One of the threads we've been pursuing as part of the Fedora Futures project is integration with asynchronous and/or very slow storage. We've taken on AWS Glacier as a prime, generally accessable example. Uploading content is slow, but can be done synchronously in one API request:
POST /:account_id/vaults/:vault_id/archives
x-amz-archive-description: Description
...Request body (aka your content)...
Where things get radically different is when requesting content back. First, you let Glacier know you'd like to retrieve your content:
POST /:account_id/vaults/:vault_id/jobs HTTP/1.1

  "Type": "archive-retrieval",
  "ArchiveId": String,
Then, you wait. and wait. and wait some more; from the documentation:
Most Amazon Glacier jobs take about four hours to complete. You must wait until the job output is ready for you to download. If you have either set a notification configuration on the vault identifying an Amazon Simple Notification Service (Amazon SNS) topic or specified an Amazon SNS topic when you initiated a job, Amazon Glacier sends a message to that topic after it completes the job. [emphasis added]


If you're iterating on some code, waiting hours to get your content back isn't realistic. So, we wrote a quick Sinatra app called Icemelt in order to mock the Glacier REST API (and, perhaps taking less time to code than retrieving content from Glacier ). We've tested it using the Ruby Fog client, as well as the official AWS Java SDK, and it actually works! Your content gets stored locally, and the delay for retrieving content is configurable (default: 5 seconds). Configuring the official SDK looks something like this:
PropertiesCredentials credentials = new PropertiesCredentials(
AmazonGlacierClient client = new AmazonGlacierClient(credentials);
And for Fog, something like: :aws_access_key_id => '',
                      :aws_secret_access_key => '', 
                      :scheme => 'http', 
                      :host => 'localhost', 
                      :port => '3000'
Right now, Icemelt skips a lot of unnecessary work (e.g. checking HMAC digests for authentication, validating hashes, etc), but, as always, patches are very welcome.