Notes

In this episode, we worked on Postgres database backups and modified the backup tool, wal-e, to use the Shiv app format.

wal-e is Postgres database backup tool that works by managing the Write-Ahead Log (WAL) that a Postgres database produces. The WAL is the log of recent changes that happened in the database. If you have access to a full WAL, then you can conceivably reproduce a Postgres database’s data.

When I switched the deployment to use a Shiv app, we now have to remove every use of the existing virtual environment in the system. The wal-e executable is installed in the bin directory of the virtual environment so we need another way to access it.

The way that the Shiv app can execute other binaries is with the SHIV_ENTRY_POINT environment variable. Using the setuptools-style link of wal_e.cmd:main, we can run the wal-e executable that is available to the Shiv app. Because I already use the envdir command, we only had to make two changes:

1. Replace {{ venv_path }}/bin/wal-e with /srv/apps/conductor.pyz.
2. Add a SHIV_ENTRY_POINT file to /etc/wal-e/env with the appropriate setuptools-style value.

Once we made the changes, I deployed things to the staging site with Ansible. From there, we verified that the cron entry was changes with:

$ sudo su -  # change to root
$ su - postgres  # change to postgres account
$ crontab -l  # list cron entries

With the wal-e updates in place, nothing used the Python virtual environment so we could finally delete it!

In the next stream, we’re going to begin the process of removing the Git clone to complete the process of simplifying deployment.