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- This video is about Fact Caching.

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So, after fact gathering,
facts are kept in-memory

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and stay there for the
duration of playbook execution.

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That is not very efficient, is it?

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But it is what is needed

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if you need your facts to
reflect the accurate state.

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If that's not needed,

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you can use plugins to
enable fact caching.

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And when you use a plugin,

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facts should be gathered
manually or as a scheduled job

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that is executed after
expiration of the fact cache.

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And next, all playbooks can
run without fact gathering

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and access their facts
from the fact cache.

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Until the cache has expired,

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then you will get an error message

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and then you will know

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that you need to do fact gathering again.

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I have listed all the
steps that are required

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to set up redis-based fact
cache, let me show you.

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So, to start with I'd like to show you,

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ansible-doc -t cache -l,

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where you can see all the different

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caching mechanisms that can be used.

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Definitely the easiest solutions
are either based on redis

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or on memcached.

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Memcached is quite workable as well.

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And hey, ansible-doc -t cache redis,

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do we have any usable information?

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Well, as you can see it's
showing some prefixes,

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but not really a very good documentation.

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So, let me be your documentation.

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To start with sudo dnf install -Y redis.

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I'm doing that on a control note

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and I'm doing that on a control
note for a simple reason,

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that that is where you are
going to run your fact cache.

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Of course, you can use a
playbook as well to do this

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but hey, this time I'm
going for the quick solution

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and I am just going to straight

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install these different components.

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So, sudo systemctl enable -- now redis

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will start the redis in-memory database.

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Redis in fact is a very simple
in-memory key value store.

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So, it's a database, an in-memory database

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as simple as simple can be.

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The next command is an important one.

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We see sudo pip3 install redis.

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This is where we are using redis

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and we are adding redis
functionality to Python.

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In order to do so we need pip3 install.

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You can safely ignore the
warning that is shown here.

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I do want this redis part

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to be available throughout all my system.

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So, I am fetching it
using sudo functionality.

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Now, this is interesting
because you will notice

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and you will notice it over and over again

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in the next lessons that
for enhanced functionality

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you need to install
additional stuff in Python

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like right here, this redis stuff.

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So, we need to wait for a little bit

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until that is done and
then we can continue.

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So, it is done.

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Let's configure this fact
cache in ansible.cfg,

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and in ansible.cfg you
do your fact caching

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under the defaults setting.

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So, use fact caching is redis

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and fact caching timeout

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is 3,600 or 36,000 if you
want, that really depends.

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This fact caching timeout

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in fact is a very important parameter

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because this is the default
timeout that you will be facing.

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And by default that will be an hour.

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If you want your fact to be
more accurate, make it shorter.

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If you want your fact cache
to be valid for a longer time,

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make it longer.

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All depends on the dynamics that you want

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from your fact cache.

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Next, I need to specify
fact caching connection

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equals localhost:6379
which is the port :0,

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where zero is the redis instance.

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And that is what we need to do

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in order to configure the redis plugin.

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Let's get back to the slides

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to learn how we are going
to use this fact cache.

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So, in order to use the fact cache,

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first step is to make sure

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that you have something
in your fact cache.

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Then you can use a checkcache.py script,

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it is provided in the plugins
Git repository directory.

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And next you can use
ansible-playbook checkfacts.yaml

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to check how it is working.

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Let me demonstrate this.

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So, for your convenience
in the Git repository,

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there is a separate subdirectory
with the name fact cache.

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And in fact cache

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we have everything that
we need for this demo.

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So, the first thing I would
like to show you is checkcache.

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Checkcache is a simple Python script.

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And in this simple Python script,

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we can see key ansible_facts
+ kubuntu.2.4 ansible 1.

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This is going to work with

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the ansible facts that have
been cached for ansible 1

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and we print ansible_memfree megabytes.

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So, what do you think is going to happen

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if I run it right now?

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I am getting a non-type
pretty curious error message

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but the thing is that we are addressing

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something that does not exist.

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Now, I am going to use sudo
systemctl status redis.

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I wanna make sure that it
is up and running, it is.

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So, now I'm going to
use ansible all -m setup

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which is going to populate the fact cache.

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And let me run this check
cache Python script again

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and there we go, we have a value, 123

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and this is obtained from the fact cache.

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That's the point I want to make here.

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So next, we can have a
look at checkfacts.yaml

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and what do we see in checkfacts.yaml?

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We see that gather facts is set to False

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and debug anyways is going to use

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the variable ansible_facts.

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That's a very simple playbook

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but it doesn't have to
be any more complicated

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to show you that fact
catching indeed is working.

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As you can see and wow,
was that fast or wasn't it?

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It sure was.

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You can see that this playbook

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is working from the fact cache.

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Do notice that at some
point sooner or later,

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the fact cache will expire.

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And if the fact cache is expired,

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you will need to run
your fact cache again.

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So, it doesn't require a
little bit of manual work.

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Fact caching is cool,

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if you don't need your facts
to contain actual values

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but there's also a couple
of things to watch out for.

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Let's go check that out.

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So, while using a fact cache,

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there's a couple of things to observe.

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First, once the fact cache expires,

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all playbooks that require facts

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and have gather_facts:False will fail,

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obviously because they will
be facing undefined variables.

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So, you might want to run a scheduled job

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to refresh the fact cache automatically.

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And to ensure that a
playbook does not use cache,

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but gather facts instead,

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you can use ansible-playbook

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whatever.yaml with --flush-cache option,

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- -flush cache option allows you to use

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the actual value of your facts.

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And that is something
you definitely have to do

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if you have playbooks that depend on

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for instance the amount
of available memory,

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that's a very dynamic value.

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You really must make sure

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that you are using the actual facts.