title: Apache Mesos - Mesos Containerizer layout: documentation

Mesos Containerizer

The MesosContainerizer provides lightweight containerization and resource isolation of executors using Linux-specific functionality such as control cgroups and namespaces. It is composable so operators can selectively enable different isolators.

It also provides basic support for POSIX systems (e.g., OSX) but without any actual isolation, only resource usage reporting.

Shared Filesystem

The SharedFilesystem isolator can optionally be used on Linux hosts to enable modifications to each container's view of the shared filesystem.

The modifications are specified in the ContainerInfo included in the ExecutorInfo, either by a framework or by using the --default_container_info agent flag.

ContainerInfo specifies Volumes which map parts of the shared filesystem (host_path) into the container‘s view of the filesystem (container_path), as read-write or read-only. The host_path can be absolute, in which case it will make the filesystem subtree rooted at host_path also accessible under container_path for each container. If host_path is relative then it is considered as a directory relative to the executor’s work directory. The directory will be created and permissions copied from the corresponding directory (which must exist) in the shared filesystem.

The primary use-case for this isolator is to selectively make parts of the shared filesystem private to each container. For example, a private “/tmp” directory can be achieved with host_path="tmp" and container_path="/tmp" which will create a directory “tmp” inside the executor‘s work directory (mode 1777) and simultaneously mount it as /tmp inside the container. This is transparent to processes running inside the container. Containers will not be able to see the host’s /tmp or any other container's /tmp.

Pid Namespace

The Pid Namespace isolator can be used to isolate each container in a separate pid namespace with two main benefits:

  1. Visibility: Processes running in the container (executor and descendants) are unable to see or signal processes outside the namespace.

  2. Clean termination: Termination of the leading process in a pid namespace will result in the kernel terminating all other processes in the namespace.

The Launcher will use (2) during destruction of a container in preference to the freezer cgroup, avoiding known kernel issues related to freezing cgroups under OOM conditions.

/proc will be mounted for containers so tools such as ‘ps’ will work correctly.

Posix Disk Isolator

The Posix Disk isolator provides basic disk isolation. It is able to report the disk usage for each sandbox and optionally enforce the disk quota. It can be used on both Linux and OS X.

To enable the Posix Disk isolator, append disk/du to the --isolation flag when starting the agent.

By default, the disk quota enforcement is disabled. To enable it, specify --enforce_container_disk_quota when starting the agent.

The Posix Disk isolator reports disk usage for each sandbox by periodically running the du command. The disk usage can be retrieved from the resource statistics endpoint (/monitor/statistics).

The interval between two dus can be controlled by the agent flag --container_disk_watch_interval. For example, --container_disk_watch_interval=1mins sets the interval to be 1 minute. The default interval is 15 seconds.

XFS Disk Isolator

The XFS Disk isolator uses XFS project quotas to track the disk space used by each container sandbox and to enforce the corresponding disk space allocation. When quota enforcement is enabled, write operations performed by tasks exceeding their disk allocation will fail with an EDQUOT error. The task will not be terminated by the containerizer.

To enable the XFS Disk isolator, append disk/xfs to the --isolation flag when starting the agent.

The XFS Disk isolator supports the --enforce_container_disk_quota flag. If enforcement is enabled, the isolator will set both the hard and soft quota limit. Otherwise, no limits will be set, Disk usage accounting will be performed but the task will be allowed to exceed its allocation.

The XFS Disk isolator requires the sandbox directory to be located on an XFS filesystem that is mounted with the pquota option. There is no need to configure projects or projid files. The range of project IDs given to the --xfs_project_range must not overlap any project IDs allocated for other uses.

The xfs_quota command can be used to show the current allocation of project IDs and quota. For example:

$ xfs_quota -x -c "report -a -n -L 5000 -U 10000"

To show which project a file belongs to, use the xfs_io command to display the fsxattr.projid field. For example:

$ xfs_io -r -c stat /mnt/mesos/

Note that the Posix Disk isolator --container_disk_watch_interval does not apply to the XFS Disk isolator.

Docker Runtime Isolator

The Docker Runtime isolator is used for supporting runtime configurations from the docker image (e.g., Entrypoint/Cmd, Env, etc.). This isolator is tied with --image_providers=docker. If --image_providers contains docker, this isolator must be used. Otherwise, the agent will refuse to start.

To enable the Docker Runtime isolator, append docker/runtime to the --isolation flag when starting the agent.

Currently, docker image default Entrypoint, Cmd, Env, and WorkingDir are supported with docker runtime isolator. Users can specify CommandInfo to override the default Entrypoint and Cmd in the image (see below for details). The CommandInfo should be inside of either TaskInfo or ExecutorInfo (depending on whether the task is a command task or uses a custom executor, respectively).

Determine the Launch Command

If the user specifies a command in CommandInfo, that will override the default Entrypoint/Cmd in the docker image. Otherwise, we will use the default Entrypoint/Cmd and append arguments specified in CommandInfo accordingly. The details are explained in the following table.

Users can specify CommandInfo including shell, value and arguments, which are represented in the first column of the table below. 0 represents not specified, while 1 represents specified. The first row is how Entrypoint and Cmd defined in the docker image. All cells in the table, except the first column and row, as well as cells labeled as Error, have the first element (i.e., /Entrypt[0]) as executable, and the rest as appending arguments.

The cgroups/net_cls Isolator

The cgroups/net_cls isolator allows operators to provide network performance isolation and network segmentation for containers within a Mesos cluster. To enable the cgroups/net_cls isolator, append cgroups/net_cls to the --isolation flag when starting the agent.

As the name suggests, the isolator enables the net_cls subsystem for Linux cgroups and assigns a net_cls cgroup to each container launched by the MesosContainerizer. The objective of the net_cls subsystem is to allow the kernel to tag packets originating from a container with a 32-bit handle. These handles can be used by kernel modules such as qdisc (for traffic engineering) and net-filter (for firewall) to enforce network performance and security policies specified by the operators. The policies, based on the net_cls handles, can be specified by the operators through user-space tools such as tc and iptables.

The 32-bit handle associated with a net_cls cgroup can be specified by writing the handle to the net_cls.classid file, present within the net_cls cgroup. The 32-bit handle is of the form 0xAAAABBBB, and consists of a 16-bit primary handle 0xAAAA and a 16-bit secondary handle 0xBBBB. You can read more about the use cases for the primary and secondary handles in the Linux kernel documentation for net_cls.

By default, the cgroups/net_cls isolator does not manage the net_cls handles, and assumes the operator is going to manage/assign these handles. To enable the management of net_cls handles by the cgroups/net_cls isolator you need to specify a 16-bit primary handle, of the form 0xAAAA, using the --cgroups_net_cls_primary_handle flag at agent startup.

Once a primary handle has been specified for an agent, for each container the cgroups/net_cls isolator allocates a 16-bit secondary handle. It then assigns the 32-bit combination of the primary and secondary handle to the net_cls cgroup associated with the container by writing to net_cls.classid. The cgroups/net_cls isolator exposes the assigned net_cls handle to operators by exposing the handle as part of the ContainerStatus —associated with any task running within the container— in the agent's /state endpoint.

The docker/volume Isolator

This is described in a separate document.

The namespaces/ipc Isolator

The IPC Namespace isolator can be used on Linux to place tasks in a distinct IPC namespace. The benefit of this is that any IPC objects created in the container will be automatically removed when the container is destroyed.

The network/cni Isolator

This is described in a separate document.

The linux/capabilities Isolator

This is described in a separate document.

The posix/rlimits Isolator

This is described in a separate document.