Getting started With Ansible - Part Two (Inventory)

In the previous article Getting started with Ansible – Part 1 of the series “Learning Ansible”, we have seen the installation of Ansible and the set up of web and database servers with CentOS and Ubuntu operating systems. We also saw the creation of a user, giving the user sudo privilege, and enabling password authentication.

So, in this article, we will learn the following.

Inventory
Groups in Inventory, Group of groups
Variables in Inventory
Execute Inventory file for a host with variables different than group variables.
Escalate privilege for the user.
Ansible modules like ping, yum, apt and service.

Inventory, Groups, and Group of groups

Inventory is a text file where we define the host information that we are going to manage through Ansible.

The default path of the inventory file is /etc/ansible/hosts/ but we can also specify a different inventory file using -I <path of inventory file> on the command line.

So in continuation of the previous article Getting started with Ansible – Part 1 where we created our project directory “superman”, we are going to create an inventory file.

Step 1

Create an inventory file using the command

vim inventory

Step 2

Add the below code in the inventory file.

websrv01 ansible_host=172.31.4.33 ansible_user=devops ansible_ssh_pass=devops123
websrv02 ansible_host=172.31.15.84 ansible_user=devops ansible_ssh_pass=devops123
websrv03 ansible_host=172.31.8.231 ansible_user=devops ansible_ssh_pass=devops123
dbsrv01 ansible_host=172.31.1.205 ansible_user=devops ansible_ssh_pass=devops123
[WebServersGroup]
websrv01
websrv02
websrv03
[DBServerGroup]
dbsrv01
[DataCenter_Ncali:children]
WebServersGroup
DBServerGroup

Now let’s understand code written in the inventory file,

Here we can see that we have specified all our hosts; i.e. the IP of 3 web servers and 1 DB server and we have also given the username and password for all of them.
We can see a group of web servers named WebServersGroup. This would be really helpful if we want to configure something on all web servers then we can simply give this WebServersGroup rather than specifying the IP of each web server host machine.
We can see a group of DB servers, though as of now we have only 1 db server but it is a good practice to keep similar kind of hosts in a group to later manage them effectively.
We can also see a group called DataCenter_Ncali: children. This is a group of groups for North California region as all our servers are present in this region so we have given the group name as DataCenter_Ncali: children. This would be helpful in case we have different hosts residing in different region.
So, if we want to run a configuration then we can club them as per region and can run the configuration by giving the appropriate group name which will contain other subgroups.
DataCenter_Ncali:children is a master group and underneath specifies the other group names.

Step 3

We have to ping each of the hosts defined in the inventory file using the ping module.

To call any module in Ansible we use -m option.

So, the command will be the following.

ansible -I inventory -m ping websrv01

Output

Few things to note here,

Now, we are getting this error because whenever we do SSH to any host then for the first time it asks whether we want to connect or not and when we say yes, it saves the fingerprint of that host machine to the local host's file. So, this is the same due to which we are getting the error above.

Step 4

Thus, the same thing is happening with ansible as well and it will also get the prompt for yes or no. So, now we have to make ansible to skip this option.

So open default ansible config file at /etc/ansible/ansible.cfg and uncomment host_key_checking=False.

Now let's try again.

ansible -I inventory -m ping websrv01

Output

And we can see it has successfully pinged web server 01 and has returned pong which means success.

Step 5

We can also see ping running on the entire group DataCenter_Ncali. This will run the ping command on all web servers and the db server.

ansible -I inventory -m ping DataCenter_Ncali

Output

We can also do the same using all or * command in the end, this will execute ping on all the hosts (not all groups) mentioned in the inventory file.

ansible -i inventory -m ping all
ansible -i inventory -m ping '*'

Variables in Inventory

Variables are used to store values in programs and as the name suggests the values can be changed throughout the program.

These values are essential to decide the code flow. Ansible variables help to determine how the tasks execute on different systems based on the values assigned to these variables.

Step 6

Above in the inventory file, we saw host-level variables like ansible_user and ansible_ssh_pass but this was repetitive as we had to define this for each host.

Instead what we can do is, we can define these details at the group level using group variables.

So, the new inventory file will look something like,

websrv01 ansible_host=172.31.4.33
websrv02 ansible_host=172.31.15.84
websrv03 ansible_host=172.31.8.231
dbsrv01 ansible_host=172.31.1.205
[WebServersGroup]
websrv01
websrv02
websrv03
[DBServerGroup]
dbsrv01
[DataCenter_Ncali:children]
WebServersGroup
DBServerGroup
[DataCenter_Ncali:vars]
ansible_user=devops
ansible_ssh_pass=devops123

We can ping it again and see if it works,

ansible -I inventory -m ping *

Great… It works 😊. So, we have learned how we can use variables at the group level.

What if some host has a different username and password?

For such cases, we can define the username and password at the host level, as host-level variables have higher priority than group-level variables.

So, suppose Db server has a different user with different username and password then we can define it at the host level for Db server.

Now, for rest hosts, the group level variables will be applied but for the host with variables at the host level, group-level variables will not be applied.

Step 7

Now let’s suppose we want to install some package on all servers. For this Ansible has a yum module.

Ping module didn’t have any arguments to pass but the yum module has.

We will run the below command:

ansible -I inventory -m yum -a “name=httpd state=present” websrv01

Output

This says we need to be a root user to perform this command.

Here execution is happening with DevOps user and DevOps user is a normal user so cannot install packages but it does have sudoers privilege. So, we can escalate the privilege from the command line.

Note
In the previous article Getting started with Ansible – Part 1, we have already given this user “DevOps” root privilege in the sudoers file.

To escalate the privilege from the command line we can use - - become.

Now the command will look like,

ansible -i inventory -m yum -a "name=httpd state=present" websrv01 --become

Output

Great… It says that httpd package has been installed on web server 01. YAY… 😊

Let’s do it again,

Now it says changed as false because it has already installed the package. So this behavior is called IDEMPOTENT behavior of configuration management tools as mentioned in Introduction to Configuration Management and Ansible.

Step 8

As our Web server 3 (Ubuntu) is Debian based so we can install package there using apt module. Here package name is apache2.

ansible -i inventory -m apt -a "name=apache2 state=present" websrv03 –become
ansible -i inventory -m apt -a "update-cache=yes" websrv03 --become

So apache2 package has been installed successfully.

Step 9

Lastly, we can start the service on the web servers and for this we have service module in Ansible.

ansible -i inventory -m service -a "name=httpd state=started enabled=yes" websrv01 --become

So, the service has been started on web server 1 and similarly, we can do it for other web servers.

Summary

So, in this article, we have seen practically how we can create inventory file, how we can change global ansible config, how to set up global variables and group and how to use different modules in Ansible.

I hope you find this article helpful. Stay tuned for more … Cheers!!

You can also check out some of my previous articles of the series “Learning Ansible” here,

2 Comments

Join the conversation! Your thoughts help the community grow.

Anil Goyal6y ago
Ubuntu@Linux-Ansible-Earth:~$ ansible -i inventory -m yum -a "name=httpd state=present" websrv01 --becomeUnhandled error: Traceback (most recent call last): File "/usr/lib/python2.7/dist-packages/ansible/config/manager.py", line 559, in update_config_data value, origin = self.get_config_value_and_origin(config, configfile) File "/usr/lib/python2.7/dist-packages/ansible/config/manager.py", line 503, in get_config_value_and_origin value = ensure_type(value, defs[config].get('type'), origin=origin) File "/usr/lib/python2.7/dist-packages/ansible/config/manager.py", line 122, in ensure_type makedirs_safe(value, 0o700) File "/usr/lib/python2.7/dist-packages/ansible/utils/path.py", line 95, in makedirs_safe raise AnsibleError("Unable to create local directories(%s): %s" % (to_native(rpath), to_native(e))) AnsibleError: Unable to create local directories(/home/ubuntu/.ansible/tmp): [Errno 13] Permission denied: '/home/ubuntu/.ansible/tmp' Traceback (most recent call last): File "/usr/bin/ansible", line 62, in <module> import ansible.constants as C File "/usr/lib/python2.7/dist-packages/ansible/constants.py", line 174, in <module> config = ConfigManager() File "/usr/lib/python2.7/dist-packages/ansible/config/manager.py", line 291, in __init__ self.update_config_data() File "/usr/lib/python2.7/dist-packages/ansible/config/manager.py", line 571, in update_config_data raise AnsibleError("Invalid settings supplied for %s: %s\n" % (config, to_native(e)), orig_exc=e) ansible.errors.AnsibleError: Invalid settings supplied for DEFAULT_LOCAL_TMP: Unable to create local directories(/home/ubuntu/.ansible/tmp): [Errno 13] Permission denied: '/home/ubuntu/.ansible/tmp'
Anil Goyal6y ago
It would have been good if you would have included some commands issues. I came across some issues while running ansible -i inventory -m service -a "name=httpd state=started enabled=yes" websrv01 --become. This one need more clarity on how to setup the environments given that you are using 4 VM consisting of centos and ubuntu.