> **Page index:** https://km.oslo.systems/llms.txt lists every page on this site, with a link to its Markdown version.

# Set up application common

All applications share a common set of infrastructure components that are set up in this guide.

The stack sets up an ECS cluster for the current `Environment` and a rule for fetching public images through the ECR repository.

## Step 1: Add and configure the `app-common` package

repo-iac/environments/dev/

```
ok pkg add app-common
```

Update `package-config.yml` with your preferences.

## Step 2: Install the package

repo-iac/environments/dev/app-common/

```
ok pkg install
```

## Step 3: Initialize and apply the `app-common` stack

repo-iac/environments/dev/app-common/

```
terraform init
terraform apply
```

## Step 4: Perform initial pull for ECR

> [!NOTE]
>
> **What's this?**
>
> If you already pulled the following image:
>
> ```
> public.ecr.aws/nginx/nginx:latest
> ```
>
> You can now pull the image via the ECR pull through cache address:
>
> ```
> ${var.account_id}.dkr.ecr.${var.region}.amazonaws.com/${var.environment}-ecr-public/nginx/nginx:latest
> ```
>
> In other words, the original upstream registry URL:
>
> ```
> public.ecr.aws/
> ```
>
> Is replaced with:
>
> ```
> ${var.account_id}.dkr.ecr.${var.region}.amazonaws.com/${var.environment}-ecr-public/
> ```

ECR pull through cache is enabled by default, but can be configured with the following:

repo-iac/environments/dev/app-common/package-config.yml

```
EcrPullThroughCache:
  Enable: true
```

Each image gets a separate ECR repository and needs a unique lifecycle policy, specify the upstream images you need:

repo-iac/environments/dev/app-common/config\_override.tf

```
pull_through_cache_repositories = [
  "aws-observability/aws-otel-collector",
  "nginx/nginx-prometheus-exporter",
  "nginx/nginx"
]
pull_through_cache_ecr_max_image_count = 15
```

Then run the following to populate ECR with the latest version of `nginx/nginx`:

repo-iac/environments/dev/app-common/bin/

```
bash _gp_ecr_pull_through_cache_init.sh -i nginx/nginx
```

If a specific tag is required, use the `-t` flag:

repo-iac/environments/dev/app-common/bin/

```
bash _gp_ecr_pull_through_cache_init.sh -i nginx/nginx -t alpine-slim
```

Repeat for each image in the list defined earlier in `config_override.tf`.

Later pulls don't require internet access.

## Step 5: Verify

To verify that a new ECS cluster has been created, run the following command:

```
aws ecs list-clusters | jq '.clusterArns'
```

The output list should contain the name of the ECS cluster you just created.

Sign in to the AWS console and navigate to **ECS**.

The list should contain the name of the ECS cluster you just created.

---

> [!TIP]
>
> **Commit your files**
>
> At this stage it is a good idea to commit your files.

You're done with the Application setup.

## Next step

Set up [CI/CD common](<https://km.oslo.systems/initial-setup-of-your-aws-environment/set-up-cicd-common/index.md>).