Glossary

This glossary provides definitions and explanations for important terms and concepts used in Nomad.

Allocation

An Allocation is a mapping between a task group in a job and a client node. A single job may have hundreds or thousands of task groups, meaning an equivalent number of allocations must exist to map the work to client machines. Allocations are created by the Nomad servers as part of scheduling decisions made during an evaluation.

Authoritative and Non-Authoritative Regions

The authoritative region is the region in a federated multi-region cluster that holds the source of true for entities replicated across regions, such as ACL tokens, policies, and roles, namespaces, and node pools.

All other regions are considered non-authoritative regions and replicate these entities by pulling them from the authoritative region.

Bin Packing

Bin Packing is the process of filling bins with items in a way that maximizes the utilization of bins. This extends to Nomad, where the clients are "bins" and the items are task groups. Nomad optimizes resources by efficiently bin packing tasks onto client machines.

Client

A Nomad client is an agent configured to run and manage tasks using available compute resources on a machine. The agent is responsible for registering with the servers, watching for any work to be assigned and executing tasks. The Nomad agent is a long lived process which interfaces with the servers.

Datacenters

Nomad models a datacenter as an abstract grouping of clients within a region. Nomad clients are not required to be in the same datacenter as the servers they are joined with, but do need to be in the same region. Datacenters provide a way to express fault tolerance among jobs as well as isolation of infrastructure.

Deployment

Deployments are the mechanism by which Nomad rolls out changes to cluster state in a step-by-step fashion. Deployments are only available for Jobs with the type service. When an Evaluation is processed, the scheduler creates only the number of Allocations permitted by the update block and the current state of the cluster. The Deployment is used to monitor the health of those Allocations and emit a new Evaluation for the next step of the update.

Driver

A Driver represents the basic means of executing your Tasks. Example Drivers include Docker, QEMU, Java, and static binaries.

Evaluation

Evaluations are the mechanism by which Nomad makes scheduling decisions. When either the desired state (jobs) or actual state (clients) changes, Nomad creates a new evaluation to determine if any actions must be taken. An evaluation may result in changes to allocations if necessary.

Job

A Job is a specification provided by users that declares a workload for Nomad. A Job is a form of desired state; the user is expressing that the job should be running, but not where it should be run. The responsibility of Nomad is to make sure the actual state matches the user desired state. A Job is composed of one or more task groups.

Node

A more generic term used to refer to machines running Nomad agents in client mode. Despite being different concepts, you may find node being used interchangeably with client in some materials and informal content.

Node Pool

Node pools are used to group nodes and can be used to restrict which jobs are able to place allocations in a given set of nodes. Example use cases for node pools include segmenting nodes by environment (development, staging, production), by department (engineering, finance, support), or by functionality (databases, ingress proxy, applications).

Regions

Nomad models infrastructure as regions and datacenters. A region will contain one or more datacenters. A set of servers joined together will represent a single region. Servers federate across regions to make Nomad globally aware.

In federated clusters one of the regions must be defined as the authoritative region.

Server

Nomad servers are the brains of the cluster. There is a cluster of servers per region and they manage all jobs and clients, run evaluations, and create task allocations. The servers replicate data between each other and perform leader election to ensure high availability. More information about latency requirements for servers can be found in Network Topology.

Task

A Task is the smallest unit of work in Nomad. Tasks are executed by drivers, which allow Nomad to be flexible in the types of tasks it supports. Tasks specify their driver, configuration for the driver, constraints, and resources required.

Task Group

A Task Group is a set of tasks that must be run together. For example, a web server may require that a log shipping co-process is always running as well. A task group is the unit of scheduling, meaning the entire group must run on the same client node and cannot be split.