Why You Should Use Fossil

Or, if not Fossil, at least some kind of modern version control
such as Git, Mercurial, or Subversion.

(Presented in outline form, for people in a hurry)

I. Benefits of Version Control

1. Immutable file and version identification

   1. Simplified and unambiguous communication between developers
   2. Detect accidental or surreptitious changes
   3. Locate the origin of discovered files

2. Parallel development

   1. Multiple developers on the same project
   2. Single developer with multiple subprojects
   3. Experimental features do not contaminate the main line
   4. Development/Testing/Release branches
   5. Incorporate external changes into the baseline

3. Historical record

   1. Exactly reconstruct historical builds
   2. Locate when and by whom faults were injected
   3. Find when and why content was added or removed
   4. Team members see the big picture
   5. Research the history of project features or subsystems
   6. Copyright and patent documentation
   7. Regulatory compliance

4. Automatic replication and backup

   1. Everyone always has the latest code
   2. Failed disk-drives cause no loss of work
   3. Avoid wasting time doing manual file copying
   4. Avoid human errors during manual backups

II. Definitions

• Project → a collection of computer files that serve some common purpose. Often the project is a software application and the individual files are source code together with makefiles, scripts, and "README.txt" files. Other examples of projects include books or manuals in which each chapter or section is held in a separate file.

  • Projects change and evolve. The whole purpose of version control is to track and manage that evolution.

  • Most projects contain many files, but it is possible to have a project consisting of just a single file.

  • Fossil requires that all the files for a project must be collected into a single directory hierarchy - a single folder possibly with layers of subfolders. Fossil is not a good choice for managing a project that has files scattered hither and yon all over the disk. In other words, Fossil only works for projects where the files are laid out such that they can be archived into a ZIP file or tarball.

• Repository → (also called "repo") a single file that contains all historical versions of all files in a project. A repo is similar to a ZIP archive in that it is a single file that stores compressed versions of many other files. Files can be extracted from the repo and new files can be added to the repo, just as with a ZIP archive. But a repo has other capabilities above and beyond what a ZIP archive can do.

  • Fossil does not care what you name your repository files, though names ending with ".fossil" are recommended.

  • A single project typically has multiple, redundant repositories on separate machines.

  • All repositories stay synchronized with one another by exchanging information via HTTP or SSH.

  • All repos for a single project redundantly store all information about that project. So if any one repo is lost due to a disk crash, all content is preserved in the surviving repos.

  • The usual arrangement is one repository per user. And since most users these days have their own computer, that means one repository per computer. But this is not a requirement. It is ok to have multiple copies of the same repository on the same computer.

  • Fossil works fine with just a single copy of the repository. But in that case there is no redundancy. If that one repository file is lost due to a hardware malfunction, then there is no way to recover the project.

  • Best practice is to keep all repositories for a user in a single folder. Folders such as "~/Fossils" or "%USERPROFILE%\Fossils" are recommended. Fossil itself does not care where the repositories are stored. Nor does Fossil require repositories to be kept in the same folder. But it is easier to organize your work if all repositories are kept in the same place.

• Check-out → a set of files that have been extracted from a repository and that represent a particular version or snapshot of the project.

  • Check-outs must be on the same computer as the repository from which they are extracted. This is just like with a ZIP archive: one must have the ZIP archive file on the local machine before extracting files from ZIP archive.

  • There can be multiple check-outs (in different folders) from the same repository.

  • The repository must be on the same computer as the check-out, but the relative locations of the repo and the check-out are arbitrary. The repository may be located inside the folder holding the check-out, but it certainly does not have to be and usually is not.

  • A special file exists in every check-out that tells Fossil from which repository the check-out was extracted, and which version of the project the check-out represents. This is the ".fslckout" file on unix systems or the "_FOSSIL_" file on Windows.

• Check-in → another name for a particular version of the project. A check-in is a collection of files inside of a repository that represent a snapshot of the project for an instant in time. Check-ins exist only inside of the repository. This contrasts with a check-out which is a collection of files outside of the repository.

  • Every check-out knows the check-in from which it was derived. But check-outs might have been edited and so might not exactly match their associated check-in.

  • Check-ins are immutable. They can never be changed. But check-outs are collections of ordinary files on disk. The files of a check-out can be edited just like any other file.

  • A check-in can be thought of as an historical snapshot of a check-out.

  • "Check-in", "version", "snapshot", and "revision" are synonyms.

  • When used as a noun, the word "commit" is another synonym for "check-in". When used as a verb, the word "commit" means to create a new check-in.

III. Basic Fossil commands

• clone → Make a copy of a repository. The original repository is usually (but not always) on a remote machine and the copy is on the local machine. The copy remembers the network location from which it was copied and (by default) tries to keep itself synchronized with the original.

• open → Create a new check-out from a repository on the local machine.

• update → Modify an existing check-out so that it is derived from a different version of the same project.

• commit → Create a new version (a new check-in) of the project that is a snapshot of the current check-out.

• revert → Undo all local edits on a check-out. Make the check-out be an exact copy of its associated check-in.

• push → Copy content found in a local repository over to a remote repository. (Fossil usually does this automatically in response to a "commit" and so this command is seldom used, but it is important to understand it.)

• pull → Copy new content found in a remote repository into a local repository. A "pull" by itself does not modify any check-out. The "pull" command only moves content between repositories. However, the "update" command will (often) automatically do a "pull" before attempting to update the local check-out.

• sync → Do both a "push" and a "pull" at the same time.

• add → Add a new file to the local check-out. The file must already be on disk. This command tells Fossil to start tracking and managing the file. This command affects only the local check-out and does not modify any repository. The new file is inserted into the repository at the next "commit" command.

• rm/mv → Short for 'remove' and 'move', these commands are like "add" in that they specify pending changes to the structure of the check-out. As with "add", no changes are made to the repository until the next "commit".

IV. The history of a project is a Directed Acyclic Graph (DAG)

• Fossil (and other distributed VCSes like Git and Mercurial, but not Subversion) represent the history of a project as a directed acyclic graph (DAG).

  • Each check-in is a node in the graph

  • If check-in Y is derived from check-in X then there is an arc in the graph from node X to node Y.

  • The older check-in (X) is call the "parent" and the newer check-in (Y) is the "child". The child is derived from the parent.

• Two users (or the same user working in different check-outs) might commit different changes against the same check-in. This results in one parent node having two or more children.

• Command: merge → combines the work of multiple check-ins into a single check-out. That check-out can then be committed to create a new check-in that has two (or more) parents.

  • Most check-ins have just one parent, and either zero or one child.

  • When a check-in has two or more parents, one of those parents is the "primary parent". All the other parent nodes are "secondary" or "merge" parents. Conceptually, the primary parent shows the main line of development. Content from the merge parents is added into the main line.

  • The "direct children" of a check-in X are all children that have X as their primary parent.

  • A check-in node with no direct children is sometimes called a "leaf".

  • The "merge" command changes only the check-out. The "commit" command must be run subsequently to make the merge a permanent part of project.

• Definition: branch → a sequence of check-ins that are all linked together in the DAG through the primary parent.

  • Branches are often given names which propagate to direct children. The tradition in Fossil is to call the main branch "trunk". In Git, it's called "master" by default, though some call it something else, like "main".

  • It is possible to have multiple branches with the same name. Fossil has no problem with this, but it can be confusing to humans, so best practice is to give each branch a unique name.

  • The name of a branch can be changed by adding special tags to the first check-in of a branch. The name assigned by this special tag automatically propagates to all direct children.

V. Why version control is important (reprise)

1. Every check-in and every individual file has a unique name - its SHA1 or SHA3-256 hash. Team members can unambiguously identify any specific version of the overall project or any specific version of an individual file.

2. Any historical version of the whole project or of any individual file can be easily recreated at any time and by any team member.

3. Accidental changes to files can be detected by recomputing their cryptographic hash.

4. Files of unknown origin can be identified using their hash.

5. Developers are able to work in parallel, review each others work, and easily merge their changes together. External revisions to the baseline can be easily incorporated into the latest changes.

6. Developers can follow experimental lines of development, then revert back to an earlier stable version if the experiment does not work out. Creativity is enhanced by allowing crazy ideas to be investigated without destabilizing the project.

7. Developers can work on several independent subprojects, flipping back and forth from one subproject to another at will, and merge patches together or back into the main line of development as they mature.

8. Older changes can be easily backed out of recent revisions, for example if bugs are found long after the code was committed.

9. Enhancements in a branch can be easily copied into other branches, or into the trunk.

10. The complete history of all changes is plainly visible to all team members. Project leaders can easily keep track of what all team members are doing. Check-in comments help everyone to understand and/or remember the reason for each change.

11. New team members can be brought up-to-date with all of the historical code, quickly and easily.

12. New developers, interns, or inexperienced staff members who still do not understand all the details of the project or who are otherwise prone to making mistakes can be assigned significant subprojects to be carried out in branches without risking main line stability.

13. Code is automatically synchronized across all machines. No human effort is wasted copying files from machine to machine. The risk of human errors during file transfer and backup is eliminated.

14. A hardware failure results in minimal lost work because all previously committed changes will have been automatically replicated on other machines.

15. The complete work history of the project is conveniently archived in a single file, simplifying long-term record keeping.

16. A precise historical record is maintained which can be used to support copyright and patent claims or regulatory compliance.