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Now here is the thing, automation is super super powerful.

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So much so that when you happen to be making a very large change across many

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devices in your network,

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you might be a little intimidated to actually go through with the change.

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Because wealth automation can really support charge your ability to make

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changes and manage your devices,

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it can also support charge your ability to completely wreck the network and

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cripple all the devices in that network.

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This is just a reality of automation. If you happen to construct an erroneous
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This is just a reality of automation. If you happen to construct an erroneous

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configuration and you suddenly,

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via automation, push that change out to 1000 devices as opposed to just one

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device sequentially.

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Well, quite obviously the consequences are a lot larger.

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So wouldn't it be cool if we could actually check the effect that our

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particular change was going to make

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on our devices without actually touching those devices.

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Now the good news is that this is one of the things that we can do with Ansible

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.

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Now what I should say is that this is not an absolute full proof solution.
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Now what I should say is that this is not an absolute full proof solution.

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There are some caveats, but we will talk about it within this particular video.

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So with that said, let's dive in and see how we can check our automation

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solutions before pushing the changes out

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to our devices for real.

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Okay, so let me just create a playbook called play2.yml going to insert mode.

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Let's have our three hyphens at the top, give the playbook a name.

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And I'll just call this play2 checking changes.

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Let's target the hosts of the, let's go to blue box again for simplicity.

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And we'll set gather facts to the value false.
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So let's define our tasks.

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So let's say I want to install engine X.

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So what I will do is I will use the Ansible built in DNF module.

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And let's specify that I want to have engine X installed on my target device.

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So I'll say the state should be present on device.

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Now right now if I go to my blue box device and I say systemctl status on

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engine X.

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We can actually see here that I don't have engine X on this system.

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Now if I happen to run this playbook as it stands right now ordinarily, what
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Now if I happen to run this playbook as it stands right now ordinarily, what

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would happen is that because Ansible notices that the engine X7 is not present,

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then it will begin installing the engine X package to ensure that it is present

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on the system.

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So what I will do here is as opposed to running it as I ordinarily would write

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here,

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I can invoke check mode by saying dash dash check on the command line right

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here.

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Watch what happens now.

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Okay, so let's hit enter.

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It actually simulates what would happen.

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So it tells us a change would be made on the system.

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But if I go to my blue box here and I rerun the command that systemctl status
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But if I go to my blue box here and I rerun the command that systemctl status

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on engine X,

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it still says that the engine X service cannot be found.

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It wasn't actually applied.

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But if I run it and remove the check parameter here and to enter,

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it looks like we have the exact same output, but because we didn't run it in

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check mode,

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if I go to my device right here and check the command once again,

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now we can see here the system is indeed installed.

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So think about it now.

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Engine X is installed on a target device.

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However, let's say weeks past and I didn't actually know if it was installed or
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However, let's say weeks past and I didn't actually know if it was installed or

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not.

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If I just happened to run this particular playbook, which checks that it should

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be present,

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but it run effectively in simulation mode and check mode that is.

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And to enter the check mode this time actually says that no changes would be

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made.

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So now we can actually logically infer via simulation that the playbook,

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if it would not actually change anything in the playbook,

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it's designed to ensure that engine X is installed on the system.

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Then without the danger of potentially making any changes on the system,

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because it is purely a simulation, we can actually infer that the package must
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because it is purely a simulation, we can actually infer that the package must

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actually be

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installed on the system as it is right now.

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Really, really handy.

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Now also what we can do is let's maybe say we wanted to maybe update a

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configuration file, okay?

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So if I go into my templates directory right here and I'll just create a little

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playbook or rather,

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rather should I say a ginger to a template and I'll just call this example.j2.

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Let's just say this template is going to say hey friends,

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and I'll say my name is, and I'll just pass in the variable name, okay?
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So super, super simple template we have right here, okay?

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Now let's go back and I'll go into a playbook three right here,

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or rather create playbook three should I say.

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And again, we'll do our usual three hyphens,

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playbook three can be checking config changes.

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Again, we'll target this one device, although you can do two,

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if you still wish, we'll have gather facts to be false, and now we define our

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tasks.

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And I'll give this task your name saying push configs using templates.

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So I'm going to use the Ansible built-in template module this time,
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and now I'm going to specify that the source is going to be example.j2,

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and the destination.

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I, where do I want the file to be copied on the target device?

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I will just have it in the 10th directory, and I'll just call the message

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greeting.txt, okay?

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So what I'm going to do here is I actually want to define this particular

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variable name.

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So I'm going to my host files right here, and I'll go into the playbook,

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and I'll give the name of let's go with John, okay?

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So check this out, if I go back, and I run my little playbook right here,
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So check this out, if I go back, and I run my little playbook right here,

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and again, I just run it in check modes, it tells me that if I run this,

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a change would be made on the system, but right now it's not given me that much

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information,

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and again, just for proof, if I do an LS, notice the greeting file was not

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actually created,

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just merely simulated.

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So this time, if we happen to make a change here, and this time we run it for

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real,

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give it a moment, it looks like we have the same thing, a change should be

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applied,

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but this time, the change actually was applied, okay?

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So we do have the greeting right here.
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So we do have the greeting right here.

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Now, this is not the interesting part that I want to show you,

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I want to show you an additional flag that we can use here, and personally,

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this is one I really love.

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So let's maybe say I happen to make the modification of the variable itself,

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okay?

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So let's go into blue box, and I'll change my name here to be Simone, okay?

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Now let's go back, if I happen to run the playbook and check mode again here,

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it shows us a change would actually be made because that is a change within the

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render

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template now, the name has changed within the templates, but the cool thing is,
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template now, the name has changed within the templates, but the cool thing is,

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if I want to see what the change actually would be versus what it is right now,

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I.e. what the difference would be, if I run this playbook, I can say dash dash,

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def with my check command.

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So let's hit enter here, and look at this, we get this summary right here,

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it says if we happen to run this particular file, or rather this particular

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playbook,

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the net effect would be we would keep this particular line here,

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that would stay the same, hey friends, but this line here, my name is German,

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no longer exists, it would be replaced with this line here, my name is Simone,
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no longer exists, it would be replaced with this line here, my name is Simone,

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so imagine you happen to have some complex configurations, maybe a whole bunch

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of DNS servers

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specified within a configuration file that you're rendering, you can

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effectively see

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what changes you are going to make via the automation system before you apply

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them for real.

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So again, like I say, if I go to my device and I can't the file, the change

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hasn't

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actually been applied yet, however, I have simulated the change and I see here

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that I'm happy that I want the fail to change, and instead to read my name as
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that I'm happy that I want the fail to change, and instead to read my name as

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Simone, so what I will do now is because I know what the change is going to be,

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I just run the playbook for real.

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So now the change will actually be applied, I go to my device, let's cat this,

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look at this right here, now we see the actual change that was simulated

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beforehand,

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and again, if I run check mode once again with the diff,

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because the render template is no longer going to make a change,

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look at this right here, it simulates that there would be no changes made,

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we're getting to see the item importance of the actual module itself here.
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So if we change the variable, we can see the change, but also if we go and

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modify the

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template itself and perhaps maybe add in an additional line, let's just imagine

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we were

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adding in a DNS server, okay, we'll just say DNS server one, two, three, four,

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okay,

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whatever, it doesn't actually matter the change.

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If I now re-run the simulation once again, it tells us a change would actually

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be made,

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and it tells us we would have an additional line of DNS server one, two, three,

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four.

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Now one thing just to be aware of, with respect to your playbooks, you may

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actually have
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some conditionals that do require particular changes to actually be running on

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the system

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in order for a particular task to execute, you may make an actual change,

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and if the answerable system detects that change on the live system,

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then another task might execute, perhaps you're running a handler, that type of

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thing.

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However, because of this, the nature of the simulation, because it won't really

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make that

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change, it might give you an erroneous output.

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So well as this can be very, very effective, understand the limitations,
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be very aware that when you use conditionals, not all the output is going to be

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100% accurate,

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so still proceed with caution is what I'm saying here.

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So just with respect to our check mode, we can actually run check mode on a

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particular task,

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as opposed to the entire playbook.

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So if I go in to my playbook right here, if I just happen to specify that this

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particular task

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has check_mode set to the value of yes, and let's do one similar here okay, let

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's create another task,
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in order to push other conflicts using template, and again I'll do something

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very similar.

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Let's use another template called example2.j2, the destination is going to be

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temp,

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and just call us one another greeting.txt.

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Notice this one here has to check mode enabled for this particular task, and

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this one does not.

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All I'm going to do here very briefly is I'll go into my templates again, and I

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'll create a file called example2.j2,

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go into insert mode, order save bonjour, as another greeting, hello from name.
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Okay, so let's save this right here, and if I go back, and all I will do is I

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'll go into the

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host files again, and all I will do is I'll update the name, this time to be

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James, okay, so let's save this.

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So if I happen to run the playbook right here, and to enter, we can see here

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the two tasks apparently have

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operated, which should mean that the greeting.txt here, that file should be

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updated to reflect the change of

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James. However, when I count the greeting file, notice it still is Samona, it

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was ran in check mode,

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however, the other task, which was not configured to run as check mode, that
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however, the other task, which was not configured to run as check mode, that

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did run for real to be really

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should have an example to .j2 rendering, creating the file, another greeting.

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txt.

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So do an LS, notice we do have this particular file, another greeting, so let's

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cut this.

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Look at this right here, this particular change was indeed applied to the live

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system.