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# Introduction to Terraform
Welcome to the intro guide to Terraform! This guide is the best place to start
with Terraform. We cover what Terraform is, what problems it can solve, how it compares
to existing software, and a quick start for using Terraform. If you are already familiar
with the basics of Terraform, the [documentation](/docs/index.html) provides more
of a reference for all available features.
Welcome to the intro guide to Terraform! This guide is the best
place to start with Terraform. We cover what Terraform is, what
problems it can solve, how it compares to existing software,
and contains a quick start for using Terraform.
If you are already familiar with the basics of Terraform, the
[documentation](/docs/index.html) provides a better reference
guide for all available features as well as internals.
## What is Terraform?
Terraform has multiple components, but as a whole, it is a tool for discovering
and configuring services in your infrastructure. It provides several
key features:
Terraform is a tool for building, changing, and versioning infrastructure
safely and efficiently. Terraform can manage existing and popular service
providers as well as custom in-house solutions.
* **Service Discovery**: Clients of Terraform can _provide_ a service, such as
`api` or `mysql`, and other clients can use Terraform to _discover_ providers
of a given service. Using either DNS or HTTP, applications can easily find
the services they depend upon.
Configuration files describe to Terraform the components needed to
run a single application or your entire datacenter.
Terraform generates an execution plan describing
what it will do to reach the desired state, and then executes it to build the
described infrastructure. As the configuration changes, Terraform is able
to determine what changed and create incremental execution plans which
can be applied.
* **Health Checking**: Terraform clients can provide any number of health checks,
either associated with a given service ("is the webserver returning 200 OK"), or
with the local node ("is memory utilization below 90%"). This information can be
used by an operator to monitor cluster health, and it is used by the service
discovery components to route traffic away from unhealthy hosts.
The infrastructure Terraform can manage includes
low-level components such as
compute instances, storage, and networking, as well as high-level
components such as DNS entries, SaaS features, etc.
* **Key/Value Store**: Applications can make use of Terraform's hierarchical key/value
store for any number of purposes including: dynamic configuration, feature flagging,
coordination, leader election, etc. The simple HTTP API makes it easy to use.
Examples work best to showcase Terraform. Please see the
[use cases](/intro/use-cases.html).
* **Multi Datacenter**: Terraform supports multiple datacenters out of the box. This
means users of Terraform do not have to worry about building additional layers of
abstraction to grow to multiple regions.
The key features of Terraform are:
Terraform is designed to be friendly to both the DevOps community and
application developers, making it perfect for modern, elastic infrastructures.
* **Infrastructure as Code**: Infrastructure is described using a high-level
configuration syntax. This allows a blueprint of your datacenter to be
versioned and treated as you would any other code. Additionally,
infrastructure can be shared and re-used.
## Basic Architecture of Terraform
* **Execution Plans**: Terraform has a "planning" step where it generates
an _execution plan_. The execution plan shows what Terraform will do when
you call apply. This lets you avoid any surprises when Terraform
manipulates infrastructure.
Terraform is a distributed, highly available system. There is an
[in-depth architecture overview](/docs/internals/architecture.html) available,
but this section will cover the basics so you can get an understanding
of how Terraform works. This section will purposely omit details to quickly
provide an overview of the architecture.
* **Resource Graph**: Terraform builds a graph of all your resources,
and parallelizes the creation and modification of any non-dependent
resources. Because of this, Terraform builds infrastructure as efficiently
as possible, and operators get insight into dependencies in their
infrastructure.
Every node that provides services to Terraform runs a _Terraform agent_. Running
an agent is not required for discovering other services or getting/setting
key/value data. The agent is responsible for health checking the services
on the node as well as the node itself.
The agents talk to one or more _Terraform servers_. The Terraform servers are
where data is stored and replicated. The servers themselves elect a leader.
While Terraform can function with one server, 3 to 5 is recommended to avoid
data loss scenarios. A cluster of Terraform servers is recommended for each
datacenter.
Components of your infrastructure that need to discover other services
or nodes can query any of the Terraform servers _or_ any of the Terraform agents.
The agents forward queries to the servers automatically.
Each datacenter runs a cluster of Terraform servers. When a cross-datacenter
service discovery or configuration request is made, the local Terraform servers
forward the request to the remote datacenter and return the result.
* **Change Automation**: Potentially complex changesets are applied to
your infrastructure with minimal human interaction.
With the previously mentioned execution
plan and resource graph, you know exactly what Terraform will change
and in what order, avoiding many possible human errors.
## Next Steps
See the page on [how Terraform compares to other software](/intro/vs/index.html)
to see how it fits into your existing infrastructure. Or continue onwards with
See the page on [Terraform use cases](/intro/use-cases.html) to see the
multiple ways Terraform can be used. Then see
[how Terraform compares to other software](/intro/vs/index.html)
to see how it fits into your existing infrastructure. Finally, continue onwards with
the [getting started guide](/intro/getting-started/install.html) to get
Terraform up and running and see how it works.
Terraform managing some real infrastructure and to see how it works.