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+---
+title: "Ansible custom facts"
+linkTitle: "Ansible custom facts"
+date: 2018-09-25
+description: >
+  How to write custom facte with ansible
+---
+
+Custom facts are actually quite easy to implement despite the lack of documentation about it.
+
+## How they work
+
+On any Ansible controlled host — that is, the remote machine that is being controlled and not the machine on which the playbook is run — you just need to create a directory at
+`/etc/ansible/facts.d`. Inside this directory, you can place one or more `*.fact` files. These are files that return JSON data, which will then be included in the raft of facts that
+Ansible gathers.
+
+The facts will be available to ansible at `hostvars.host.ansible_local.<fact_name>`.
+
+## A simple example
+
+Here is the simplest example of a fact, let's suppose we make it `/etc/ansible/facts.d/mysql.fact` :
+{{< highlight sh >}}
+#!/bin/sh
+set -eu
+
+echo '{"password": "xxxxxx"}'
+{{< /highlight >}}
+
+This will give you the fact `hostvars.host.ansible_local.mysql.password` for this machine.
+
+## A more complex example
+
+A more interesting example is something I use with small webapps. In the container that hosts the frontent I use a small ansible role to generate a mysql password on its first run, and
+provision a database with a user that has access to it on a mysql server. This fact ensures that on subsequent runs we will stay idempotents. Here is how it works.
+
+First the fact from before, only slightly modified :
+{{< highlight sh >}}
+#!/bin/sh
+set -eu
+
+echo '{"password": "{{mysql_password}}"}'
+{{< /highlight >}}
+
+This fact is deployed with the following tasks :
+{{< highlight yaml >}}
+- name: Generate a password for mysql database connections if there is none
+  set_fact: mysql_password="{{ lookup('password', '/dev/null length=15 chars=ascii_letters') }}"
+  when: (ansible_local.mysql_client|default({})).password is undefined
+
+- name: Deploy mysql client ansible fact to handle the password
+  template:
+    src: ../templates/mysql_client.fact
+    dest: /etc/ansible/facts.d/
+    owner: root
+    mode: 0500
+  when: (ansible_local.mysql_client|default({})).password is undefined
+
+- name: reload ansible_local
+  setup: filter=ansible_local
+  when: (ansible_local.mysql_client|default({})).password is undefined
+
+- name: Ensures mysql database exists
+  mysql_db:
+    name: '{{ansible_hostname}}'
+    state: present
+  delegate_to: "{{mysql_server}}"
+
+- name: Ensures mysql user exists
+  mysql_user:
+    name: '{{ansible_hostname}}'
+    host: '{{ansible_hostname}}'
+    priv: '{{ansible_hostname}}.*:ALL'
+    password: '{{ansible_local.mysql_client.password}}'
+    state: present
+  delegate_to: '{{mysql_server}}'
+{{< /highlight >}}
+
+## Caveat : a fact you deploy is not immediately available
+
+Note that installing a fact does not make it exist before the next inventory run on the host. This can be problematic especially if you rely on facts caching to speed up ansible. Here
+is how to make ansible reload facts using the setup tasks (If you paid attention you already saw me use it above).
+{{< highlight yaml >}}
+- name: reload ansible_local
+  setup: filter=ansible_local
+{{< /highlight >}}
+
+## References
+
+- https://medium.com/@jezhalford/ansible-custom-facts-1e1d1bf65db8