Michael Schurter ca0b19a050 Add documentation and changelog entry

2017-03-06 20:05:05 -08:00

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layout	page_title	sidebar_current	description
docs	Environment - Runtime	docs-runtime-environment	Learn how to configure the Nomad runtime environment.

Runtime Environment

Some settings you specify in your job specification are passed to tasks when they start. Other settings are dynamically allocated when your job is scheduled. Both types of values are made available to your job through environment variables.

Summary

Variable	Description
`NOMAD_ALLOC_DIR`	Path to the shared alloc directory
`NOMAD_TASK_DIR`	Path to the local task directory
`NOMAD_SECRETS_DIR`	Path to the task's secrets directory
`NOMAD_MEMORY_LIMIT`	The task's memory limit in MB
`NOMAD_CPU_LIMIT`	The task's CPU limit in MHz
`NOMAD_ALLOC_ID`	The allocation ID of the task
`NOMAD_ALLOC_NAME`	The allocation name of the task
`NOMAD_ALLOC_INDEX`	The allocation index; useful to distinguish instances of task groups
`NOMAD_TASK_NAME`	The task's name
`NOMAD_JOB_NAME`	The job's name
`NOMAD_IP_`	The IP of the port with the given label
`NOMAD_PORT_`	The port value with the given label
`NOMAD_ADDR_`	The IP:Port pair of the port with the given label
`NOMAD_ADDR__	The allocated address, given as IP:Port for the given label of other tasks in the same group
`NOMAD_HOST_PORT_`	The host port for the given label if the port is port mapped
`NOMAD_META_`	The metadata of the task
`VAULT_TOKEN`	The task's Vault token. See [Vault Integration](/docs/vault-integration/index.html) for more details

Port labels and task names will have any dashes - in their names replaced by underscores _ when they're used in environment variable names such as NOMAD_ADDR_<task>_<label>.

Task Identifiers

Nomad will pass both the allocation ID and name as well as the task and job's names. These are given as NOMAD_ALLOC_ID, NOMAD_ALLOC_NAME, NOMAD_ALLOC_INDEX, NOMAD_JOB_NAME, and NOMAD_TASK_NAME. The allocation ID and index can be useful when the task being run needs a unique identifier or to know its instance count.

Resources

When you request resources for a job, Nomad creates a resource offer. The final resources for your job are not determined until it is scheduled. Nomad will tell you which resources have been allocated after evaluation and placement.

CPU and Memory

Nomad will pass CPU and memory limits to your job as NOMAD_CPU_LIMIT and NOMAD_MEMORY_LIMIT. Your task should use these values to adapt its behavior to fit inside the resource allocation that nomad provides. For example, you can use the memory limit to inform how large your in-process cache should be, or to decide when to flush buffers to disk.

Both CPU and memory are presented as integers. The unit for CPU limit is 1024 = 1GHz. The unit for memory is 1 = 1 megabyte.

Writing your applications to adjust to these values at runtime provides greater scheduling flexibility since you can adjust the resource allocations in your job specification without needing to change your code. You can also schedule workloads that accept dynamic resource allocations so they can scale down/up as your cluster gets more or less busy.

Networking

Nomad assigns IPs and ports to your jobs and exposes them via environment variables. See the Networking page for more details.

Task Directories

Nomad makes the following directories available to tasks:

alloc/: This directory is shared across all tasks in a task group and can be used to store data that needs to be used by multiple tasks, such as a log shipper.
local/: This directory is private to each task. It can be used to store arbitrary data that should not be shared by tasks in the task group.
secrets/: This directory is private to each task, not accessible via the nomad fs command or filesystem APIs and where possible backed by an in-memory filesystem. It can be used to store secret data that should not be visible outside the task.

These directories are persisted until the allocation is removed, which occurs hours after all the tasks in the task group enter terminal states. This gives time to view the data produced by tasks.

Depending on the driver and operating system being targeted, the directories are made available in various ways. For example, on docker the directories are bound to the container, while on exec on Linux the directories are mounted into the chroot. Regardless of how the directories are made available, the path to the directories can be read through the NOMAD_ALLOC_DIR, NOMAD_TASK_DIR, and NOMAD_SECRETS_DIR environment variables.

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