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Comment:Rewrite the "fossil versus git" document.
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User & Date: drh 2015-12-16 16:13:16
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Further edits to the fossil-v-git.wiki page. check-in: 7c32a784 user: drh tags: trunk
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<title>Fossil Versus Git</title>

<h2>1.0 Don't Stress!</h2>

If you start out using one DVCS and later decide you like the other better,
it is [./inout.wiki | easy to change].

But it also helps to be informed about the differences between

[http://git-scm.com | Git] and Fossil.  See the table below for
a high-level summary and the text that follows for more details.

Keep in mind that you are reading this on a Fossil website,
so the information here
might be biased in favor of Fossil.  Ask around with people who have
used both Fossil and Git for other opinions.

<h2>2.0 Executive Summary:</h2>

<blockquote><center><table border=1 cellpadding=5>
<tr><th width="50%">GIT</th><th width="50%">FOSSIL</th></tr>
<tr><td>File versioning only</td>
    <td>Versioning, Tickets, Wiki, and Technotes</td></tr>
<tr><td>Sharding</td><td>Replicating</td></tr>
<tr><td>Developer branches</td><td>Feature branches</td></tr>
<tr><td>Complex</td><td>Intuitive</td></tr>
<tr><td>Separate web tools</td><td>Integrated Web interface</td></tr>

<tr><td>Lots of little tools</td><td>Single executable</td></tr>
<tr><td>Pile-of-files repository</td>
    <td>Single-file relational database</td></tr>
<tr><td>One check-out per repository</td>
    <td>Many check-outs per repository</td></tr>

<tr><td>Uses "<tt>rebase</tt>"</td><td>Immutable</td></tr>
<tr><td>GPL</td><td>BSD</td></tr>
</table></center></blockquote>

<h2>3.0 Discussion</h2>

<h3>3.1 Feature Set</h3>

Git provides file versioning services only, whereas Fossil adds an
integrated [./wikitheory.wiki | wiki],
[./bugtheory.wiki | ticketing &amp; bug tracking],
[./embeddeddoc.wiki | embedded documentation], and
[./event.wiki | Technical notes].
These additional capabilities are available for Git as 3rd-party
user-installed add-ons, but with Fossil they are integrated into
the design.  One way to describe Fossil is that it is
"[https://github.com/ | github]-in-a-box".

<h3>3.2 Sharding versus Replicating</h3>

Git makes it easy for each repository in a project to hold a subset of
the branches for that project.  In fact, it is entirely possible and not
uncommon for no repository in the project to hold all the different code
versions for a project.  Instead the information is distributed.
Individual developers have one or more private branches.  A hierarchy
of integrators merge changes from individual developers into collaborative
branches, until all the changes are merged together at the top-level master
branch.  And all of this can be accomplished without having to have all the
code in any one repository.  Developers or groups of developers can share
only those branches that they want to share and keep other branches of the
project private.  This is analogous to sharding a distributed database.

Fossil allows private branches, but its default mode is to share everything.
And so in a Fossil project, all repositories tend to contain all of the
content at all times.  This is analogous to replication in a
distributed database.

The Git model works best for large projects, like the
Linux kernel for which Git was designed.
Linus Torvalds does not need or want to see a thousand
different branches, one for each contributor.  Git allows intermediary
"gate-keepers" to merge changes from multiple lower-level developers
into a single branch and only present Linus with a handful of branches
at a time.  Git encourages a programming model where each developer
works in his or her own branch and then merges changes up the hierarchy
until they reach the master branch.

Fossil is designed for smaller and non-hierarchical teams where all
developers are operating directly on the master branch, or at most
a small number of well-defined branches.
The [./concepts.wiki#workflow | autosync] mode of Fossil makes it easy
for multiple developers to work on a single branch and maintain
linear development on that branch and avoid needless forking
and merging.

<h3>3.3 Branches</h3>

Git (and especially GitHub) encourages a workflow where each developer
has his or her own branch or branches.  Developers then send "pull requests"
to have their changes be merged into "official" branches by integrators.
For example, the Linux kernel team has a hierarchy of integrators with
Linus Torvalds at the root.  Individual developers each have their own
private branches of the source tree into which they make their own changes.
They then encourage first-tier integrators to pull those changes.  The
first-tier integrators merge together changes from multiple contributors
then try to get second-tier integrators to pull their branches.  The
changes merge up the hierarchy until (hopefully) they are pulled into
"Linus's branch", at which time they become part of the "official" Linux.

In Git, each branch is "owned" by the person who creates it and works
on it.  The owner might pull changes from others, but the owner is always
in control of the branch.  Branches are developer-centric.

Fossil, on the other hand, encourages a workflow where branches are
associated with features or releases, not individual developers.
All developers share all branches in common, and two
or more developers can and often do intersperse commits onto the same branch.
Branches do not belong to individuals.  All branches are read/write
accessible to all developers at all times.  There is no need
for integrators to merge together changes from various independent
developers.  Instead, all of the developers work together cooperatively
and the changes stay integrated naturally.

So to a first approximation, branches in Git are developer-centric whereas
branches in Fossil are feature-centric.

The Git approach scales much better for large projects like the Linux
kernel with thousands of contributors who in many cases don't even know
each other's names.  The integrators serve a gatekeeper role to help keep
undesirable code out of the official Linux source tree.  On the other hand,
not many projects are as big or as loosely organized as the Linux kernel.
Most projects have a small team of developers who all know each other
well and trust each other, and who enjoy working together collaboratively
without the overhead and hierarchy of integrators.

One consequence of the "everybody-sees-everything" focus of Fossil is that
branch names are global and are part of the distributed and synchronized
content of a Fossil repository, rather than being private and user-specific
as they are in Git.

<h3>3.4 Complexity</h3>

Git is a complex system.  It can be tricky to use and requires a fair
amount of knowledge and experience to master.  Fossil strives to be
a much simpler system that can be learned and mastered much more quickly.
Fossil strives to have fewer "gotchas" and quirks that can trip up a
developer.

The ideal VCS should just get out of the way of the developer and allow
the developer to focus 100% of their thinking on the project under
development.  One should not have to stop and think about how to operate
the VCS.  Of course, no VCS is ideal.  Every VCS requires the developer
to think about version control to some extent.  But one wants to minimize
the thinking about version control.

Git requires the developer to maintain a more complex mental model than
most other DVCSes.  Git takes longer to learn.  And you have to spend
more time thinking about what you are doing with Git.

Fossil strives for simplicity.  Fossil wants to be easy to learn and to
require little thinking about how to operating it.
[./quotes.wiki | Reports from the field]
indicate that Fossil is mostly successful at this effort.

Fossil will <u>never</u> get you into anything like the
"disconnected head state" which has frustrated so many Git users.

<h3>3.5 Web Interface</h3>

Git has a web interface, but it requires a fair amount of setup and an
external web server.  Fossil comes with a fully functional
[./webui.wiki | built-in web-server]
and a really simple mechanism (the "[/help/ui|fossil ui]" command) to
automatically start the web server and bring up a web browser to navigate
it.  The web interface for Fossil is not only easier to set up, it is also
more powerful and easier to use.  The web interface to Fossil is a practical
replacement to the 3rd-party "GUI Tools" that users often employ to operate
Git.

<h3>3.6 Implementation Strategy</h3>

Git consists of a collection of many little programs.  Git needs to be
"installed" using some kind of installer or package tool.  Git can be
tricky to install and get working, especially for users without
administrative privileges.

Fossil is a single self-contained executable.  To "install" Fossil one
has merely to download a precompiled binary and place that binary
somewhere on $PATH.  To uninstall Fossil, simply delete the binary.
To upgrade Fossil, replace the old binary with a new one.

Fossil is designed to be trivial to install, uninstall, and upgrade so
that developers can spend more time working on their own projects and
much less time configuring their version control system.

<h3>3.7 Repository Storage</h3>

A Git repository is a "pile-of-files" in the ".git" directory at the
root of the working checkout.  There is a one-to-one correspondence
between repositories and working checkouts.  A power-loss or system crash
in the middle of Git operation can damage or corrupt the Git repository.

A Fossil repository consists of a single disk file.  A single Fossil
repository can serve multiple simultaneous working checkouts.
A Fossil repository is an SQLite database, so it is highly resistant
to damage from a power-loss or system crash - incomplete transactions
are simply rolled back after the system reboots.

<h3>3.8 Check-outs Per Repository</h3>

In Git, a check-out and a repository are joined in a fundamental way
so that only a single version of the project history, or a single branch,
can be open at once.  If you have a project with multiple branches and
you want to have two or more branches open at the same time (perhaps to
do performance comparisons, or maybe to run simultaneous builds using
different compile-time options) then in Git you actually have to create
a new clone of the repository for each open checkout.

In Fossil, the repository and the check-out are distinct entities and
so a single repository can support multiple simultaneous checkouts.
This feature is <em>extensively</em> used by the Fossil developers
themselves.  Perhaps we are biased, but we not understand how anyone
can work efficiently with just one check-out per repository.

<h3>3.9 Audit Trail</h3>

Git features the "rebase" command which can be used to change the
sequence of check-ins in the repository.  Rebase can be used to "clean up"
a complex sequence of check-ins to make their intent easier for others
to understand.  This is important if you view the history of a project
as part of the documentation for the project.

Fossil takes an opposing view.  Fossil views history as sacrosanct and
stubbornly refuses to change it.
Fossil allows mistakes to be corrected (for example, check-in comments
can be revised, and check-ins can be moved onto new branches even after
the check-in has occurred) but the correction is an addition to the repository
and the original actions are preserved and displayed alongside
the corrections, thus preserving an historically accurate audit trail.
This is analogous to an accounting practice of marking through an incorrect
entry in a ledger and writing a correction beside it.


To put it another way, Git remembers what you should have done whereas
Fossil remembers what you actually did.

The lack of a "rebase" command and the inability to rewrite history
is considered a feature of Fossil, not an omission or bug.

<h3>3.10 License</h3>

Both Git and Fossil are open-source.  Git is under
[http://www.gnu.org/licenses/gpl.html | GPL] whereas Fossil is
under the
[http://en.wikipedia.org/wiki/BSD_licenses | two-clause BSD license].
The different licenses parallel, to some extent, the different philosophies
of Git and Fossil.
There are exceptions on both sides, but to a first approximation, Git
works better for GPL projects and Fossil works better for BSD projects.

The GPL is designed to provide a very contributor-friendly environment.
No legal paperwork is needed to contribute to a GPL project because
the GPL is cleverly designed so that the act of contributing
to the project (or even reading the code for the project) constitutes
an acceptance of the licensing terms.  GPL encourages a bazaar-style
development model, with lots of anonymous programmers contributing
drive-by patches.  The theory is that with many eyeballs, all bugs
are shallow.  Surprisingly, this has actually been demonstrated to
work in many well-known projects.

The BSD-style licenses are more user-friendly.  BSD-style licenses
place fewer restrictions on the users of the software at the expense 
of making it more difficult to contribute changes or enhancements.
To protect against IP claims,
every contributor to a BSD-style project must sign legal documents in
which they agree to release their contributions under the same license.
(Some BSD-licensed projects omit this formality, but do so at their peril.)
A BSD-style license encourages a more cathedral-style approach to development.
There is a small team of developers.  Drive-by patches and anonymous
contributors are discouraged and/or prohibited.  Contributors are expected
to be experts and be available to support their changes for the long-term.












































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<title>Fossil Versus Git</title>

<h2>1.0 Don't Stress!</h2>

If you start out using one DVCS and later decide you like the other better,
it is [./inout.wiki | easy to change].

Fossil and [http://git-scm.com | Git] are very similar in many respects,
but there are also important differences.
See the table below for
a high-level summary and the text that follows for more details.

Keep in mind that you are reading this on a Fossil website,
so the information here
might be biased in favor of Fossil.  Ask around with people who have
used both Fossil and Git for other opinions.

<h2>2.0 Executive Summary:</h2>

<blockquote><center><table border=1 cellpadding=5>
<tr><th width="50%">GIT</th><th width="50%">FOSSIL</th></tr>
<tr><td>File versioning only</td>
    <td>Versioning, Tickets, Wiki, and Technotes</td></tr>
<tr><td>Ad-hoc, pile-of-files key/value database</td>
    <td>Relational SQL database</td></tr>
<tr><td>Bazaar-style development</td><td>Cathedral-style development</td></tr>
<tr><td>Designed for Linux development</td>
    <td>Designed for SQLite development</td></tr>
<tr><td>Lots of little tools</td><td>Stand-alone executable</td></tr>


<tr><td>One check-out per repository</td>
    <td>Many check-outs per repository</td></tr>
<tr><td>Remembers what you should have done</td>
    <td>Remembers what you actually did</td></tr>
<tr><td>GPL</td><td>BSD</td></tr>
</table></center></blockquote>

<h2>3.0 Discussion</h2>

<h3>3.1 Feature Set</h3>

Git provides file versioning services only, whereas Fossil adds an
integrated [./wikitheory.wiki | wiki],
[./bugtheory.wiki | ticketing &amp; bug tracking],
[./embeddeddoc.wiki | embedded documentation], and
[./event.wiki | Technical notes].
These additional capabilities are available for Git as 3rd-party and/or
user-installed add-ons, but with Fossil they are integrated into
the design.  One way to describe Fossil is that it is
"[https://github.com/ | github]-in-a-box".

If you clone Git's self-hosting repository you get just Git's source code.
If you clone Fossil's self-hosting repository, you get the entire
Fossil website - source code, documentation, ticket history, and so forth.

For developers who choose to self-host projects (rather than using a
3rd-party service such as GitHub) Fossil is much easier to set up, since
the stand-alone Fossil executable together with a 2-line CGI script
suffice to instantiate a full-featured developer website.  To accomplish
the same using Git requires locating, installed, configuring, integrating, 
and managing a wide assortment of separate tools.  Standing up a developer
website using Fossil can be done in minutes, whereas doing the same using
Git requires hours or days.

<h3>3.2 Database</h3>

The baseline data structures for Fossil and Git are the same (modulo
formatting details).  Both systems store check-ins as immutable
objects referencing their immediate ancestors and named by their SHA1 hash.

The difference is that Git stores its objects as individual files 
in the ".git" folder or compressed into
a bespoke "pack-files", whereas Fossil stores its objects in a 
relational ([https://www.sqlite.org/|SQLite]) database file.  To put it
another way, Git uses an ad-hoc pile-of-files key/value database whereas
Fossil uses a proven, general-purpose SQL database.  This
difference is more than an implementation detail.  It
has important consequences.

With Git, one can easily locate the ancestors of a particular check-in
by following the pointers embedded the check-in object, but it is very 
difficult to go the other direction and locate the descendants of a
check-in.  It is so difficult, in fact, that neither native Git nor
GitHub provide this capability.  With Git, if you are looking at some
historical check-in then you cannot ask
"what came next" or "what are the children of this check-in".

Fossil, on the other hand, parses essential information about check-ins
(such as parentage) into a relation database that can be easily 
queried using concise SQL statements to find both ancestors and 
descendents of a check-in.

Leaf check-ins in Git that lack a "ref" become "detached", making them
difficult to locate and subject to garbage collection.  This
"detached head" problem has caused untold grief for countless
Git users.  With Fossil, all check-ins are easily located using
a variety of attributes (parents, children, committer, date, full-text
search of the check-in comment) and so detached heads are simply not possible.

The ease with which check-ins can be located and queried in Fossil
has resulted in a huge variety of reports and status screens 
([./webpage-ex.md|examples]) that show project state
in ways that help developers
maintain enhanced awareness and comprehension
and avoid errors.

<h3>3.3 Cathedral vs. Bazaar</h3>

Fossil and Git promote different development styles.  Git promotes a
"bazaar" development style in which numerous anonymous developers make
small and sometimes haphazard contributions.  Fossil
promotes a "cathedral" development model in which the project is
closely supervised by an highly engaged architect and implemented by 
a clique of developers.

Nota Bene:  This is not to say that Git cannot be used for cathedral-style
development or that Fossil cannot be used for bazaar-style development.
They can be.  But those modes are not their design intent nor the their
low-friction path.

Git encourages a style in which individual developers work in relative
isolation, maintaining their
own branches and the occasionally rebasing and pushing selected changes up
to the main repository.  Developers using Git often have their own
private branches that nobody else ever sees.  Work becomes siloed.
This is exactly what one wants when doing bazaar-style development.

Fossil, in contrast, strives to keep all changes from all contributors
mirrored in the main repository (in separate branches) at all times.
Work in progress from one developer is readily visible to all other
developers and to the project leader, well before the code is ready
to integrate.  Fossil places a lot of emphasis on reporting the state
of the project, and the changes underway by all developers, so that
all developers and especially the project leader can maintain a better
mental picture of what is happening, and better situational awareness.

<h3>3.4 Linux vs. SQLite</h3>

Git was specifically designed to support the development of Linux.
Fossil was specifically designed to support the development of SQLite.

Both SQLite and Linux are important pieces of software.
SQLite is found on far more systems than Linux.  (Almost every Linux
system uses SQLite, but there are many non-Linux systems such as
iPhones, PlayStations, and Windows PC that use SQLite.)  On the other
hand, for those systems that do use Linux, Linux is a far more important
component.

Linux uses a bazaar-style development model.  There are thousands and
thousands of contributors, most of whom do not know each others names.
Git is designed for this scenario.

SQLite uses cathedral-style development.  95% of the code in SQLite
comes from just three programmers, 64% from just the lead developer.
And all SQLite developers know each other well and interact daily.
Fossil is designed for this development model.

<h3>3.5 Lots of little tools vs. Self-contained system</h3>

Git consists of many small tools, each doing one small part of the job,
which can be recombined (by experts) to perform powerful operations.
Git has a lot of complexity and many dependencies and requires an "installer"
script or program to get it running.

Fossil is a single self-contained stand-alone executable with hardly 
any dependencies.  Fossil can be (and often is) run inside a 
minimally configured chroot jail.  To install Fossil,
one merely puts the executable on $PATH.

The designer of Git says that the unix philosophy is to have lots of 
small tools that collaborate to get the job done.  The designer of 
Fossil says that the unix philosophy is "it just works".  Both 
individuals have written their DVCSes to reflect their own view 
of the "unix philosophy".

<h3>3.6 One vs. Many Check-outs per Repository</h3>

A "repository" in Git is a pile-of-files in the ".git" subdirectory
of a single check-out.  The check-out and the repository are inseperable.

With Fossil, a "repository" is a single SQLite database file 	
that can be stored anywhere.  There
can be multiple active check-outs from the same repository, perhaps
open on different branches or on different snapshots of the same branch.
Long-running tests or builds can be running in one check-out while
changes are being committed in another.

<h3>3.7 What you should have done vs. What you actually did</h3>

Git puts a lot of emphasis on maintaining
a "clean" check-in history.  Extraneous and experimental branches by
individual developers often never make it into the main repository.  And
branches are often rebased before being pushed, to make
it appear as if development had been linear.  Git strives to record what
the development of a project should have looked like had there been no
mistakes.

Fossil, in contrast, puts more emphasis on recording exactly what happened,
including all of the messy errors, dead-ends, experimental branches, and
so forth.  One might argue that this
makes the history of a Fossil project "messy".  But another point of view
is that this makes the history "accurate".  In actual practice, the 
superior reporting tools available in Fossil mean that the added "mess"
is not a factor.

One commentator has mused that Git records history according to
the victors, whereas Fossil records history as it actually happened.

<h3>3.8 GPL vs. BSD</h3>

Git is covered by the GPL license whereas Fossil is covered by 
a two-clause BSD license.

Consider the difference between GPL and BSD licenses:  GPL is designed
to make writing easier at the expense of making reading harder.  BSD is
designed to make reading easier and the expense of making writing harder.

To a first approximation, the GPL license grants the right to read 
source code to anyone who promises to give back enhancements.  In other
words, the act of reading GPL source code (a prerequiste for making changes)
implies acceptance of the license which requires updates to be contributed
back under the same license.  (The details are more complex, but the
foregoing captures the essence of the idea.)  A big advantage of the GPL
is that anybody can contribute to the code without having to sign additional
legal documentation because they have implied their acceptance of the GPL
license by the very act of reading the source code.  This means that a GPL
project can legally accept anonymous and drive-by patches.

The BSD licenses, on the other hand, make reading much easier than the GPL,
because the reader need not surrender proprietary interest
in their own enhancements.  On the flip side, BSD and similarly licensed
projects must obtain legal affidavits from authors before
new content can be added into the project.  Anonymous and drive-by
patches cannot be accepted.  This makes signing up new contributors for
BSD licensed projects harder.

The licenses on the implementations of Git and Fossil only apply to the
implementations themselves, not to the projects which the systems store.
Nevertheless, one can see a more GPL-oriented world-view in Git and a
more BSD-oriented world-view in Fossil.  Git encourages anonymous contributions
and siloed development, which are hallmarks of the GPL/bazaar approach to
software, whereas Fossil encourages a more tightly collaborative,
cliquish, cathedral-style approach more typical of BSD-licensed projects.

<h2>4.0 Missing Features</h2>

Most of the capabilities found in Git are also available in Fossil and
the other way around. For example, both systems have local check-outs,
remote repositories, push/pull/sync, bisect capabilities, and a "stash".
Both systems store project history as a directed acyclic graph (DAG) 
of immutable check-in objects.

But there are a few capabilities in one system that are missing from the
other.

<h3>4.1 Features found in Fossil but missing from Git</h3>

  *  <b>The ability to show descendents of a check-in.</b>

   Both Git and Fossil can easily find the ancestors of a check-in.  But
   only Fossil shows the descendents.  (It is possible to find the
   descendents of a check-in in Git using the log, but that is sufficiently
   difficult that nobody ever actually does it.)

  *  <b>Wiki, Embedded documentation, Trouble-tickets, and Tech-Notes</b>

   Git only provides versioning of source code.  Fossil strives to provide
   other related configuration management services as well.

  *  <b>Named branches</b>

   Branches in Fossil have persistent names that are propagated 
   to collaborators via [/help?cmd=push|push] and [/help?cmd=pull|pull].
   All developers see the same name on the same branch.  Git, in contrast,
   uses only local branch names, so developers working on the
   same project can (and frequently do) use a different name for the
   same branch.  

  *  <b>The [/help?cmd=all|fossil all] command</b>

   Fossil keeps track of all repositories and check-outs and allows
   operations over all of them with a single command.  For example, in
   Fossil is possible to request a pull of all repositories on a laptop
   from their respective servers, prior to taking the laptop off network.
   Or it is possible to do "fossil all status" to see if there are any
   uncommitted changes that were overlooked prior to the end of the workday.

  *  <b>The [/help?cmd=ui|fossil ui] command</b>

   Fossil supports an integrated web interface.  Some of the same features
   are available using third-party add-ons for Git, but they do not provide
   nearly as many features and they are not nearly as convenient to use.


<h2>4.2 Features found in Git but missing from Fossil</h2>

  *  <b>Rebase</b>

   Because of its emphasis on recording history exactly as it happened,
   rather than as we would have liked it to happen, Fossil deliberately
   does not provide a "rebase" command.  One can rebase manually in Fossil,
   with sufficient perserverence, but it not something that can be done with
   a single command.

  *  <b>Push or pull a single branch</b>

   The [/help?cmd=push|fossil push], [/help?cmd=pull|fossil pull], and
   [/help?cmd=sync|fossil sync] commands do not provide the capability to
   push or pull individual branches.  Pushing and pulling in Fossil is
   all or nothing.  This is in keeping with Fossil's emphasis on maintaining
   a complete record and on sharing everything between all developers.