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+# Cloud init
+
+1. [TL;DR](#tldr)
+2. [Merge 2 or more files or parts](#merge-2-or-more-files-or-parts)
+   1. [In Terraform](#in-terraform)
+3. [Further readings](#further-readings)
+4. [Sources](#sources)
+
+## TL;DR
+
+```sh
+# Get the current status.
+cloud-init status
+cloud-init status --wait
+
+# Verify that cloud-init received the expected user data.
+cloud-init query userdata
+
+# Assert the user data we provided is a valid cloud-config.
+# From version 22.2, drops the 'devel' command.
+cloud-init devel schema --system --annotate
+cloud-init devel schema --config-file '/tmp/user-data'
+
+# Check the raw logs.
+cat '/var/log/cloud-init.log'
+
+# Parse and organize cloud-init.log events by stage.
+cloud-init analyze show
+
+# Manually run a single cloud-config module onceafter the instance has booted.
+sudo cloud-init single --name 'cc_ssh' --frequency 'always'
+
+# Clean up everything so cloud-init can re-run.
+sudo cloud-init clean
+
+# Re-run all.
+sudo cloud-init init
+```
+
+```yaml
+#cloud-config
+
+# Sources:
+# - https://github.com/trajano/terraform-docker-swarm-aws/blob/master/common.cloud-config
+
+# Add the Docker repository
+# https://cloudinit.readthedocs.io/en/latest/topics/modules.html#yum-add-repo
+# https://cloudinit.readthedocs.io/en/latest/topics/examples.html#adding-a-yum-repository
+#
+# Got from the official installation guide at
+# https://docs.docker.com/engine/install/rhel/#install-using-the-repository :
+#   yum install -y yum-utils && \
+#   yum-config-manager --add-repo \
+#     https://download.docker.com/linux/rhel/docker-ce.repo && \
+#   cat /etc/yum.repos.d/docker-ce.repo
+yum_repos:
+  docker-ce:
+    name: Docker CE Stable - $basearch
+    enabled: true
+    baseurl: https://download.docker.com/linux/rhel/$releasever/$basearch/stable
+    priority: 1
+    gpgcheck: true
+    gpgkey: https://download.docker.com/linux/rhel/gpg
+
+# Upgrade the instance
+# Deactivated as this could take a long time if the image is old
+# https://cloudinit.readthedocs.io/en/latest/topics/modules.html#package-update-upgrade-install
+# https://cloudinit.readthedocs.io/en/latest/topics/examples.html#run-apt-or-yum-upgrade
+package_upgrade: false
+package_reboot_if_required: false
+
+# Install required packages
+# This will always update the list of packages, regardless of package_update's value.
+# https://cloudinit.readthedocs.io/en/latest/topics/modules.html#package-update-upgrade-install
+# https://cloudinit.readthedocs.io/en/latest/topics/examples.html#install-arbitrary-packages
+#
+# docker-ce already depends on docker-ce-cli and containerd.io
+packages:
+packages:
+  - docker-ce
+  - jq
+  - unzip
+```
+
+## Merge 2 or more files or parts
+
+FIXME
+
+See [Merging User-Data sections] for details.
+
+```yaml
+#cloud-config
+packages:
+  - jq
+  - unzip
+
+---
+merge_how:
+ - name: list
+   settings: [append]
+ - name: dict
+   settings: [no_replace, recurse_list]
+
+packages:
+  - parallel
+
+---
+packages:
+  - vim
+
+merge_type: 'list(append)+dict(recurse_array)+str()'
+```
+
+### In Terraform
+
+1. create a data resource containing the files in order, one per part:
+
+   ```hcl
+   # https://registry.terraform.io/providers/hashicorp/cloudinit/latest/docs
+   # https://github.com/chrusty/terraform-multipart-userdata/blob/master/example/cloudinit.tf
+   data "cloudinit_config" "vm" {
+     gzip = true
+     base64_encode = true
+
+     part {
+       content      = file("files/first.yaml")
+       content_type = "text/cloud-config"
+     }
+     …
+     part {
+       content      = file("files/n-th.yaml")
+       content_type = "text/cloud-config"
+       filename     = "n-th.yaml"
+       merge_type   = "dict(recurse_array,no_replace)+list(append)"
+     }
+   }
+   ```
+
+1. give its rendered form as input to a vm's userdata attribute or an output resource:
+
+   ```hcl
+   resource "azurerm_linux_virtual_machine" "vm" {
+     user_data = data.cloudinit_config.vm.rendered
+     …
+   }
+
+   output "cloudinit_config" {
+     value = data.cloudinit_config.vm.rendered
+   }
+   ```
+
+## Further readings
+
+- [Website]
+- [Modules]
+- [Examples]
+- [Merging User-Data sections]
+- [cloud-init multipart encoding issues]
+- [Test cloud-init with a multipass container]
+
+## Sources
+
+- [Debugging cloud-init]
+- [Tutorial]
+- [Cloud-Init configuration merging]
+
+<!-- cloud-init documentation -->
+[debugging cloud-init]: https://cloudinit.readthedocs.io/en/latest/topics/debugging.html
+[examples]: https://cloudinit.readthedocs.io/en/latest/topics/examples.html
+[merging user-data sections]: https://cloudinit.readthedocs.io/en/latest/topics/merging.html
+[modules]: https://cloudinit.readthedocs.io/en/latest/topics/modules.html
+[tutorial]: https://cloudinit.readthedocs.io/en/latest/topics/tutorial.html
+[website]: https://cloud-init.io/
+
+<!-- external references -->
+[cloud-init configuration merging]: https://jen20.dev/post/cloudinit-configuration-merging/
+[cloud-init multipart encoding issues]: https://github.com/hashicorp/terraform/issues/4794
+[test cloud-init with a multipass container]: https://medium.com/open-devops-academy/test-cloud-init-with-a-multipass-containers-e3e3bb740604