Even though I’m full steam ahead on the Docker train (it should really be a ship, to keep in line with the container metaphors), it was time to look at what everyone else is using for provisioning and configuration management.
Here’s my overview of getting Ansible up and running using my laptop as the control node and a few EC2 nodes, as well as a brief description of the components. And judging by the quality of the documentation, which is quite meaningful in my book, this is one excellent piece of software.
Local installation on macOS using Homebrew
- Install locally via
brew install ansible
- Inventory stored in
- YAML format seems cooler and easier to manage
- Hosts / group variables can be installed in subdirectories, e.g.
- ping all hosts:
ansible all -m ping
- print environment variables for all hosts, specifying user and private key in script command:
ansible all -a "printenv" -u ec2-user --private ~/.ssh/my-private-key.pem
The Ansible “inventory” describes the available hosts. Default location for inventory file is
all: children: webservers: hosts: 220.127.116.11: ansible_user: ec2-user ansible_ssh_private_key_file: ~/.ssh/my-private-key.pem ansible_become: true 18.104.22.168: ansible_user: ec2-user ansible_ssh_private_key_file: ~/.ssh/my-private-key.pem ansible_become: true vars: ntp_server: ntp.atlanta.example.com proxy: proxy.atlanta.example.com dbservers: hosts: 22.214.171.124: ansible_user: ec2-user ansible_ssh_private_key_file: ~/.ssh/my-private-key.pem ansible_become: true
Patterns represent the targets for the script execution, i.e. which hosts to communicate with. The generic format is:
ansible <pattern_goes_here> -m <module_name> -a <arguments>
An example execution would be:
ansible webservers -m service -a "name=httpd state=restarted"
Patterns — Ansible Documentation
The pattern can specify:
- Individual Hosts
- All hosts
- Groups (=sets of hosts)
- Groups of hosts that exclude other groups, e.g. hosts that are in group A but not in Group B
- The intersection of two groups, i.e. hosts that have to be in two groups
- Pretty much anything else that’s possible in Python
Ad-hoc commands are commands that are not part of a playbook. They are executed using the
-a flag in the ansible script, usually in combination with a specific Ansible module indicated by the
Here’s an example to print out the PATH in a single host:
ansible dbserver -m shell -a "echo $PATH"
Copy files to servers and set correct permissions
One excellent use case listed in the tutorial for ad-hoc commands is copying a test file to all servers and set the correct permissions. This assumes that the user on the host has the permission to adjust file permissions.
This example creates a sample text file and sends it to all hosts using the
$ touch test.txt $ echo 'This is just a test file' > test.txt $ ansible all -m copy -a "src=/Users/danny/Desktop/test.txt dest=/tmp"
Change permissions for sample text file
This example uses the
file module to change the permissions of a file on specific hosts:
$ ansible webservers -m file -a "dest=/tmp/test.txt mode=600"
Managing packages on hosts
Assuming your server is running a CentOS-based distro, you can use the
yum module to install, remove, check, or update modules: Managing Packages
Managing services on hosts
You can start/stop or restart services easily on target hosts, e.g. make sure the cron daemon is running:
ansible dbservers -m service -a "name=crond state=started"
Deployments using Git
I have to admit that this is blowing my mind a bit, especially after trying a lot of different ways to deploy code. My last pre-Docker deployment method was AWS CodeDeploy, which requires a CodeDeploy agent to be installed on all targets. Granted, the Ansible git module requires Git to be installed, and most likely configured to pull files securely, but it seems a lot more straightforward to pull code and adjust the permissions accordingly for security.
Playbooks are described as the “instruction manuals” in the Ansible toolbox. Comparing them to a containerized environment on AWS, I would describe them as coming closest to a task definition, which contains the Docker images required for different services.
Playbooks are written in YAML and contains one more or “plays” in a list.
Plays is Ansible’s sports analogy, where each play maps a host or group of hosts to roles, which consists of tasks. Here’s an example with just one play:, which contains three tasks: Update Apache to the latest version, write the config file, and make sure apache is running.
--- - hosts: webservers vars: http_port: 80 max_clients: 200 remote_user: root tasks: - name: ensure apache is at the latest version yum: name=httpd state=latest - name: write the apache config file template: src=/srv/httpd.j2 dest=/etc/httpd.conf notify: - restart apache - name: ensure apache is running (and enable it at boot) service: name=httpd state=started enabled=yes handlers: - name: restart apache service: name=httpd state=restarted
Task items can also be broken up into dictionaries for better readability:
--- - hosts: webservers vars: http_port: 80 max_clients: 200 remote_user: root tasks: - name: ensure apache is at the latest version yum: name: httpd state: latest - name: write the apache config file template: src: /srv/httpd.j2 dest: /etc/httpd.conf notify: - restart apache - name: ensure apache is running service: name: httpd state: started handlers: - name: restart apache service: name: httpd state: restarted
Each play contains a list of tasks. Tasks are executed in order, one at a time, against all machines matched by the host pattern, before moving on to the next task. It is important to understand that, within a play, all hosts are going to get the same task directives. It is the purpose of a play to map a selection of hosts to tasks.
The goal of each task is to execute a module, with very specific arguments. Variables, as mentioned above, can be used in arguments to modules.
When running the playbook, which runs top to bottom, hosts with failed tasks are taken out of the rotation for the entire playbook. If things fail, simply correct the playbook file and rerun.
- Every task should have a human-readable name, although not required
- Most tasks take
key=valuearguments, or a yaml dictionary:
tasks: - name: make sure apache is running service: name=httpd state=started
tasks: - name: make sure apache is running service: name: httpd state: started
shellmodule are the only tasks which take a list of arguments, and care about return codes, unless the
ignore_errorsflag is enabled.
Handlers: Basic event listener system for running operations on change
- Task can contain
notifysections, which define the
- Handlers are essentially lists of tasks that get triggered by the
- Handlers are referenced by a globally unique name
- Handlers are notified by notifiers
- Handlers will only run once per play, even if contained by multiple tasks
Here’s an example of a tasks that notifies two handlers.
- name: template configuration file template: src=template.j2 dest=/etc/foo.conf notify: - restart memcached - restart apache handlers: - name: restart memcached service: name=memcached state=restarted - name: restart apache service: name=apache state=restarted
Listening handlers: Easy way to trigger multiple handlers
Instead of having the task specify all the handlers required to successfully restart the web server, handlers can listen to generic topics. This allows handlers to be combined as groups.
The following example has two handlers that listens to the
restart web services topic:
handlers: - name: restart memcached service: name=memcached state=restarted listen: "restart web services" - name: restart apache service: name=apache state=restarted listen: "restart web services" tasks: - name: restart everything command: echo "this task will restart the web services" notify: "restart web services"
Sample playbook: WordPress on Amazon AMI
There are a number of good starter playbooks on Github.. I’m giving the
wordpress-nginx playbook a try on an Amazon AMI and found out that it needed some tweaking. The base AMI is
Amazon Linux AMI 2017.09.1.20171103 x86_64 HVM.
- The Amazon AMI has renamed its MySQL-python package to
default_serverin the NGINX configuration conflicts with the one setup in the playbook, so the default one needs to be removed
Here’s the sample code that worked in combination with this specific version of the Amazon Linux AMI 2017.09.1.20171103 x86_64 HVM.