As previously described, the BlobSync library (Github, Nuget, Blog) can be used to update Azure block blobs without having to upload the entire file/blobs. It perform an intelligent delta calculation and uploads the minimal data possible.
So, what’s next?
To show possible use cases for BlobSync, this post will outline how it is easily possible to create a backup application that not only uploads the minimal data required but also keeps a series of backups so you can always restore a previously saved blob.
The broad design of the program is as follows:
- Allow uploading (updating) of blobs.
- Allow downloading (updating of local files) of blobs
- Allow multiple versions of blobs to exist and prune what we don’t want.
For this I’m using Visual Studio 2013, other versions may work fine but YMMV. The version of BlobSync I’m using is the latest available at time of writing (0.3.0) and can be installed through Nuget as per any other package (for those who are new to Nuget, please see the Nuget documentation).
Of the three requirements listed above only the last one really adds any new functionality above BlobSync. For the upload/download I really am just using a couple of equivalent methods in BlobSync. For the multiple versions we need to figure out which approach to use.
What I decided on (and has been working well) is that for updating of an existing blob, the following process is used:
- Each blob will have a piece of metadata which has the latest version number of the blob
- On upload the existing blob is copied to another blob with the name <original blob name>.v<latest version number>. (along with paired signature blob)
- New delta is uploaded against existing blob.
For example, say we have a blob called “myfile”. This means we also have a “myfile.0.sig” which is the paired signature blob.
When we upload a new version of myfile the following happens:
- copy myfile to myfile.v.1
- copy myfile.0.sig to myfile.v.1.0.sig
- upload delta against myfile
This means that myfile is now the latest version and myfile.v.1 is the version that previously existed. If we repeat this process then again myfile will be the latest and what used to be myfile will now be myfile.v.2 and so on. It should be noted that the copying of the blobs is performed by the brilliantly useful Azure CopyBlob API which allows Azure it copy the blob itself and doesn’t require any traffic between the application and Azure Blob Storage. This is a BIG time saver!
Now that we’d have myfile, myfile.v.1 and myfile.v.2 we should also be able to use this new project to download any version of the file. More importantly be able to just download the deltas to reduce bandwidth usage (since that is the aim of the game).
So this is the high level design in mind… you might want to look at the implementation.