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

Job-related variables

Network-related Variables

Consul-related Variables

Task Identifiers

Nomad will pass both the allocation ID and name, the deployment ID that created the allocation, the job ID and name, the parent job ID as well as the task and group's names. These are given as NOMAD_ALLOC_ID, NOMAD_ALLOC_NAME, NOMAD_ALLOC_INDEX, NOMAD_JOB_NAME, NOMAD_JOB_ID, NOMAD_JOB_PARENT_ID, NOMAD_GROUP_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, NOMAD_MEMORY_LIMIT, and NOMAD_MEMORY_MAX_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 up or down as your cluster gets more or less busy.

Networking

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

Task Directories

Nomad creates a working directory for each allocation on a client. The allocation working directory contains a task working directory for each task in the allocation.

Nomad makes the following directories available to tasks, relative to the task working directory:

• 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 alloc fs command or filesystem APIs. Where possible it is backed by an in-memory filesystem and mounted noexec. 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 chroot is built in the task working directory, and the directories are mounted into that 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.

For more details on the task directories, see the Filesystem internals.

Meta

The job specification also allows you to specify a meta block to supply arbitrary configuration to a task. This allows you to easily provide job-specific configuration even if you use the same executable unit in multiple jobs. These key-value pairs are passed through to the job as NOMAD_META_<key>=<value> environment variables. Any character in a key other than [A-Za-z0-9_.] will be converted to _. Both the original case and an uppercased version are injected. The uppercased version will be deprecated in a future release.

Currently there is no enforcement that the meta keys be lowercase, but using multiple keys with the same uppercased representation will lead to undefined behavior.

Host environment variables

Nomad passes the environment variables defined in the client host to tasks when using the exec, raw_exec, and java task drivers. The variables that are passed to the tasks can be controlled using the client configuration env.denylist.