One of the explicit goals of ASDF is to be as future proof as possible. This involves being able to add features as needed while still allowing older libraries that may not understand those new features to reasonably make sense of the rest of the file.
The ASDF standard includes three categories of versions, all of which may advance independently of one another.
Standard version: The version of the standard as a whole. This version provides a convenient handle to refer to a particular snapshot of the ASDF standard at a given time. This allows libraries to advertise support for “ASDF standard version X.Y.Z”.
File format version: Refers to the version of the blocking scheme and other details of the low-level file layout. This is the number that appears on the
#ASDFheader line at the start of every ASDF file and is essential to correctly interpreting the various parts of an ASDF file.
Schema versions: Each schema for a particular YAML tag is individually versioned. This allows schemas to evolve, while still allowing data written to an older version of the schema to be validated correctly.
Schemas provided by third parties (i.e. not in the ASDF specification itself) are also strongly encouraged to be versioned as well.
Version numbers all follow the same convention according to the Semantic Versioning 2.0.0 specification.
major version: The major version number advances when a backward incompatible change is made. For example, this would happen when an existing property in a schema changes meaning. (An exception to this is that when the major version is 0, there are no guarantees of backward compatibility.)
minor version: The minor version number advances when a backward compatible change is made. For example, this would happen when new properties are added to a schema.
patch version: The patch version number advances when a minor change is made that does not directly affect the file format itself. For example, this would happen when a misspelling or grammatical error in the specification text is made that does not affect the interpretation of an ASDF file.
pre-release version: An optional fourth part may also be present following a hyphen to indicate a pre-release version in development. For example, the pre-release of version
Relationship of version numbers
The major number in the standard version is incremented whenever the major number in the file format version is incremented.
Schema versions are created and adjusted independently of the standad
version and the file format version. New schemas are created with version
1.0.0 and are updated according to the Semantic Versioning conventions
An update to any of the schema versions will be reflected in a bump of the standard version as well, although the version numbers will not necessarily match. Bumping a particular schema version will also require new versions of any of the schemas that make reference to it.
For example, schema
Foo has version
1.0.0 in version
1.2.0 of the
Standard. We make a backwards compatible change to
Foo and bump its version
Bar contains a reference to
Foo. The current
1.1.0, and we must now bump it to
reflect the new reference to
Foo-1.1.0. We also bump the Standard version
1.3.0 to reflect the changes to these schemas.
Handling version mismatches
Given these conventions, the ASDF standard recommends certain behavior of ASDF libraries. ASDF libraries should, but are not required, to support as many existing versions of the file format and schemas as possible, and use the version numbers in the file to act accordingly.
For future-proofing, the library should gracefully handle version numbers that are greater than those understood by the library. The following applies to both kinds of version numbers that appear in the file: the file format version and schema versions.
When encountering a major version that is greater than the understood version, by default, an exception should be raised. This behavior may be overridden through explicit user interaction, in which case the library will attempt to handle the element using the conventions of the most recent understood version.
When encountering a minor version that is greater than the understood version, a warning should be emitted, and the library should attempt to handle the element using the conventions of the most recent understood version.
When encountering a patch version that is greater than the understood version, silently ignore the difference and handle the element using the conventions of the most recent understood version.
When writing ASDF files, it is recommended that libraries provide both of the following modes of operation:
Upgrade the file to the latest versions of the file format and schemas understood by the library.
Preserve the version of the ASDF standard used by the input file.
Writing out a file that mixes versions of schema from different versions of the ASDF standard is not recommended, though such a file should be accepted by readers given the rules above.