The Digital Succession Identifier (DSI) is a persistent identifier for bibliographic references. This specification of DSI is specific to the initial implementation using git.
For a non-technical background and motivation for this technology,
refer to
A digital succession contains digital objects,
which are fixed finite collections of bits.
Examples of digital objects include computer files and file system directories (folders).
Both
In addition, a digital succession is digitally signed.
In this specification, the digital signature is made using an SSH signing key via
In the git implementation, the genesis record is a signed initial git commit with an
empty tree (and no parent). To expand the digital succession, additional git commits
are made using the same signing key. The git tree of each commit is not a
digital object in the succession. Instead, it is a record of all the
digital object editions in the succession. The top level directory consists of
subdirectories named as non-negative integers. Each subdirectory
contains either an entry named
In the simplest scenario, edition numbers are positive integers. However, multilevel edition numbering may be used for more advanced usage. Multilevel numbering is commonly used in the numbering of chapters, sections, and subsections (e.g. chapter 2, section 2.4, subsection 2.4.3) as well as software release versions (e.g. software release 2.19.2).
An edition number prefix, such as
The textual representation of a Digital Succession Identifier (DSI) is a base identifier followed by an optional slash followed by an optional edition number (possibly multilevel). The edition number is represented as non-negative integers separated by periods.
The base DSI is calculated from the git initial commit (genesis record) of a digital
succession. Git-compatible software can calculate a 20-byte binary hash that identifies the digital succession.
This 20-byte binary hash is usually represented textually as a 40-digit
hexadecimal number. However, for a DSI, this 20-byte binary hash has a 27-character
representation in standard base64url format (RFC 4648)
It is worth noting that when a new digital succession (git initial commit) is created, a user can choose not to use it and instead immediately create a new one with a different base64url text representation. There is little cost in not using empty digital successions and recreating new genesis records (git initial commits) until an acceptable base64url identifier is found.
To support future enhancements, there are three paths to extending this textual representation. These paths involve using a base DSI where:
a character is neither a slash (
the number of characters is different than 27, or
the 27th character is one of the 48 base64url characters that never appear
as the last character of a base64url encoding of 40 bytes
(i.e., any base64url character that is not
A significant design choice is to not directly rely on git commit history to determine the succession of editions. Git commit history accurately records the actions performed with git, but it can be inflexible and confusing for establishing a clean linear history. Software Heritage automatically preserves git commits, which compounds the risk that git commits do not correspond well, or even prevent, an intended clean linear history. There may be situations where having merge commits and non-linear git commit history will be convenient.
Having edition history separate from git history also provides a potential path for an enhancement akin to retractions of specific editions.
Edition numbers are similar to release versions of software projects. The usual practice in software development is to use git tags to identify releases. In contrast, this specification takes a different approach. Edition numbers are recorded with file paths in git trees rather than git tags.
The main reason for this different approach has to do with how digital successions are recorded and copied. A highly desirable feature for digital successions is the ease of copying without errors. It is convenient to copy digital successions from multiple sources and store them in a single git repository. In this specification, a complete digital succession is captured by just a chain of commits (initial commit to latest commit). In contrast, entire software projects, which often include release tags, are usually copied by cloning an entire repository. If edition numbers were recorded as git annotated tags, copying digital successions properly would be more complicated and error-prone (due to missing tags).
Git repository branches are a convenient tool for managing digital successions. However, branch names are not part of the digital succession record.
The textual identifier is in base64url (Base64 with URL and filename safe alphabet) as
specified in RFC 4648
As mentioned in the introduction to the textual representation, there is no obligation to use the Digital Succession Identifier (DSI) generated by a brand new digital succession. If a DSI is unacceptable to the creator of a new digital succession, a new DSI can be easily generated. Helper tools could perform this function for users by default.
The main con to base64url is that it can contain characters that are more prone to copy errors when transmitted via human sight. Certain fonts and handwriting can make poor or no distinction between certain character pairs. For example, some popular sans serif fonts make no visual distinction between capital I and lowercase l.
The main pro to base64url is that it is 27 characters rather than 40. Given that DSIs are used in contexts similar to DOIs, it is likely more acceptable to adopters to use 27 characters, which is very typical for a long DOI, rather than 40 characters. Displaying an ID of 27 characters rather than 40 characters also fits better on web pages rendered on mobile devices. 27 characters are less likely to be hidden by showing ellipsis, horizontal scroll regions, or shrunk into extremely small fonts.
This design decision is partly made on the belief that the copy-via-human-sight issue is increasingly mitigated by a few technology trends:
Computer systems are relied upon more and more via more reliable copying mechanisms such as clicking on hyperlinks, copy-and-paste, and QR codes instead of visual copying and handwriting.
Websites tend to switch to using appropriate fonts for code like base64url IDs when output into HTML. The same applies to tools that automatically produce PDFs from computer data.
Human-to-computer interfaces, such as input fields of Internet-connected software, tend to add features like autocomplete and search disambiguation for typos when sufficient need arises for such a feature.
The term edition is used to refer to a digital object in a digital succession. Part of the motivation is to avoid confusion with the many uses of the term version. For instance, version can be used to refer to versions of a digital object that are not added as editions.
This language is partly due to the initial application of digital successions for
scientific articles. Although there are rarely
new versions of scientific articles after they are published, it is well established
that scientific textbooks are published in multiple
In the initial application of digital successions, documents are the digital object.
A single edition of a document might be presented in different HTML and PDF
A multilevel edition number should not have more than four levels (components). Edition number components (per level) must not be negative and should not exceed four decimal digits.
Thank you to Valentin Lorentz for questions about design choices
and pointing out an important shortcoming in how GPG digital signatures were used in the
initial implementation of the hidos library (version 0.3)
This specification is heavily influenced by the concept of
For a discussion on various concepts and proposed terminology about persistent
identifiers, see
Reference SSH signing keys instead of GPG/PGP signing keys.
Add section about multilevel edition numbering.
Add design rationale behind git tree paths instead of git tags.
Note future expansion paths for text representation.