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  3. Understanding Stack Outputs

Understanding Stack Outputs

Now let’s explore stack outputs. Stack outputs are values exported by a given stack. These values are shown during an update, can be retrieved with the Pulumi CLI, and are displayed in Pulumi Cloud once you’ve exported them. Examples include resource IDs, computed IP addresses, and DNS names. They’re useful in many ways, such as when you want to run CLI commands that reference those values.

To illustrate how stack outputs work, let’s set one programmatically. At the end of index.js index.ts __main__.py main.go Program.cs Program.fs Program.vb App.java Pulumi.yaml in my-first-app, add the following line:

export const url = pulumi.interpolate`http://localhost:${frontendPort}`;

pulumi.export("url", pulumi.Output.format("http://localhost:{0}", frontend_port))

Now, run pulumi up:

$ pulumi up

Previewing update (dev)

View in Browser: https://app.pulumi.com/***/my-first-app/dev/previews/...
...

Updating (dev)

View in Browser: https://app.pulumi.com/***/my-first-app/dev/updates/3
...

    pulumi:pulumi:Stack my-first-app-dev running
    pulumi:pulumi:Stack my-first-app-dev

Outputs:
    url: "http://localhost:3001"
...

Notice that there is now a stack output for the value of the key url.

We can get this value by running pulumi stack output <key> on any particular stack.

$ pulumi stack output url
http://localhost:3001

We can use it to open our website in a browser:

$ open $(pulumi stack output url)

Before moving on, destroy the dev stack, as we’ll no longer need it for this tutorial:

$ pulumi destroy --yes
...

Destroying (dev)

     Type                         Name                  Status
 -   pulumi:pulumi:Stack          my-first-app-staging  deleted (0.16s)
 -   ├─ docker:index:Container    backendContainer      deleted (0.43s)
 -   ├─ docker:index:Container    mongoContainer        deleted (0.51s)
 -   ├─ docker:index:Container    frontendContainer     deleted (0.31s)
 -   ├─ docker:index:RemoteImage  mongoImage            deleted (0.60s)
 -   ├─ docker:index:RemoteImage  frontendImage         deleted (1s)
 -   ├─ docker:index:RemoteImage  backendImage          deleted (1s)
 -   └─ docker:index:Network      network               deleted (2s)

Outputs:
  - url: "http://localhost:3001"

Resources:
    - 8 deleted

Duration: 6s

Making a stack configurable

One of the benefits of using Pulumi stacks is being able to configure each one independently. In this example, you’ll learn how to do so by applying different settings to the dev and staging stacks.

For the dev stack, your work is already done; the stack itself may be empty (we just destroyed it above), but its configuration settings are still there in Pulumi.dev.yaml in case you need them again. Let’s take a look. First, make sure the dev stack is still active:

$ pulumi stack select dev

Then run the following command to show all of its configuration values:

$ pulumi config
KEY              VALUE
backendPort      3000
database         cart
frontendPort     3001
mongoHost        mongodb://mongo:27017
mongoPort        27017
nodeEnvironment  development
protocol         http://

Now let’s set the configuration for the staging stack — only this time, we’ll use a different value for the frontendPort:

$ pulumi stack select staging

$ pulumi config set frontendPort 3002
$ pulumi config set backendPort 3000
$ pulumi config set mongoPort 27017
$ pulumi config set mongoHost mongodb://mongo:27017
$ pulumi config set database cart
$ pulumi config set nodeEnvironment development
$ pulumi config set protocol http://

In addition to setting configuration values individually, you can also create a new stack and apply the settings from an existing stack in one step using --copy-config-from:

pulumi stack init staging --copy-config-from dev

You should now have two stack-configuration files in your project folder: Pulumi.dev.yaml and Pulumi.staging.yaml. If you take a look at them, you’ll see each one has the value for frontendPort set (along with some other values we set in the Fundamentals tutorial):

$ cat Pulumi.staging.yaml

config:
  my-first-app:backendPort: "3000"
  my-first-app:database: cart
  my-first-app:frontendPort: "3002"
  my-first-app:mongoHost: mongodb://mongo:27017
  my-first-app:mongoPort: "27017"
  my-first-app:nodeEnvironment: development
  my-first-app:protocol: http://

Now, if you run pulumi up on the staging stack, you’ll see that the frontend port is 3002:

$ pulumi up

Previewing update (staging)
...
...

Outputs:
    url: "http://localhost:3002"
...

Next up, we’ll explore how to share outputs with other stacks. Let’s go!

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