Question

I am thinking of implementing a rake task that would sync certain files in my repository to S3. The catch is that I only want to update the files when they are changed in my Repo. So if file A gets modified and B stays the same, only file A will be synchronized to S3 during my next app deploy.

What is a reliable way to determine that a file has been modified? I am thinking of using git to determine whether the file has been changed locally…. is there any other way to do this? Does S3 provide similar functionality to this?

Answer 1

S3 does not presently support conditional PUTs, which would be the ideal solution, but you can get this behavior with two requests instead. Your sync operation would look something like:

• For each file that you want on S3:
  • Calculate the MD5 of the local file.
  • Issue a HEAD request for that S3 object.
  • Issue a PUT request if the object’s Content-MD5 differs or the object does not exist.

That said, this sounds a lot like something you’d do with assets, in which case you’d be reinventing the wheel. The Rails 3 asset pipeline addresses this problem well — in particular, fingerprinting assets and putting the hash in the URL allows you to serve them with insanely long max-age values since they’re immutable — and the asset_sync gem can already put your assets on S3 automatically.