terraform/website/source/docs/internals/graph.html.md

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2014-07-26 06:37:22 +02:00
---
layout: "docs"
page_title: "Resource Graph"
sidebar_current: "docs-internals-graph"
---
# Resource Graph
Terraform builds a
[dependency graph](http://en.wikipedia.org/wiki/Dependency_graph)
from the Terraform configurations, and walks this graph to
generate plans, refresh state, and more. This page documents
the details of what are contained in this graph, what types
of nodes there are, and how the edges of the graph are determined.
<div class="alert alert-block alert-warning">
<strong>Advanced Topic!</strong> This page covers technical details
of Terraform. You don't need to understand these details to
effectively use Terraform. The details are documented here for
those who wish to learn about them without having to go
spelunking through the source code.
</div>
## Graph Nodes
There are only a handful of node types that can exist within the
graph. We'll cover these first before explaining how they're
determined and built:
* **Resource Node** - Represents a single resource. If you have
the `count` metaparameter set, then there will be one resource
node for each count. The configuration, diff, state, etc. of
the resource under change is attached to this node.
* **Provider Configuration Node** - Represents the time to fully
configure a provider. This is when the provider configuration
block is given to a provider, such as AWS security credentials.
* **Resource Meta-Node** - Represents a group of resources, but
does not represent any action on its own. This is done for
convenience on dependencies and making a prettier graph. This
node is only present for resources that have a `count`
parameter greater than 1.
When visualizing a configuration with `terraform graph`, you can
see all of these nodes present.
## Building the Graph
Building the graph is done in a series of sequential steps:
1. Resources nodes are added based on the configuration. If a
diff (plan) or state is present, that meta-data is attached
to each resource node.
1. Resources are mapped to provisioners if they have any
defined. This must be done after all resource nodes are
created so resources with the same provisioner type can
share the provisioner implementation.
1. Explicit dependencies from the `depends_on` meta-parameter
are used to create edges between resources.
1. If a state is present, any "orphan" resources are added to
the graph. Orphan resources are any resources that are no
longer present in the configuration but are present in the
state file. Orphans never have any configuration associated
with them, since the state file does not store configuration.
1. Resources are mapped to providers. Provider configuration
nodes are created for these providers, and edges are created
such that the resources depend on their respective provider
being configured.
1. Interpolations are parsed in resource and provider configurations
to determine dependencies. References to resource attributes
are turned into dependencies from the resource with the interpolation
to the resource being referenced.
1. Create a root node. The root node points to all resources and
is created so there is a single root to the dependency graph. When
traversing the graph, the root node is ignored.
1. If a diff is present, traverse all resource nodes and find resources
that are being destroyed. These resource nodes are split into two:
one node that destroys the resource and another that creates
the resource (if it is being recreated). The reason the nodes must
be split is because the destroy order is often different from the
create order, and so they can't be represented by a single graph
node.
1. Validate the graph has no cycles and has a single root.
## Walking the Graph
To walk the graph, a standard depth-first traversal is done. Graph
walking is done with as much parallelism as possible: a node is walked
as soon as all of its dependencies are walked.