Backing Up Your Data With Fog

Fog, in case you haven’t heard of it, is a fantastic cloud computing library written in Ruby. It provides a unified interface to several popular cloud computing platforms(including Amazon, Rackspace, Linode, and others), making it easy to interact with them from Ruby. It currently supports four types of cloud services: storage, compute, DNS, and CDN. Fog has become very popular lately, and serves as the backbone for Chef’s cloud computing functionality, which is how I first became aware of it.

I recently used Fog to write a backup script in Ruby to automatically send encrypted database backups from a database server running at Rackspace to Amazon’s S3 storage service. Here’s how I did it.


My script runs as the second step in a process. The first step is a shell script that calls pg_dump to dump a PostgreSQL database and then encrypts the file using GnuPG, dropping them in a backup directory on the database server.

My Fog-based script’s job is to make sure that all of the files in the backup directory get moved to S3.

Writing Files

Fogsync (my script), looks at all of the files in that directory and makes sure that they all exist in a bucket on S3. If they don’t, it copies them up there. Additionally, it deletes old backups from S3. For this customer, we keep backups for 14 days, so all backups older than that get deleted.

Let’s look at how it works:

fog =
  :provider => 'AWS',
  :aws_access_key_id => MY_ACCESS_KEY,
  :aws_secret_access_key => MY_SECRET
directory =  fog.directories.get("MY_DIRECTORY")

files = Dir["/var/backup/*.gpg"]
for file in files do
  name = File.basename(file)
  unless directory.files.head(name)
    directory.files.create(:key => name, :body => open(file))

Here’s what this snippet does:

  1. Creates a connection to AWS. The syntax is basically the same for connecting to all of the cloud platforms, just the parameter names are changed.

  2. Uses ‘head’ to check if the file exists and, optionally, get some metadata about it (size, modify date, etc). Think of this as the cloud equivalent to the unix stat command. You don’t want to use the ‘get’ command, as that will return the whole file, which would take a very long time if the files are large cough*voice of experience*cough.

  3. Creates the file in the given directory (“bucket” in S3 terms) if it doesn’t exist already.

If you’ve used S3, you’ll notice that Fog uses slightly different terms for things than S3 does. Because Fog works across a number of different storage providers, it uses more general terms. While this might be confusing at first if you’re familiar with a specific provider’s nomenclature, but the tradeoff is that if you want to move from one provider to another, the only thing you have to change is the code that sets up the connection (the call to in this example).

Deleting files

      oldest = - 14 (our date)
      directory =  fog.directories.get(MY_DIRECTORY)
      files = directory.files
      files.each do |f|
        file_date = Date.parse(f.last_modified.to_s)
        if file_date < oldest

This is fairly straightforward as well. Get all the files in the directory and check their age, deleting the ones that are older than we want to keep.

So that, in a nutshell, is how to use Fog. This is a simplified example of course, in my production code the parameters are all pulled from configuration files, and the script emails a report of what it did, in addition to having a lot more error handling.

If you do any scripting with cloud computing, you owe it to yourself to check out Fog.