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

# Runtime Environment

Some settings you specify in your [jobspec](/docs/jobspec/) 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.

## 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 evaulation 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 `1 = 1 megabytes`.

Writing your applications to adjust to these values at runtime provides greater
scheduling flexibility since you can adjust the resource allocations in your
jobspec 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.

### IPs and Named Ports

Each task will receive port allocations on a single IP address. The IP is made
available through `NOMAD_IP.`

If you requested reserved ports in your jobspec and your task is successfully
scheduled, these ports are available for your use. Ports from `reserved_ports`
in the job spec are not exposed through the environment. If you requested
dynamic ports in your jobspec these are made known to your application via
environment variables `NOMAD_PORT_{LABEL}`. For example
`dynamic_ports = ["HTTP"]` becomes `NOMAD_PORT_HTTP`.

Some drivers such as Docker and QEMU use port mapping. If a driver supports port
mapping and you specify a numeric label, the label will be automatically used as
the private port number. For example, `dynamic_ports = ["5000"]` will have a
random port mapped to port 5000 inside the container or VM. These ports are also
exported as environment variables for consistency, e.g. `NOMAD_PORT_5000`.

Please see the relevant driver documentation for details.

## Meta

The jobspec 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}`,
where `key` is UPPERCASED from the jobspec.

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