What does "Error: Cycle". means in Terraform?

The Terraform docs for some weird reason do not explain what "Error: Cycle" means. I've looked everywhere but there is no mention of it on the official docs.

As part of Terraform's work, it analyzes the dependencies between your resource blocks, data blocks, and other configuration constructs in order to determine a suitable order to process them so that the necessary input data will be available.

For example, consider the following contrived simple configuration:

resource "null_resource" "foo" {
}

resource "null_resource" "bar" {
  triggers = {
    foo = null_resource.foo.id
  }
}

Terraform will analyze the above and notice that the configuration of null_resource.bar contains a reference to null_resource.foo, and therefore the operations related to null_resource.foo must happen before null_resource.bar. We can visualize that as a graph where an arrow represents "must happen after", or "depends on":

• Operations for null_resource.bar must happen after operations for null_resource.foo.

Consider now what happens if we modify that configuration like this:

resource "null_resource" "foo" {
  triggers = {
    bar = null_resource.bar.id
  }
}

resource "null_resource" "bar" {
  triggers = {
    foo = null_resource.foo.id
  }
}

Now null_resource.foo also refers to null_resource.bar. There are now two "must happen after" relationships implied by this configuration:

• Operations for null_resource.bar must happen after operations for null_resource.foo.
• Operations for null_resource.foo must happen after operations for null_resource.bar.

The two statements above contradict one another: null_resource.bar cannot be processed both before and after null_resource.foo. Terraform will respond to this situation by reporting a dependency cycle, using the error message you've seen:

Cycle: null_resource.foo, null_resource.bar

When Terraform returns this error, the solution is to remove at least one of the "must happen after" arrows (dependencies) from the configuration so that it's no longer contradictory. Without seeing your configuration I can't suggest what specific change might achieve that in your case, but it's likely that somewhere in your configuration you have two mutually-dependent resources like this, or perhaps a resource referring to itself.

If you are sighted, depending on how complicated your configuration is, it might help to ask Terraform to produce a graph similar to the ones I've included above in this answer, but highlighting the cycles. To do this, you can use the terraform graph command, like this:

terraform graph -draw-cycles

The output of this command is a description of the graph in the format accepted by Graphviz. If you don't have Graphviz installed on your local computer, you could instead copy-paste the output into Graphviz Online to produce a graph image. The -draw-cycles command causes Terraform to mark the arrows that are related to the cycle being reported using the color red. If you cannot visually distinguish red from black, you may wish to first edit the generated Graphviz code to replace red with some other color you can distinguish.

The graph visualization of the configuration tends to become unusable for non-trivial configurations because there are so many graphs and edges, so if your configuration has many objects it may be better to follow the dependencies through the configuration itself.