Astro

Astro is a tool for managing multiple Terraform executions as a single command.

Features:

Declarative configuration for modules to execute
Dependencies between modules
Fast, concurrent executions of Terraform operations
Safe Terraform upgrades and state file migrations

NOTE: Astro is currently experimental.

Getting started

Installation

Install Astro using go get (Go >1.12 required):

GO111MODULE=on go get github.com/uber/astro/astro/cli/astro

This will install a binary called astro in your $GOPATH/bin.

Alternatively, you can download precompiled binaries from the Github releases page.

Note that from version 0.6.0 tvm, a tool to download and install specific versions of Terraform for your platforms, is packaged together with astro.

Configuration

Astro looks for a configuration file called astro.yaml in the current or parent directories. It is recommended to place this file in the same top-level directory of your project where the Terraform code exists (e.g. terraform/astro.yaml).

An example astro configuration could look like:

---

terraform:
  version: 0.11.7

hooks:
  startup:
    - command: assume-role --role terraform
      set_env: true

modules:
  - name: app
    path: core/app
    deps:
      - module: users
      - module: vpc
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: "{{.aws_region}}/app-{{.environment}}.tfstate"
        region: us-east-1
    variables:
      - name: region
      - name: environment
        values: [dev, prod]

  - name: database
    path: core/database
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: "{{.aws_region}}/database-{{.environment}}.tfstate"
        region: us-east-1
    variables:
      - name: region
      - name: environment
        values: [dev, prod]

  - name: mgmt
    path: core/mgmt
    deps:
      - module: vpc
        variables:
          environment: mgmt  # depends on vpc/mgmt
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: "{{.aws_region}}/mgmt-{{.environment}}.tfstate"
        region: us-east-1
    variables:
      - name: region

  - name: users
    path: core/users
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: global/users
        region: us-east-1

  - name: vpc
    path: core/vpc
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: "{{.aws_region}}/vpc-{{.environment}}.tfstate"
        region: us-east-1
    variables:
      - name: region
      - name: environment
        values: [mgmt, dev, prod]

Planning

You can run a plan across all modules by doing:

astro plan --region us-east-1

--region in this example is one of the variables defined in the module configuration above with no predefined value, so it must be provided at the command line.

Astro will show the results of the plan for each execution:

> astro plan --region us-east-1
users: OK No changes (7s)
vpc-mgmt-us-east-1: OK No changes (15s)
vpc-dev-us-east-1: OK No changes (31s)
vpc-prod-us-east-1: OK No changes (28s)
database-dev-us-east-1: OK No changes (9s)
database-prod-us-east-1: OK No changes (10s)
app-dev-us-east-1: OK No changes (10s)
app-prod-us-east-1: OK No changes (11s)
mgmt-us-east-1: OK No changes (43s)
>

If there is a change, the plan will be shown, e.g.:

> astro plan --region us-east-1 --modules app
app-dev-us-east-1: OK Changes (10s)

  ~ module.app.aws_s3_bucket.app-data
      versioning.0.enabled: "false" => "true"

app-prod-us-east-1: OK Changes (11s)

  ~ module.app.aws_s3_bucket.app-data
      versioning.0.enabled: "false" => "true"
>

Upgrading

Upgrading Terraform is as easy as changing the version in the config, e.g.:

diff --git a/terraform/astro.yaml b/terraform/astro.yaml
index 5725a36d..c0ef720f 100644
--- a/terraform/astro.yaml
+++ b/terraform/astro.yaml
@@ -1,7 +1,7 @@
 ---

 terraform:
-  version: 0.10.5
+  version: 0.11.7

 modules:
  - name: app

Astro will automatically download the new version when it needs it next.

Detaching from the remote

Older versions of Terraform had the ability to disable the remote state, which was useful for performing safe upgrades or migrations.

Astro restores this ability using the --detach command to plan, e.g.:

astro plan --detach

This will create a session directory with a sandbox containing a copy (hard links) of the Terraform code, along with a local copy of the state file:

> ls terraform/.astro/01CGC80C81CJFPFCCM0F1FRKDJ/app/sandbox/core/app/terraform.tfstate
terraform/.astro/01CGC80C81CJFPFCCM0F1FRKDJ/app/sandbox/core/app/terraform.tfstate

If you need to test anything, you can change directory within the sandbox without affecting the remote.

Hooks

Astro can run run external commands both at startup or before the execution of a module. If set_env is true, Astro will parse command output for NAME=value pairs, and set those as environment values.

This can be useful, for example, when using an assume-role script to assume an AWS role that requires MFA authentication. If the script outputs AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and AWS_SESSION_TOKEN to standard output, then it can be used as a startup hook by Astro to transparently change role before running Terraform.

Use cases

Dynamic environments

When running a terraform plan or terraform apply, you can specify custom variables at the command line (using -var foo=bar). This can be used to dynamically deploy to a particular environment, or region, for example.

Astro allows you to specify these variables at runtime, or filter a set of predefined ones.

In the example configuration above, the "app" and "database" modules are deployed to two different environments ("dev" and "prod") by invoking Terraform with different -var environment=<value> flags set.

What is happening behind the scenes is the module configuration generates a list of "executions", which is a Cartesian product of each set of possible variable values, plus the user-provided values at run time.

Each execution is then run in parallel, taking into considerations dependencies that modules may have on one another.

Targeted deploys

Given a list of predefined environments, the user can "filter" which executions are run. For example, the following would run only the executions with enviroment=dev:

astro plan --enviroment dev

The result would be:

> astro plan --region us-east-1 --environment dev
vpc-dev-us-east-1: OK No changes (31s)
database-dev-us-east-1: OK No changes (9s)
app-dev-us-east-1: OK No changes (10s)
>

Remapping CLI flags

Astro is meant to be used every day by operators. If your Terraform variable names are long-winded to type at the CLI, you can remap them to something simpler. For example, instead of typing --environment dev, you may wish to shorten this to --env dev.

You can specify a flags: block in your project configuration, like:

flags:
  environment:
    name: env
    description: Environment to deploy to