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

Moved to a separate document.

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.