Fossil  Hashes: Fossil Artifact Identification

All artifacts in Fossil are identified by a unique hash, currently using the SHA3 algorithm by default, but historically using the SHA1 algorithm:

Algorithm<Raw Bits Hexadecimal digits
SHA3-256 256 64
SHA1 160 40

There are many types of artifacts in Fossil: commits (a.k.a. check-ins), tickets, ticket comments, wiki articles, forum postings, file data belonging to check-ins, etc. (More info...).

There is a loose hierarchy of terms used instead of “hash” in various parts of the Fossil UI, which we cover in the sections below.

Names

Several Fossil interfaces accept a wide variety of check-in names: commit artifact hashes, ISO8601 date strings, branch names, etc. Fossil interfaces that accept any of these options usually document the parameter as “NAME”, so we will use that form to refer to this specialized use.

Artifact hashes are only one of many different types of NAME. We use the broad term “NAME” to refer to the whole class of options. We use more specific terms when we mean one particular type of NAME.

Versions

When an artifact hash refers to a specific commit, Fossil sometimes calls it a “VERSION,” a “commit ID,” or a “check-in ID.” We may eventually settle on one of these terms, but all three are currently in common use within Fossil’s docs, UI, and programming interfaces.

A VERSION is a specific type of artifact hash, distinct from, let us say, a wiki article artifact hash.

A unique prefix of a VERSION hash is itself a VERSION. That is, if your repository has exactly one commit artifact with a hash prefix of “abc123”, then that is a valid version string as long as it remains unambiguous.

UUIDs

Fossil uses the term “UUID” as a short alias for “artifact hash” in its internals. There are a few places where this leaks out into external interfaces, which we cover in the sections below. Going forward, we prefer one of the terms above in public interfaces instead.

Whether this short alias is correct is debateable.

One argument is that since "UUID" is an acronym for “Universally Unique Identifier,” and both SHA1 and SHA3-256 are larger and stronger than the 128-bit algorithms used by “proper” UUIDs, Fossil artifact hashes are more universally unique. It is therefore quibbling to say that Fossil UUIDs are not actually UUIDs. One wag suggested that Fossil artifact hashes be called MUIDs: multiversally unique IDs.

The common counterargument is that the acronym “UUID” was created for a particular type of universally-unique ID, with particular ASCII and bitfield formats, and with particular meaning given to certain of its bits. In that sense, no Fossil “UUID” can be used as a proper UUID.

Be warned: attempting to advance the second position on the Fossil discussion forum will get you nowhere at this late date. We’ve had the debates, we’ve done the engineering, and we’ve made our evaluation. It’s a settled matter: internally within Fossil, “UUID” is defined as in this section’s leading paragraph.

To those who remain unconvinced, “fixing” this would require touching almost every source code file in Fossil in a total of about a thousand separate locations. (Not exaggeration, actual data.) This would be a massive undertaking simply to deal with a small matter of terminology, with a high risk of creating bugs and downstream incompatibilities. Therefore, we are highly unlikely to change this ourselves, and we are also unlikely to accept a patch that attempts to fix it.

Repository DB Schema

The primary place where you find "UUID" in Fossil is in the blob.uuid table column, in code dealing with that column, and in code manipulating other data that refers to that column. This is a key lookup column in the most important Fossil DB table, so it influences broad swaths of the Fossil internals.

For example, C code that refers to SQL result data on blob.uuid usually calls the variable zUuid. That value may then be inserted into a table like ticket.tkt_uuid, creating a reference back to blob.uuid, and then be passed to a function like uuid_to_rid(). There is no point renaming a single one of these in isolation: it would create needless terminology conflicts, making the code hard to read and understand, risking the creation of new bugs.

You may have local SQL code that digs into the repository DB using these column names. While you may rest easy, assured now that we are highly unlikely to ever rename these columns, the Fossil repository DB schema is not considered an external user interface, and internal interfaces are subject to change at any time. We suggest switching to a more stable API: the JSON API, timeline.rss, TH1, etc.

TH1 Scripting Interfaces

Some TH1 interfaces expose Fossil internals flowing from blob.uuid, so “UUID” is a short alias for “artifact hash” in TH1. For example, the $tkt_uuid variable — available when customizing the ticket system — is a ticket artifact hash, exposing the ticket.tkt_uuid column, which has a SQL relation to blob.uuid.

TH1 is a longstanding public programming interface. We cannot rename its interfaces without breaking existing TH1 Fossil customizations. We are also unlikely to provide a parallel set of variables with “better” names, since that would create a mismatch with respect to the internals they expose, creating a different sort of developer confusion in its place.

JSON API Parameters and Outputs

The JSON API frequently uses the term “UUID” in the same sort of way, most commonly in artifact and timeline APIs. As with TH1, we can’t change this without breaking code that uses the JSON API as originally designed, so we take the same stance.

manifest.uuid

If you have the manifest setting enabled, Fossil writes a file called manifest.uuid at the root of the check-out tree containing the commit hash for the current checked-out version. Because this is a public interface that existing code depends on, we are unwilling to rename the file.