Understanding Stack References

We’ve created some resources. Now, let’s see how we can use outputs outside of Pulumi. In this part, we’re going to explore more about stacks, stack outputs, and stack references. Stack outputs are, as you might guess, the values exported from any given stack. These values can also be obtained from the Pulumi Service, and they’re extremely useful when you want to run commands with the CLI that reference those values. Note, though, that stack outputs are for the current stack only. If you want to get values from another stack, you want to use stack references, which bridge different stacks through inter-stack dependencies.

Stack references allow you to access the outputs of one stack from another stack. Inter-stack dependencies allow one stack to reference the outputs of another stack.

For this section, we are going to create a new Pulumi program that will bring in the stack outputs from the program we just created.

Let’s start by making our new Pulumi program in a new directory:

mkdir my-second-app
cd my-second-app
$ pulumi new python -y

Let’s go ahead and create a staging stack here as well:

$ pulumi stack init staging

Now comes the fun part! Let’s add a little code to pull in the values from the my-first-app stacks, based on the corresponding environment.

Add this code to the index.js index.ts `__main__.py` main.go MyStack.cs Program.fs MyStack.vb file inside of my-second-app.

import pulumi

config = pulumi.Config()
stack = pulumi.get_stack()
org = config.require("org")

stack_ref = pulumi.StackReference(f"{org}/my-first-app/{stack}")

pulumi.export("shopUrl", stack_ref.get_output("url"))

The org environment variable is new, as is the stack_ref declaration. That declaration sets up an instance of the StackReference class, which needs the fully qualified name of the stack as an input. Here, the org is the organization associated with your account, the my-first-app is the name of the project you’ve been working in, and the stack is the stack that you want to reference. If you have an individual account, the org is your account name. The export then grabs the url output from the other stack.

Set the org environment variable, which is the organization associated with your account, and change the <YOURNAME> to your username/account name for Pulumi:

pulumi config set org <YOURNAME>

Run pulumi up. You’ll see the value gets exported from the other project’s stack to reference in this new project’s stack:

     Type                 Name               Status
     pulumi:pulumi:Stack  my-second-app-staging

Outputs:
  + shopUrl: "http://localhost:3002"

These exported values are incredibly useful when using Pulumi stacks. For example, let’s say you have two systems that depend on one another, perhaps a frontend application with a database and a complex backend API. You might have two separate staging environments that you want to have reference one another. You can use stack references to share automatically generated connection strings from the staged API to the staged frontend application to see how they might work together.

Next up, we’re going to change gears and start exploring how Pulumi handles secrets.