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    <h4>If you're new to Mesos</h4>
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       information about downloading, building, and deploying Mesos.</p>

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    <h1>Overview</h1>

<h2>Motivation</h2>

<p>A <a href="http://kubernetes.io/docs/user-guide/pods">pod</a> can be defined as
a set of containers co-located and co-managed on an agent that share
some resources (e.g., network namespace, volumes) but not others
(e.g., container image, resource limits). Here are the use cases for
pod:</p>

<ul>
<li>Run a side-car container (e.g., logger, backup) next the main
application controller.</li>
<li>Run an adapter container (e.g., metrics endpoint, queue consumer)
next to the main container.</li>
<li>Run transient tasks inside a pod for a operations which are
short-lived and whose exit does not imply that a pod should
exit (e.g., a task which backs up data in a persistent volume).</li>
<li>Provide performance isolation between latency-critical application
and supporting processes.</li>
<li>Run a group of containers sharing volumes and network namespace
while some of them can have their own mount namespace.</li>
<li>Run a group of containers with the same life cycle, e.g, one
container&rsquo;s failure would cause all other containers being
cleaned up.</li>
</ul>


<p>In order to have first class support for running &ldquo;pods&rdquo;, two new
primitives are introduced in Mesos: <code>Task Group</code> and <code>Nested Container</code>.</p>

<h2>Background</h2>

<p>Mesos has the concept of Executors and Tasks. An executor can launch
multiple tasks while the executor runs in a container. An agent can
run multiple executors. The pod can be implemented by leveraging the
executor and task abstractions. More specifically, the executor runs
in the top level container (called executor container) and its tasks
run in separate nested containers inside this top level container,
while the container image can be specified for each container.</p>

<h2>New Primitives: Task Group and Nested Container</h2>

<h3>Task Group</h3>

<p>The concept of Task Group addresses the limitation of the existing
Scheduler and Executor APIs, which cannot send a group of tasks to
an executor <code>atomically</code>. Even though a scheduler can launch multiple
tasks for the same executor in a LAUNCH offer operation, these tasks
are delivered to the executor one at a time via separate LAUNCH events.
It cannot guarantee atomicity since any individual task might be dropped
due to different reasons (e.g., network partition). Therefore, the Task
Group provides <code>all-or-nothing</code> semantics to ensure a group of tasks
are delivered <code>atomically</code> to an executor.</p>

<h3>Nested Container</h3>

<p>The concept of Nested Container describes containers nested under an
executor container. They share the network namespace and volumes while
they may have their own container images and resource limits. By
introducing the new agent API for nested container in the following
section, executors no longer need to implement their own containerization.
Instead, executors can reuse the containerizer (in the agent) to launch
nested container. Both authorized operators or executors will be allowed
to create nested containers.</p>

<h1>Task Group API</h1>

<h2>Framework API</h2>

<pre><code>message TaskGroupInfo {
  repeated TaskInfo tasks = 1;
}

message Offer {
  ...

  message Operation {
    enum Type {
      ...
      LAUNCH_GROUP = 6;
      ...
    }
    ...

    message LaunchGroup {
      required ExecutorInfo executor = 1;
      required TaskGroupInfo task_group = 2;
    }
    ...

    optional LaunchGroup launch_group = 7;
  }
}
</code></pre>

<p>By using the TaskGroup Framework API, frameworks can launch a task
group with the <a href="/documentation/latest/./app-framework-development-guide/">default executor</a>
or a custom executor. The group of tasks can be specified through an
offer operation <code>LaunchGroup</code> when accepting an offer. The
<code>ExecutorInfo</code> indicates the executor to launch the task group, while
the <code>TaskGroupInfo</code> includes the group of tasks to be launched
atomically.</p>

<p>To use the default executor for launching the task group, the framework should:
* Set <code>ExecutorInfo.type</code> as <code>DEFAULT</code>.
* Set <code>ExecutorInfo.resources</code> for the resources needed for the executor.</p>

<p>Please note that the following fields in the <code>ExecutorInfo</code> are not allowed to set when using the default executor:
* <code>ExecutorInfo.command</code>.
* <code>ExecutorInfo.container.type</code>, <code>ExecutorInfo.container.docker</code> and <code>ExecutorInfo.container.mesos</code>.</p>

<p>To allow containers to share a network namespace:
* Set <code>ExecutorInfo.container.network</code>.</p>

<p>To allow containers to share an ephemeral volume:
Specify the <code>volume/sandbox_path</code> isolator.
* Set <code>TaskGroupInfo.tasks.container.volumes.source.type</code> as <code>SANDBOX_PATH</code>.
* Set <code>TaskGroupInfo.tasks.container.volumes.source.sandbox_path.type</code> as <code>PARENT</code> and the path relative to the parent container&rsquo;s sandbox.</p>

<h2>Executor API</h2>

<pre><code>message Event {
  enum Type {
    ...
    LAUNCH_GROUP = 8;
    ...
  }
  ...

  message LaunchGroup {
    required TaskGroupInfo task_group = 1;
  }
  ...

  optional LaunchGroup launch_group = 8;
}
</code></pre>

<p>A new event <code>LAUNCH_GROUP</code> is added to Executor API. Similar to the
Framework API, the <code>LAUNCH_GROUP</code> event guarantees a group of tasks
are delivered to the executor atomically.</p>

<h1>Nested Container API</h1>

<h2>New Agent API</h2>

<pre><code>package mesos.agent;

message Call {
  enum Type {
    ...
    // Calls for managing nested containers underneath an executor's container.
    NESTED_CONTAINER_LAUNCH = 14;  // See 'NestedContainerLaunch' below.
    NESTED_CONTAINER_WAIT = 15;    // See 'NestedContainerWait' below.
    NESTED_CONTAINER_KILL = 16;    // See 'NestedContainerKill' below.
  }

  // Launches a nested container within an executor's tree of containers.
  message LaunchNestedContainer {
    required ContainerID container_id = 1;
    optional CommandInfo command = 2;
    optional ContainerInfo container = 3;
  }

  // Waits for the nested container to terminate and receives the exit status.
  message WaitNestedContainer {
    required ContainerID container_id = 1;
  }

  // Kills the nested container. Currently only supports SIGKILL.
  message KillNestedContainer {
    required ContainerID container_id = 1;
  }

  optional Type type = 1;
  ...
  optional NestedContainerLaunch nested_container_launch = 6;
  optional NestedContainerWait nested_container_wait = 7;
  optional NestedContainerKill nested_container_kill = 8;
}

message Response {
  enum Type {
    ...
    NESTED_CONTAINER_WAIT = 13;    // See 'NestedContainerWait' below.
  }

  // Returns termination information about the nested container.
  message NestedContainerWait {
    optional int32 exit_status = 1;
  }

  optional Type type = 1;
  ...
  optional NestedContainerWait nested_container_wait = 14;
}
</code></pre>

<p>By adding the new Agent API, any authorized entity, including the
executor itself, its tasks, or the operator can use this API to
launch/wait/kill nested containers. Multi-level nesting is supported
by using this API. Technically, the nested level is up to 32 since
it is limited by the maximum depth of <a href="https://github.com/torvalds/linux/commit/f2302505775fd13ba93f034206f1e2a587017929">pid namespace</a> and
<a href="http://man7.org/linux/man-pages/man7/user_namespaces.7.html">user namespace</a> from the Linux Kernel.</p>

<p>The following is the workflow of how the new Agent API works:</p>

<ol>
<li><p>The executor sends a <code>NESTED_CONTAINER_LAUNCH</code> call to the agent.</p>

<pre><code>                                +---------------------+
                                |                     |
                                |     Container       |
                                |                     |
 +-------------+                | +-----------------+ |
 |             |     LAUNCH     | |                 | |
 |             | &lt;------------+ | |    Executor     | |
 |             |                | |                 | |
 | Mesos Agent |                | +-----------------+ |
 |             |                |                     |
 |             |                |                     |
 |             |                |                     |
 +-------------+                |                     |
                                +---------------------+
</code></pre></li>
<li><p>Depending on the <code>LaunchNestedContainer</code> from the executor, the
agent launches a nested container inside of the executor container
by calling <code>containerizer::launch()</code>.</p>

<pre><code>                                 +---------------------+
                                 |                     |
                                 |     Container       |
                                 |                     |
  +-------------+                | +-----------------+ |
  |             |     LAUNCH     | |                 | |
  |             | &lt;------------+ | |    Executor     | |
  |             |                | |                 | |
  | Mesos Agent |                | +-----------------+ |
  |             |                |                     |
  |             |                | +---------+         |
  |             | +------------&gt; | |Nested   |         |
  +-------------+                | |Container|         |
                                 | +---------+         |
                                 +---------------------+
</code></pre></li>
<li><p>The executor sends a <code>NESTED_CONTAINER_WAIT</code> call to the agent.</p>

<pre><code>                                +---------------------+
                                |                     |
                                |     Container       |
                                |                     |
 +-------------+                | +-----------------+ |
 |             |      WAIT      | |                 | |
 |             | &lt;------------+ | |    Executor     | |
 |             |                | |                 | |
 | Mesos Agent |                | +-----------------+ |
 |             |                |                     |
 |             |                | +---------+         |
 |             |                | |Nested   |         |
 +-------------+                | |Container|         |
                                | +---------+         |
                                +---------------------+
</code></pre></li>
<li><p>Depending on the <code>ContainerID</code>, the agent calls <code>containerizer::wait()</code>
to wait for the nested container to terminate or exit. Once the container
terminates or exits, the agent returns the container exit status to the
executor.</p>

<pre><code>                                 +---------------------+
                                 |                     |
                                 |     Container       |
                                 |                     |
  +-------------+      WAIT      | +-----------------+ |
  |             | &lt;------------+ | |                 | |
  |             |                | |    Executor     | |
  |             | +------------&gt; | |                 | |
  | Mesos Agent |  Exited with   | +-----------------+ |
  |             |  status 0      |                     |
  |             |                | +--XX-XX--+         |
  |             |                | |   XXX   |         |
  +-------------+                | |   XXX   |         |
                                 | +--XX-XX--+         |
                                 +---------------------+
</code></pre></li>
</ol>


<h1>Future Work</h1>

<ul>
<li>Authentication and authorization on the new Agent API.</li>
<li>Command health checks inside of the container&rsquo;s mount namespace.</li>
<li>Resource isolation for nested containers.</li>
<li>Resource statistics reporting for nested containers.</li>
<li>Multiple task groups.</li>
</ul>


<h1>Reference</h1>

<ul>
<li><a href="http://kubernetes.io/docs/user-guide/pods/">Kubernetes Pods</a></li>
<li><a href="https://github.com/docker/docker/issues/8781">Docker Pods</a></li>
</ul>


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