, virtio-blk will be used. In all other cases a 9pfs virtio mount point will be used. cc-agent uses this mount point as the root filesystem for the container processes.

Overall architecture

Hypervisor

Clear Containers use QEMU/KVM to create virtual machines where containers will run:

QEMU/KVM

Although Clear Containers can run with any recent QEMU release, Clear Containers boot time and memory footprint are significantly optimized by using a specific QEMU version called qemu-lite.

Clear Containers supports various machine types, including pc, pc-lite and q35. Clear Containers defaults to using the pc machine type. In the past pc-lite was utilized which provided the following improvements:

  • Removed many of the legacy hardware devices support so that the guest kernel does not waste time initializing devices of no use for containers.
  • Skipped the guest BIOS/firmware and jumped straight to the Clear Containers kernel.

In order to provide some advanced features which were lacking in pc-lite like hotplug support, the default was changed to pc.

In the future, Clear Containers plan to move to a 2.9 based version of QEMU, available at the Clear Containers QEMU repository. This transition has been delayed until after the release of Clear Containers 3.0 due to regressions, as described in runtime issue 407. Once support for features like hotplug are available in q35, the project will look to transition to this machine type.

Assets

Additional resources are required to create each QEMU virtual machine instance, specifically a guest kernel and a root filesystem image known as the "mini O/S".

The locations of these resources are passed to QEMU by the runtime (see Configuration).

Guest kernel

The guest kernel is passed to the QEMU hypervisor and used to boot the virtual machine. It is highly optimised for kernel boot time and minimal memory footprint, providing only those services required by a container workload.

Root filesystem image

The root filesystem image, sometimes referred to as the "mini O/S", is a highly optimised container bootstrap system based on Clear Linux. It provides an extremely minimal environment and has a highly optimised boot path.

The only services running in the context of the mini O/S are the init daemon (systemd) and the Agent. The real workload the user wishes to run executes within a confined context which uses the container manager image as its root filesystem.

For example, when docker run -ti ubuntu date is run:

  • The hypervisor will boot the mini-OS image using the guest kernel.
  • systemd, running inside the mini-OS context, will launch the cc-agent in the same context.
  • The agent will create a new confined context to run the specifed command in (date in this example).
  • The agent will then execute the command (date in this example) inside this new context, first setting the root filesystem to the expected Ubuntu* root filesystem.

Agent

cc-agent is a daemon running in the guest as a supervisor for managing containers and processes running within those containers.

The cc-agent execution unit is the pod. A cc-agent pod is a container sandbox defined by a set of namespaces (NS, UTS, IPC and PID). cc-runtime can run several containers per pod to support container engines that require multiple containers running inside a single VM. In the case of docker, cc-runtime creates a single container per pod.

cc-agent uses a communication protocol defined by the hyperstart project. This was chosen to maintain backward compatibility with the hyperstart agent used in the 2.x Clear Containers architecture.

The cc-agent interface consists of:

  • A control serial channel over which the cc-agent sends and receives specific commands for controlling and managing pods and containers. Detailed information about the commands can be found at cc-agent API.
  • An I/O serial channel for passing the container processes output streams (stdout, stderr) back to cc-proxy and receiving the input stream (stdin) for them. As all streams for all containers are going through one single serial channel, the cc-agent prepends them with container specific sequence numbers. There are at most two sequence numbers per container process: one for stdout and stdin, and another one for stderr.

cc-agent supports the following commands:

  • StartPodCmd: Sets up a pod in a newly created VM.
  • NewContainerCmd: Creates a new container within a pod. This needs to be sent after the StartPodCmd has been issued for starting a pod. This command also starts the container process.
  • ExecCmd: Executes a new process within an already running container.
  • KillContainerCmd: cc-shim uses this to send signals to a container process.
  • WinsizeCmd: cc-shim uses this to change the terminal size of the terminal associated with a container.
  • RemoveContainerCmd: Removes a container from the pod. This command will fail if the container is in a running state.
  • Destroypod: Removes all containers within a pod . All containers need to be in a stopped state for this command to succeed. The command also frees resources associated with the pod.

Each control message is composed of a command code and a payload required for the command:

  ┌────────────────┬────────────────┬──────────────────────────────┐
  │  Command Code  │     Length     │ Payload(request or response) │
  │   (32 bits)    │    (32 bits)   │     (data length bytes)      │
  └────────────────┴────────────────┴──────────────────────────────┘
  • Command Code is the predefined id associated with a command.
  • Length is the size of the entire message in bytes and encoded in network order.
  • Payload is the JSON-encoded command request or response data.

Each stream message is composed of a stream sequence code and a payload containing the stream data.

  ┌────────────────┬────────────────┬──────────────────────────────┐
  │  Sequence Code │     Length     │          Payload             │
  │   (64 bits)    │    (64 bits)   │     (data length bytes)      │
  └────────────────┴────────────────┴──────────────────────────────┘
  • Sequence code is the 64 bit sequence number assigned by cc-agent for a stream.
  • Length is the size of the entire stream message in bytes and encoded in network order.
  • Payload is the stream data.

The cc-agent makes use of libcontainer to manage the lifecycle of the container. This way the cc-agent reuses most of the code used by runc.

Runtime

cc-runtime is an OCI compatible container runtime and is responsible for handling all commands specified by the OCI runtime specification and launching cc-shim instances.

cc-runtime heavily utilizes the virtcontainers project, which provides a generic, runtime-specification agnostic, hardware-virtualized containers library.

Configuration

The runtime uses a TOML format configuration file called configuration.toml. By default this file is installed in the /usr/share/defaults/clear-containers directory and contains various settings such as the paths to the hypervisor, the guest kernel and the mini-OS image.

Most users will not need to modify the configuration file.

The file is well commented and provides a few "knobs" that can be used to modify the behavior of the runtime.

The configuration file is also used to enable runtime debug output (see https://github.com/clearcontainers/runtime#debugging).

Significant OCI commands

Here we will describe how cc-runtime handles the most important OCI commands.

create

When handling the OCI create command, cc-runtime goes through the following steps:

  1. Create the networking container namespace on the host, according to the container OCI configuration file. We only support networking namespaces for now, but will support other namespaces later.
  2. Run all the prestart OCI hooks in the host namespaces created in step 1, as described by the OCI container configuration file.
  3. Set up the container networking namespace. This is when all networking interfaces are created and setup inside the namespace, according to the selected pod networking model (CNM for Docker or CNI for Kubernetes).
  4. Create and start the virtual machine that will run the container process. The VM will run inside the host namespaces created during step 1, and its systemd instance will spawn the cc-agent daemon.
  5. Register the virtual machine with cc-proxy.
  6. The cc-proxy waits for the agent to signal that it is ready and then returns a token. This token uniquely identifies a process within a container inside the virtual machine.
  7. Spawn the cc-shim process providing two arguments: cc-shim --token $(token) --uri $(uri)
    • The proxy URI, which can be either a UNIX or a TCP socket.
    • The token for the container process it needs to monitor.
  8. The cc-shim connects to the proxy and signals which container process it is going to monitor by passing its token through the cc-proxy connectShim command.

Docker create

At this point, the virtual machine that will run the containers workloads is up and running, and the cc-agent is ready to process container life cycle commands. The pod inside the virtual machine is not created, and the containers are not running yet. This will be done through the OCI start command

start

With namespace containers, start launches a traditional Linux container process in its own set of namespaces. With Clear Containers, the main task of cc-runtime is to ask the cc-agent to create a pod inside the virtual machine and then start all containers within this pod. In practice, this means cc-runtime will run through the following steps:

  1. cc-runtime connects to cc-proxy and sends it the attach command to let it know which VM we want to use to create a pod and start the new container.
  2. cc-runtime sends an agent STARTPOD command via cc-proxy. This creates the container pod.
  3. cc-runtime sends an agent NEWCONTAINER command via cc-proxy in order to create and start a new container in a given pod. The command is sent to cc-proxy who forwards it to the right agent instance running in the appropriate guest.

Docker start

exec

OCI exec allows you to run an additional command within an already running container. With Clear Containers, this translates into sending a EXECCMD command to the agent so that it executes a command into a running container belonging to a certain pod. All I/O streams from the executed command will be passed back to containerd or conmon through a newly created cc-shim.

The exec code path is partly similar to the create one and cc-runtime goes through the following steps:

  1. cc-runtime connects to cc-proxy and sends it the attach command to let it know which VM to run the exec command.
  2. cc-runtime receives a token from cc-proxy based on this connection.
  3. cc-runtime spawns a cc-shim process providing two arguments: cc-shim --token $(token) --uri $(uri)
    • The proxy URI, which can be either a UNIX or a TCP socket.
    • The token for the container process it needs to monitor. The cc-shim process will forward the output streams (stderr and stdout) to either containerd-shim in the Docker use cases or conmon for the Kubernetes deployements.
  4. cc-runtime sends an agent EXECMD command to start the command in the right container. The command is sent to cc-proxy who forwards it to the right agent instance running in the appropriate guest.

Now the exec'ed process is running in the virtual machine, sharing the UTS, PID, mount and IPC namespaces with the container's init process.

kill

When sending the OCI kill command, container runtimes should send a UNIX signal to the container process. In the Clear Containers context, this means cc-runtime needs to handle 2 resources:

  1. It needs to send the signal to the container running on the VM.
  2. It needs to signal the shim since we don't want to leave dangling cc-shim instances on the host in case the container process terminates inside the VM.

To do so, cc-runtime goes through the following steps:

  1. cc-runtime connects to cc-proxy and sends it the attach command to let it know on which VM the container it is trying to kill is running.
  2. cc-runtime sends an agent KILLCONTAINER command to kill the container running on the guest. The command is sent to cc-proxy who forwards it to the right agent instance running in the appropriate guest.

After step #2, if the container process terminates in the VM, cc-proxy will forward the process exit code to the shim. The shim will then disconnect from cc-proxy and terminates as well.

One corner case cc-runtime also needs to handle is when it receives a signal for a container that's not yet running. In that case, for SIGTERM or SIGKILL signals, cc-runtime will kill the cc-shim. All other signals are ignored.

delete

delete is about deleting all resources held by a stopped/killed container. Running containers can not be deleted unless the OCI runtime is explictly being asked to. In that case it will first kill the container and only then delete it.

The delete code path differs significantly between having to delete one container inside a pod (as is typical in Docker) and having to delete an entire pod (such as from Kubernetes). In the former case, cc-runtime will only send a SIGKILL signal to the container process. In the latter case, all components are deleted: the pod, its containers, all cc-shim instances and the virtual machine itself.

Below we will focus on describing the entire pod deletion process and we will look at the generic, multi-containers pod setup. Docker is just a particular case for single container pods.

  1. cc-runtime connects to cc-proxy and sends it the attach command to let it know on which pod the container it is trying to to delete is running.
  2. cc-runtime terminates all container processes within a pod by sending the KILLCONTAINER command to all of them.
  3. As each container process termination will be forwarded to all associated cc-shim instances, cc-runtime waits for all of them to terminate as well.
  4. cc-runtime sends the UnregisterVM command to cc-proxy, to let it know that the virtual machine that used to host the pod should no longer be used.
  5. cc-runtime explicitly shuts the virtual machine down.
  6. The host namespaces are cleaned up and destroyed. In particular, cc-runtime offloads the networking namespace cleanup path by calling into the specific networking model (CNM or CNI) removal method.
  7. All remaining pod related resources on the host are deleted.

Proxy

cc-proxy is a daemon offering access to the VM cc-agent to multiple cc-shim and cc-runtime clients. Only a single instance of cc-proxy per host is necessary as it can be used for several different VMs. Its main role is to:

  • Arbitrate access to the cc-agent control channel between all the cc-runtime instances and the cc-shim ones.
  • Route the I/O streams and signals between the various cc-shim instances and the cc-agent.

The cc-proxy API is available through a single socket for all cc-shim and cc-runtime instances to connect to. cc-proxy can be configured to use a UNIX or a TCP socket, and by default will handle connection over a UNIX socket.

The protocol on the cc-proxy socket supports the following commands:

  • RegisterVM: Used by cc-runtime to let cc-proxy know about a newly created virtual machine. When registering a new VM, cc-runtime will ask cc-proxy for a token and pass that token to the cc-shim instance it will eventually create for handling the container process inside that VM.
  • UnregisterVM: Does the opposite of what RegisterVM does, indicating to the proxy it should release resources created by RegisterVM.
  • AttachVM: It can be used to associate clients to an already known VM. Optionally AttachVM senders can ask cc-proxy for a token. In the OCI exec command case, this token will be used by the cc-shim instance that monitors the executed process inside the container.
  • Hyper: This payload will forward a hyperstart command to the cc-agent.
  • ConnectShim: Used by cc-shim instances to connect to cc-proxy and let it know which container process it wants to monitor. cc-shim sends the token they've been given by cc-runtime as part of the ConnectShim payload. With that information, cc-proxy builds its I/O and signal multiplexing routes between the VM process containers and the host cc-shim instances.
  • DisconnectShim: Used by cc-shim instances to disconnect themselves from cc-proxy and let it know that they stop monitoring their container process.

For more details about cc-proxy's protocol, theory of operations or debugging tips, please read the cc-proxy README or the proxy api godoc.

Shim

A container process reaper, such as Docker's containerd-shim or CRI-O's conmon, is designed around the assumption that it can monitor and reap the actual container process. As the container process reaper runs on the host, it cannot directly monitor a process running within a virtual machine. At most it can see the QEMU process, but that is not enough. With Clear Containers, cc-shim acts as the container process that the container process reaper can monitor. Therefore cc-shim needs to handle all container I/O streams (stdout, stdin and stderr) and forward all signals the container process reaper decides to send to the container process.

cc-shim has an implicit knowledge about which VM agent will handle those streams and signals and thus acts as an encapsulation layer between the container process reaper and the cc-agent. cc-shim:

  • Connects to cc-proxy using a token obtained by calling the cc-proxy ConnectShim command. The token is passed from cc-runtime to cc-shim when the former spawns the latter and is used to identify the true container process that the shim process will be shadowing or representing.
  • Fragments and encapsulates the standard input stream from the container process reaper into cc-proxy stream frames:
                  1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
┌───────────────────────────┬───────────────┬───────────────┐
│          Version          │ Header Length │   Reserved    │
├───────────────────────────┼─────┬─┬───────┼───────────────┤
│          Reserved         │ Res.│E│  0x2  │    Opcode     │
├───────────────────────────┴─────┴─┴───────┴───────────────┤
│                      Payload Length                       │
├───────────────────────────────────────────────────────────┤
│                                                           │
│                         Payload                           │
│                                                           │
│      (variable length, optional and opcode-specific)      │
│                                                           │
└───────────────────────────────────────────────────────────┘
  • De-encapsulates and assembles standard output and error cc-proxy stream frames into an output stream that it forwards to the container process reaper.
  • Translates all UNIX signals (except SIGKILL and SIGSTOP) into cc-proxy Signal frames that it sends to the container via cc-proxy.

As an example, assuming that running the pwd command from a containers standard input will generate the output "/tmp" on the containers standard output. If the cc-agent assigned to this process uses sequence number 8888 for stdin and stdout and 8889 for stderr, with cc-shim and Clear Containers, this example would look like:

cc-shim

Networking

Containers will typically live in their own, possibly shared, networking namespace. At some point in a container lifecycle, container engines will set up that namespace to add the container to a network which is isolated from the host network, but which is shared between containers

In order to do so, container engines will usually add one end of a virtual ethernet (veth) pair into the container networking namespace. The other end of the veth pair is added to the container network.

This is a very namespace-centric approach as QEMU can not handle veth interfaces. Instead it typically creates TAP interfaces for adding connectivity to a virtual machine.

To overcome that incompatibility between typical container engines expectations and virtual machines, cc-runtime networking transparently bridges veth interfaces with TAP ones:

Clear Containers networking

Clear Containers supports both CNM and CNI for networking management.

CNM

High-level CNM Diagram

CNM lifecycle

  1. RequestPool

  2. CreateNetwork

  3. RequestAddress

  4. CreateEndPoint

  5. CreateContainer

  6. Create config.json

  7. Create PID and network namespace

  8. ProcessExternalKey

  9. JoinEndPoint

  10. LaunchContainer

  11. Launch

  12. Run container

Detailed CNM Diagram

Runtime network setup with CNM

  1. Read config.json

  2. Create the network namespace

  3. Call the prestart hook (from inside the netns)

  4. Scan network interfaces inside netns and get the name of the interface created by prestart hook

  5. Create bridge, TAP, and link all together with network interface previously created

CNI

CNI Diagram

Runtime network setup with CNI

  1. Create the network namespace.

  2. Get CNI plugin information.

  3. Start the plugin (providing previously created network namespace) to add a network described into /etc/cni/net.d/ directory. At that time, the CNI plugin will create the cni0 network interface and a veth pair between the host and the created netns. It links cni0 to the veth pair before to exit.

  4. Create network bridge, TAP, and link all together with network interface previously created.

  5. Start VM inside the netns and start the container.

Storage

Container workloads are shared with the virtualized environment through 9pfs. The devicemapper storage driver is a special case. The driver uses dedicated block devices rather than formatted filesystems, and operates at the block level rather than the file level. This knowledge is used to directly use the underlying block device instead of the overlay file system for the container root file system. The block device maps to the top read-write layer for the overlay. This approach gives much better I/O performance compared to using 9pfs to share the container file system.

The approach above does introduce a limitation in terms of dynamic file copy in/out of the container using the docker cp operations. The copy operation from host to container accesses the mounted file system on the host-side. This is not expected to work and may lead to inconsistencies as the block device will be simultaneously written to from two different mounts. The copy operation from container to host will work, provided the user calls sync(1) from within the container prior to the copy to make sure any outstanding cached data is written to the block device.

docker cp [OPTIONS] CONTAINER:SRC_PATH HOST:DEST_PATH
docker cp [OPTIONS] HOST:SRC_PATH CONTAINER:DEST_PATH

Clear Containers has the ability to hotplug and remove block devices, which makes it possible to use block devices for containers started after the VM has been launched.

Users can check to see if the container uses the devicemapper block device as its rootfs by calling mount(8) within the counter. If the devicemapper block device is used, / will be mounted on /dev/vda. Users can disable direct mounting of the underlying block device through the runtime configuration.

Appendices

DAX

Clear Containers utilises the Linux kernel DAX (Direct Access filesystem) feature to efficiently map some host-side files into the guest VM space. In particular, Clear Containers uses the QEMU nvdimm feature to provide a memory-mapped virtual device that can be used to DAX map the virtual machine's root filesystem into the guest memory address space.

Mapping files using DAX provides a number of benefits over more traditional VM file and device mapping mechanisms:

  • Mapping as a direct access devices allows the guest to directly access the host memory pages (such as via eXicute In Place (XIP)), bypassing the guest page cache. This provides both time and space optimisations.
  • Mapping as a direct access device inside the VM allows pages from the host to be demand loaded using page faults, rather than having to make requests via a virtualised device (causing expensive VM exits/hypercalls), thus providing a speed optimisation.
  • Utilising MAP_SHARED shared memory on the host allows the host to efficiently share pages.

Clear Containers uses the following steps to set up the DAX mappings:

  1. QEMU is configured with an nvdimm memory device, with a memory file backend to map in the host-side file into the virtual nvdimm space.
  2. The guest kernel command line mounts this nvdimm device with the DAX feature enabled, allowing direct page mapping and access, thus bypassing the guest page cache.

DAX

Information on the use of nvdimm via QEMU is available in the QEMU source code

Previous Releases

This section provides a brief overview of architectural features enabled in prior and current versions of Clear Containers.

Version 3.0

  • Moved from hyperstart to cc-agent as an agent inside the VM.
  • Moved from qemu-lite to pc QEMU machine type.
  • Rewrite of runtime in go, leveraging virtcontainers.
  • New simplified protocol between cc-shim, cc-proxy and cc-runtime.
  • KSM throttling.
  • virtio-blk for block based graph drivers.

Version 2.2

Summary:

  • Support for devicemapper storage.

Further details:

Version 2.1

Summary:

  • Introduction of hyperstart as an agent inside the VM.
  • Creation of cc-shim and cc-proxy. Major features this enables:
    • Collection of workload exit codes (cc-shim)
    • Full support for terminal/signal control (cc-proxy)

Further details:

Version 2.0

Summary:

  • Clear Containers V2.0 is OCI compatible, and integrates seamlessly into Docker 1.12 via the OCI runtime method.
  • Moved from lkvm/kvmtool to QEMU for more extended functionality.
  • Used nvdimm to DAX map host files into the guest.

Code:

Version 1.0

Summary:

  • Initial instantiation of Clear Containers.
  • Using lkvm/kvmtool as VM supervisor on the host.
  • Not OCI compatible - 1.0 is a compiled replacement runtime for Docker (an "execution d river") and required a different build of docker to be installed on the host system.
  • Utilised a virtual PCI device to DAX map host files into the guest.

Code:

Resources

The 2.x architecture document is available at: