Terminology

These words are used throughout the spec. Here's their canonical definition:

Acontainer is a generic word for a set of files, folders, and symlinks. It isalso a protobuf message that encodes information about a set of files, folders andsymlinks such as their path (relative to the container's root), permissions and sizes,but not their contents.

Adiff is the result of a comparison between two containers,old andnew.It contains a list of operations required to buildnew's files fromold's files,re-using data when possible.

Apatch contains all the information needed to apply an upgrade: both theoldandnew container, and their diff. Patches are stored in astandard file format.

Asignature contains one container and a series of hashes corresponding to fixed-size blocksof the container's files. Signatures are stored in astandard file format.

/dev/null is the empty container: it contains no files, folders or symlinks.A diff against/dev/null is a suitable way to store the initial version ofa container.

Anarchive is a compressed container stored in a single file (e.g.build.zip,build.7z, etc.). They're the preferred way of storing a particular version ofa container, and are suitable for diffing, although at the cost of on-the-flydecompression.

Ablock is a fixed-size series of bytes that belong to a file. Folders and symlinksdo not have blocks. Files usually end withshort blocks, which are smaller thanregular blocks, except if their size is a multiple of the block size.