140 lines
7.6 KiB
Markdown
140 lines
7.6 KiB
Markdown
---
|
||
layout: "docs"
|
||
page_title: "Architecture"
|
||
sidebar_current: "docs-internals-architecture"
|
||
description: |-
|
||
Learn about the internal architecture of Nomad.
|
||
---
|
||
|
||
# Architecture
|
||
|
||
Nomad is a complex system that has many different pieces. To help both users and developers of Nomad
|
||
build a mental model of how it works, this page documents the system architecture.
|
||
|
||
~> **Advanced Topic!** This page covers technical details
|
||
of Nomad. You do not need to understand these details to
|
||
effectively use Nomad. The details are documented here for
|
||
those who wish to learn about them without having to go
|
||
spelunking through the source code.
|
||
|
||
# Glossary
|
||
|
||
Before describing the architecture, we provide a glossary of terms to help
|
||
clarify what is being discussed:
|
||
|
||
* **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.
|
||
|
||
* **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.
|
||
|
||
* **Driver** – A Driver represents the basic means of executing your **Tasks**.
|
||
Example Drivers include Docker, Qemu, Java, and static binaries.
|
||
|
||
* **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.
|
||
|
||
* **Client** - A Client of Nomad is a machine that tasks can be run on. All clients run the
|
||
Nomad agent. 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.
|
||
|
||
* **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.
|
||
|
||
* **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.
|
||
|
||
* **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. Servers federate across regions to make Nomad globally aware.
|
||
|
||
* **Regions and Datacenters** - Nomad models infrastructure as regions and
|
||
datacenters. Regions may contain multiple datacenters. Servers are assigned to
|
||
a specific region, managing state and making scheduling decisions within that
|
||
region. Multiple regions can be federated together. For example, you may
|
||
have a `US` region with the `us-east-1` and `us-west-1` datacenters,
|
||
connected to the `EU` region with the `eu-fr-1` and `eu-uk-1` datacenters.
|
||
Requests that are made between regions are forwarded to the appropriate servers.
|
||
Data is _not_ replicated between regions.
|
||
|
||
* **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.
|
||
|
||
# High-Level Overview
|
||
|
||
Looking at only a single region, at a high level Nomad looks like this:
|
||
|
||
[](/assets/images/nomad-architecture-region.png)
|
||
|
||
Within each region, we have both clients and servers. Servers are responsible for
|
||
accepting jobs from users, managing clients, and [computing task placements](/docs/internals/scheduling/scheduling.html).
|
||
Each region may have clients from multiple datacenters, allowing a small number of servers
|
||
to handle very large clusters.
|
||
|
||
In some cases, for either availability or scalability, you may need to run multiple
|
||
regions. Nomad supports federating multiple regions together into a single cluster.
|
||
At a high level, this setup looks like this:
|
||
|
||
[](/assets/images/nomad-architecture-global.png)
|
||
|
||
Regions are fully independent from each other, and do not share jobs, clients, or
|
||
state. They are loosely-coupled using a gossip protocol, which allows users to
|
||
submit jobs to any region or query the state of any region transparently. Requests
|
||
are forwarded to the appropriate server to be processed and the results returned.
|
||
Data is _not_ replicated between regions.
|
||
|
||
The servers in each region are all part of a single consensus group. This means
|
||
that they work together to elect a single leader which has extra duties. The leader
|
||
is responsible for processing all queries and transactions. Nomad is optimistically
|
||
concurrent, meaning all servers participate in making scheduling decisions in parallel.
|
||
The leader provides the additional coordination necessary to do this safely and
|
||
to ensure clients are not oversubscribed.
|
||
|
||
Each region is expected to have either three or five servers. This strikes a balance
|
||
between availability in the case of failure and performance, as consensus gets
|
||
progressively slower as more servers are added. However, there is no limit to the number
|
||
of clients per region.
|
||
|
||
Clients are configured to communicate with their regional servers and communicate
|
||
using remote procedure calls (RPC) to register themselves, send heartbeats for liveness,
|
||
wait for new allocations, and update the status of allocations. A client registers
|
||
with the servers to provide the resources available, attributes, and installed drivers.
|
||
Servers use this information for scheduling decisions and create allocations to assign
|
||
work to clients.
|
||
|
||
Users make use of the Nomad CLI or API to submit jobs to the servers. A job represents
|
||
a desired state and provides the set of tasks that should be run. The servers are
|
||
responsible for scheduling the tasks, which is done by finding an optimal placement for
|
||
each task such that resource utilization is maximized while satisfying all constraints
|
||
specified by the job. Resource utilization is maximized by bin packing, in which
|
||
the scheduling tries to make use of all the resources of a machine without
|
||
exhausting any dimension. Job constraints can be used to ensure an application is
|
||
running in an appropriate environment. Constraints can be technical requirements based
|
||
on hardware features such as architecture and availability of GPUs, or software features
|
||
like operating system and kernel version, or they can be business constraints like
|
||
ensuring PCI compliant workloads run on appropriate servers.
|
||
|
||
# Getting in Depth
|
||
|
||
This has been a brief high-level overview of the architecture of Nomad. There
|
||
are more details available for each of the sub-systems. The [consensus protocol](/docs/internals/consensus.html),
|
||
[gossip protocol](/docs/internals/gossip.html), and [scheduler design](/docs/internals/scheduling/scheduling.html)
|
||
are all documented in more detail.
|
||
|
||
For other details, either consult the code, ask in IRC or reach out to the mailing list.
|
||
|