Merge pull request #1 from volcan01010/centos7-python2

Add first draft CentOS 7 - Python 2

Author: volcan01010

_posts/2020-02-21-centos7-python2-end-of-life.md

@@ -0,0 +1,106 @@
+---
+title:  "CentOS 7, Ansible and the end of Python 2"
+author: Dr John A Stevenson
+categories:
+  - devops
+tags:
+  - Python
+  - CentOS
+  - Ansible
+  - continuous-integration
+---
+
+The [sunsetting of Python 2](https://www.python.org/doc/sunset-python-2/) on 1 January 2020 has started to cause problems with administering our CentOS 7 servers.
+We use [Ansible](https://www.ansible.com/) for automatic configuration and
+a few of our long-running jobs recently began to fail as a result Python 2 / Python 3 mistmatches.
+Officially, CentOS 7 is [supported until
+2024-06-30](https://wiki.centos.org/About/Product), but I think the end of
+Python 2 will hasten its demise.
+
+The notes below explain how we solved our issues with CentOS 7 and Ansible.
+Hopefully they will be helpful to others.
+
+
+## Background
+
+When you configure a machine with Ansible, it connects to it via SSH and runs
+shell scripts and/or Python scripts to apply the settings that you requested.
+In CentOS 7, these scripts are run by Python 2.7.
+This causes the following problems with Python tools installed using the `pip`
+package manager.  
+
+
+  - An increasing number of packages are Python 3 only, and others e.g. `docker-compose`, are not being updated for Python 2.  As Ansible's `pip` module uses the system Python 2 interpreter by default, it may fail or get an out-of-date version.
+  - Other Ansible modules (e.g. `docker-compose, `nsupdate`), rely on Python
+    libraries installed on the system.  By default, Ansible will try to use
+the Python 2 version.  Again, these may be old or may not even exist.
+
+
+## Making Ansible work with Python 3
+
+The solution to this is to make Ansible use Python 3 on the target system.  The
+following steps allow this:
+
+### Ensure Python 3 and pip are present
+
+The following tasks ensure the server is able to use Python 3.
+
+```yaml
+---
+- name: Install latest python 3 and pip3
+  yum:
+    name:
+      - python36
+      - libselinux-python
+    state: present
+
+- name: Install SELinux for Python 3
+  pip:
+    name:
+      - selinux
+    state: present
+    executable: pip3
+  vars:
+    ansible_python_interpreter: /usr/bin/python3
+```
+
+Note that `pip3` is automatically installed with Python 3.6, and that the
+SELinux dependencies are required by some Ansible modules.
+
+### Use pip3 to install packages
+
+The Ansible `pip` module has a flag to specify `pip3`.  Use this to install
+packages to the system's Python 3.
+
+```yaml
+- name: Install ETLHelper
+  pip:
+    name:
+      - etlhelper
+    state: present
+    executable: pip3
+```
+
+### Tell Ansible to use Python 3 interpreter where required
+
+Ansible uses the `ansible_python_interpreter` variable to define with Python to
+use.  This must be specified wherever a module uses a Python 3 dependency e.g.
+
+```yaml
+- name: Use dnsupdate to update ip host
+  nsupdate:
+    server: "{{ dns_server }}"
+    zone: "{{ dns_path }}"
+    record: "{{ ansible_hostname }}"
+    value: "{{ ansible_default_ipv4.address }}"
+    ttl: 600
+  vars:
+    ansible_python_interpreter: /usr/bin/python3
+```
+
+Ansible reads variable definitions according to a [defined
+hierarchy](https://docs.ansible.com/ansible/latest/user_guide/playbooks_variables.html#variable-precedence-where-should-i-put-a-variable).
+In the example above, the variable sets Python 3 as interpreter for a single
+task and Python 2 is used elsewhere.  We find this preferable as it is most
+explicit and some other modules, most notably `yum`, will only work with Python
+2.