open-nomad/website/source/docs/drivers/docker.html.md

layout	page_title	sidebar_current	description
docs	Drivers: Docker	docs-drivers-docker	The Docker task driver is used to run Docker based tasks.

Docker Driver

Name: docker

The docker driver provides a first-class Docker workflow on Nomad. The Docker driver handles downloading containers, mapping ports, and starting, watching, and cleaning up after containers.

Task Configuration

The docker driver supports the following configuration in the job specification:

• image - (Required) The Docker image to run. The image may include a tag or custom URL. By default it will be fetched from Docker Hub.

• command - (Optional) The command to run when starting the container.

• network_mode - (Optional) The network mode to be used for the container. Valid options are net, bridge, host or none. If nothing is specified, the container will start in bridge mode. The container network mode is not supported right now.

Port Mapping

Nomad uses port binding to expose services running in containers using the port space on the host's interface. For example, Nomad host running on 1.2.3.4 may allocate port 22333 to a task, so you would access that service via 1.2.3.4:22333.

Nomad provides automatic and manual mapping schemes for Docker. You can use either or both schemes for a task. Nomad binds both tcp and udp protocols to ports used for Docker containers. This is not configurable.

Note: You are not required to map any ports, for example if your task is running a crawler or aggregator and does not provide a network service. Tasks without a port mapping will still be able to make outbound network connections.

Automatic Port Mapping

Typically when you create a Docker container you configure the service to start listening on a port (or ports) when you start the container. For example, redis starts listening on 6379 when you Docker run redis. Nomad supports this by mapping the random port to the port inside the container.

You need to tell Nomad which ports your container is using so Nomad can map allocated ports for you. You do so by specifying a numeric port value for the dynamic_ports option in your job specification.

dynamic_ports = ["6379"]
# or
dynamic_ports = [6379]

This instructs Nomad to create a port mapping from the random port on the host to the port inside the container. So in our example above, when you contact the host on 1.2.3.4:22333 you will actually hit the service running inside the container on port 6379. You can see which port was actually bound by reading the NOMAD_PORT_6379 environment variable.

In most cases, the automatic port mapping will be the easiest to use, but you can also use manual port mapping (described below).

Manual Port Mapping

The dynamic_ports option takes any alphanumeric string as a label, so you could also specify a label for the port like http or admin to designate how the port will be used.

In this case, Nomad doesn't know which container port to map to, so it maps 1:1 with the host port. For example, 1.2.3.4:22333 will map to 22333 inside the container.

dynamic_ports = ["http"]

Your process will need to read the NOMAD_PORT_HTTP environment variable to determine which port to bind to.

Client Requirements

Nomad requires Docker to be installed and running on the host alongside the Nomad agent. Nomad was developed against Docker 1.8.2.

By default Nomad communicates with the Docker daemon using the daemon's unix socket. Nomad will need to be able to read/write to this socket. If you do not run Nomad as root, make sure you add the Nomad user to the Docker group so Nomad can communicate with the Docker daemon.

For example, on ubuntu you can use the usermod command to add the vagrant user to the docker group so you can run Nomad without root:

sudo usermod -G docker -a vagrant

For the best performance and security features you should use recent versions of the Linux Kernel and Docker daemon.

Client Configuration

The docker driver has the following configuration options:

• docker.endpoint - Defaults to unix:///var/run/docker.sock. You will need to customize this if you use a non-standard socket (http or another location).

Client Attributes

The docker driver will set the following client attributes:

• driver.Docker - This will be set to "1", indicating the driver is available.

Resource Isolation

CPU

Nomad limits containers' CPU based on CPU shares. CPU shares allow containers to burst past their CPU limits. CPU limits will only be imposed when there is contention for resources. When the host is under load your process may be throttled to stabilize QOS depending how how many shares it has. You can see how many CPU shares are available to your process by reading NOMAD_CPU_LIMIT. 1000 shares are approximately equal to 1Ghz.

Please keep the implications of CPU shares in mind when you load test workloads on Nomad.

Memory

Nomad limits containers' memory usage based on total virtual memory. This means that containers scheduled by Nomad cannot use swap. This is to ensure that a swappy process does not degrade performance for other workloads on the same host.

Since memory is not an elastic resource, you will need to make sure your container does not exceed the amount of memory allocated to it, or it will be terminated or crash when it tries to malloc. A process can inspect its memory limit by reading NOMAD_MEMORY_LIMIT, but will need to track its own memory usage. Memory limit is expressed in megabytes so 1024 = 1Gb.

IO

Nomad's Docker integration does not currently provide QOS around network or filesystem IO. These will be added in a later release.

Security

Docker provides resource isolation by way of cgroups and namespaces. Containers essentially have a virtual file system all to themselves. If you need a higher degree of isolation between processes for security or other reasons, it is recommended to use full virtualization like QEMU.