<title>Fossil Concepts</title>
<h1 align="center">Fossil Concepts</h1>

<h2>1.0 Introduction</h2>

[./index.wiki | Fossil] is a
[http://en.wikipedia.org/wiki/Software_configuration_management | software configuration management] system.
Fossil is software that is designed to control and track the
development of a software project and to record the history
of the project.
There are many such systems in use today.  Fossil strives to
distinguish itself from the others by being extremely simple
to setup and operate.

This document is intended as a quick introduction to the concepts
behind Fossil.

See also:

  *  [./whyusefossil.wiki#definitions|Definitions]
  *  [./quickstart.wiki|Quick start guide]

<h2>2.0 Composition Of A Project</h2>

<verbatim type="pikchr float-right">
R1: cylinder "Remote" "Repository" fill 0xadd8e6 rad 70%
R2: cylinder same "Remote" "Repository" at 2.5*R1.wid right of R1
    spline <-> from R1.e to 0.6<R1.se,R2.sw> then to 0.4<R1.ne,R2.nw> then to R2.w
    text "HTTP" at .5<R1.ne,R2.nw>
R3: cylinder same "Local" "Repository" fill 0x90ee90 \
    at dist(R1.e,R2.w) below .5<R1,R2>
    spline <-> from .5<R1.s,R1.se> to 0.6<R1.s,R3.w> to 0.5<R1.se,R3.n> to .5<R3.nw,R3.n> "HTTP" \
      behind R1
    spline <-> from R2.sw to .6<R2.sw,R3.n> to .5<R2.s,R3.e> to R3.ne "HTTP" ljust
T1: line from 1.0cm heading 200 from R3.sw go 2.2cm heading 150 then 2.2cm west close \
       fill 0xffff00 "Local" below "Source Tree" below
T2: line from 1.0cm heading 160 from R3.se same "Local" below "Source Tree" below
    line <-> from R3.sw to T1.start
    line <-> from R3.se to T2.start
</verbatim>

A software project normally consists of a "source tree".
A source tree is a hierarchy of files that are used to generate
the end product.  The source tree changes over time as the
software grows and expands and as features are added and bugs
are fixed.  A snapshot of the source tree at any point in time
is called a "version" or "revision" or a "baseline" of the product.
In Fossil, we use the name "check-in".

A "repository" is a database that contains copies of all historical
check-ins for a project.  Check-ins are normally stored in the
repository in a highly space-efficient compressed format (delta encoding).
But that is an implementation detail that you the user need not worry over.
Think of the repository as a safe place where all your old check-ins are
securely stored away and available for retrieval whenever you need
them.

A repository in Fossil is a single file on your disk.  This file
might be rather large (dozens or hundreds of megabytes for a large
or long running project) but it is nevertheless just a file.  You
can move it around, rename it, write it out to a memory stick, or
do anything else you normally do with files.

Each source tree that is controlled by Fossil is associated with
a single repository on the local disk drive.  You can tie two or more
source trees to a single repository if you want (though one
tree per repository is the most common configuration.)  So a
single repository can be associated with many source trees, but
each source tree is associated with only one repository.

Fossil source trees may not overlap.  A Fossil source tree is identified
by a file named "_FOSSIL_" (or ".fslckout", but this article will always
use the name "_FOSSIL_") in the root directory of the source tree.  Every
file that is a sibling of _FOSSIL_ and every file in every subfolder is
considered potentially a part of the source tree.  The _FOSSIL_ file
contains (among other things) the pathname of the repository with which
the source tree is associated.  On the other hand, the repository has
no record of its source trees.  So you are free to delete a source tree
or move it around without consequence.  But if you move or rename or
delete a repository, then any source trees associated with that repository
will no longer be able to locate their repository and will stop working.

When multiple developers are working on the same project, each
developer typically has his or her own local repository and an associated
source tree in which to work.  Developers share their work by
"syncing" the content of their local repositories either directly
or through a central server.  Changes can "push" from the local
repository into a remote repository.  Or changes can "pull" from a
remote repository into a local repository.  Or one can do a "sync"
which is a shortcut for doing both a push and a pull at the same time.
Fossil also has the concept of "cloning".  A "clone" is like a "pull",
except that instead of beginning with an existing local repository,
a clone begins with nothing and creates a new local repository that
is a duplicate of a remote repository.

Communication between repositories is via HTTP.  Remote
repositories are identified by URL.  You can also point a web browser
at a repository and get human-readable status, history, and tracking
information about the project.

<h3><a id="artifacts"></a>2.1 Identification Of Artifacts</h3>

A particular version of a particular file is called an "artifact".  Each
artifact has a universally unique name which is the <a
href="http://en.wikipedia.org/wiki/SHA1">SHA1</a> or <a
href="http://en.wikipedia.org/wiki/SHA3">SHA3-256</a> hash of the
content of that file expressed as either 40 or 64 characters of
lower-case hexadecimal. (See the [./hashpolicy.wiki|hash policy
document] for information on which algorithm is used, when.) Such a hash
is referred to as the Artifact ID. These hash algorithms were created
with Fossil's purpose in mind: to provide a highly forgery-resistant
identifier for a blob of data, such as a file. Given any file, it is
simple to find the artifact ID for that file. But given an artifact ID,
it is computationally intractable to generate a file that will have that
same artifact ID.

Artifact IDs look something like this:

<blockquote><b>
6089f0b563a9db0a6d90682fe47fd7161ff867c8<br>
59712614a1b3ccfd84078a37fa5b606e28434326<br>
19dbf73078be9779edd6a0156195e610f81c94f9<br>
b4104959a67175f02d6b415480be22a239f1f077<br>
997c9d6ae03ad114b2b57f04e9eeef17dcb82788
</b></blockquote>

When referring to an artifact using Fossil, you can use a unique
prefix of the artifact ID that is four characters or longer.  This saves
a lot of typing.  When displaying artifact IDs, Fossil will usually only
show the first 10 digits since that is normally enough to uniquely
identify a file.

Changing (or adding or removing) a single byte in a file results
in a completely different artifact ID.  And since the artifact ID is the name of
the artifact, making any change to a file results in a new artifact.
In this way, artifacts are immutable.

A repository is really just an unordered collection of
artifacts.  New artifacts can be added to the repository, but
existing artifacts can never be removed.  (Well, almost never. There
is a [./shunning.wiki | "shunning"] mechanism that allows spam or other
inappropriate content to be removed if absolutely necessary, but such
removal is discouraged.)
Fossil is designed in
such a way that it can be handed a set of artifacts in any
order and it can figure out the relationship between those
artifacts and reconstruct the complete development history of
a software project.

<h3>2.2 Manifests</h3>

Associated with every check-in is a special file called the
[./fileformat.wiki#manifest| "manifest"].  The manifest is a
listing of all other files in
that source tree.  The manifest contains the (complete) artifact ID
of the file and the name of the file as it appears on disk,
and thus serves as a mapping from artifact ID to disk name.  The artifact ID
of the manifest is the identifier for the entire check-in.  When
you look at a "timeline" of changes in Fossil, the ID associated
with each check-in or commit is really just the artifact ID of the
manifest for that check-in.

<p>The manifest file is not normally a real file on disk.  Instead,
the manifest is computed in memory by Fossil whenever it needs it.
However, the "fossil setting manifest on" command will cause the
manifest file to be materialized to disk, if desired.  Both Fossil
itself, and SQLite cause the manifest file to be materialized to disk
so that the makefiles for these project can read the manifest and
embed version information in generated binaries.

<p>Fossil automatically generates a manifest whenever you "commit"
a new check-in.  So this is not something that you, the developer,
need to worry with.  The format of a manifest is intentionally
designed to be simple to parse, so that if
you want to read and interpret a manifest, either by hand or
with a script, that is easy to do.  But you will probably never
need to do so.</p>

<p>In addition to identifying all files in the check-in, a
manifest also contains a check-in comment, the date and time
when the check-in was established, who created the check-in,
and links to other check-ins from which the current check-in
is derived.  There is also a couple of checksums used to verify
the integrity of the check-in.  And the whole manifest might
be PGP clearsigned.</p>

<a name="keyconc"></a>
<h3>2.3 Key concepts</h3>

<ul>
<li>A <b>check-in</b> is a set of files arranged
    in a hierarchy.</li>
<li>A <b>repository</b> keeps a record of historical check-ins.</li>
<li>Repositories share their changes using <b>push</b>, <b>pull</b>,
    <b>sync</b>, and <b>clone</b>.</li>
<li>A particular <u>version</u> of a particular file is an <b>artifact</b>
    that is identified by an <b>artifact ID</b>.</li>
<li>Artifacts tracked by Fossil are inherently immutable.</li>
<li>Fossil automatically generates a <b>manifest</b> file that identifies
    every artifact in a check-in.</li>
<li>The artifact ID of the manifest is the identifier of the check-in.</li>
</ul>

<h2>3.0 Fossil - The Program</h2>

Fossil is software.  The implementation of Fossil is in the form
of a single executable named "fossil" (or "fossil.exe" on Windows).
To install Fossil on your system,
all you have to do is obtain a copy of this one executable file (either
by downloading a
<a href="https://fossil-scm.org/home/uv/download.html">pre-compiled version</a>
or [./build.wiki | compiling it yourself]) and then
putting that file somewhere on your PATH.

Fossil is completely self-contained.  It is not necessary to
install any other software in order to use Fossil.  You do <u>not</u> need
CVS, gzip, diff, rsync, Python, Perl, Tcl, Java, Apache, PostgreSQL, MySQL,
SQLite, patch, or any similar software on your system in order to use
Fossil effectively.  You will want to have some kind of text editor
for entering check-in comments.  Fossil will use whatever text editor
is identified by your VISUAL environment variable.  Fossil will also
use GPG to clearsign your manifests if you happen to have it installed,
but Fossil will skip that step if GPG missing from your system.
You can optionally set up Fossil to use external "diff" programs,
though Fossil has an excellent built-in "diff" algorithm that works
fine for most people.  If you happen to have Tcl/Tk installed on your
system, Fossil will use it to generate a graphical "diff" display when
you use the --tk option to the "diff" command, but this too is entirely
optional.


To uninstall Fossil, simply delete the executable.

To upgrade an older version of Fossil to a newer version, just
replace the old executable with the new one.  You might need to
run "<b>fossil all rebuild</b>" to restructure your repositories after
an upgrade.  Running "all rebuild" never hurts, so when upgrading it
is a good policy to run it even if it is not strictly necessary.

To use Fossil, simply type the name of the executable in your
shell, followed by one of the various built-in commands and
arguments appropriate for that command.  For example:

<blockquote><b>
fossil help
</b></blockquote>

In the next section, when we say things like "use the <b>help</b>
command" we mean to use the command name "help" as the first
token after the name of the Fossil executable, as shown above.

<a name="workflow"></a>
<h2>4.0 Workflow</h2>

<verbatim type="pikchr float-right">
    down
R1: cylinder "Remote" "Repository" fill 0xadd8e6 rad 70%
    move 150%
R2: cylinder same "Local" "Repository" fill 0x90ee90
    move 120%
T1: line go 2.2cm heading 150 then 2.2cm west close \
       fill 0xffff00 "Local" below "Source Tree" below
    arrow from R2.n+(-0.25cm,+0.25cm) to R1.s+(-0.25cm,-0.25cm) \
       "push " rjust
    arrow from R1.s+(+0.25cm,-0.25cm) to R2.n+(+0.25cm,+0.25cm) \
       " pull" ljust " clone" ljust
    arrow from T1.start+(-0.25cm,+0cm) to R2.s+(-0.25cm,-0.25cm) \
       "commit " rjust
    arrow from R2.s+(+0.25cm,-0.25cm) to T1.start+(+0.25cm,+0cm) \
       " open" ljust " update" ljust " merge" ljust
</verbatim>


Fossil has two modes of operation: <i>"autosync"</i> and
<i>"manual-merge"</i>
Autosync mode is reminiscent of CVS or SVN in that it automatically
keeps your changes in synchronization with your co-workers through
the use of a central server.  The manual-merge mode is the standard workflow
for GIT or Mercurial in that your local repository develops
independently of your coworkers and you share and merge your changes manually.
An interesting feature of Fossil is that it supports both autosync
and manual-merge work flows.

The default setting for Fossil is to be in autosync mode.  You
can change the autosync setting or check the current autosync
setting using commands like:

<blockquote>
<b>fossil setting autosync on<br>
fossil setting autosync off<br>
<b>fossil settings</b>
</blockquote>

By default, Fossil runs with autosync mode turned on.  The
authors finds that projects run more smoothly in autosync mode since
autosync helps to prevent pointless forking and merging and helps keeps
all collaborators working on exactly the same code rather than on their
own personal forks of the code.  In the author's view, manual-merge mode
should be reserved for disconnected operation.

<h3>4.1 Autosync Workflow</h3>

<ol>
<li>
Establish a local repository using either the <b>new</b> command
to start a new project, or the <b>clone</b> command to make a clone
of a repository for an existing project.
</li>

<li>
Establish one or more source trees using
the <b>open</b> command with the name of the repository file as its
argument.
</li>

<li>
The <b>open</b> command in the previous step populates your local source
tree with a copy of the latest check-in.  Usually this is what you want.
In the rare cases where it is not, use the <b>update</b> command to
switch to a different check-in.
Use the <b>timeline</b> or <b>leaves</b> commands
to identify alternative check-ins to switch to.
</li>

<li>
Edit the code.  Add new files to the source tree using the <b>add</b>
command.  Omit files from future check-ins using the <b>rm</b> command.
(Even when you remove files from future check-ins, those files continue
to exist in historical check-ins.)  Test your changes.
</li>

<li>
Create a new check-in using the <b>commit</b> command.  You will be prompted
for a check-in comment and also for your GPG key if you have GPG installed.
The commit copies the edits you have made in your local source
tree into your local repository.  After your commit completes, Fossil will
automatically <b>push</b> your changes back to the server
you cloned from or whatever server you most recently synced with.
</li>

<li>
When your coworkers make their own changes, you can merge those changes
into your local local source tree using the <b>update</b> command.
In autosync mode, <b>update</b> will first go back to the server you
cloned from or with which you most recently synced, and pull down all
recent changes into your local repository.  Then it will merge recent
changes into your local source tree.  If you do an <b>update</b> and
find that it messes something up in your source tree (perhaps a co-worker
checked in incompatible changes) you can use the <b>undo</b> command
to back out the changes.
</li>

<li>
Repeat all of the above until you have generated great software.
</li>
</ol>

<h3>4.2 Manual-Merge Workflow</h3>

When autosync is disabled, the <b>commit</b> command is decoupled from
<b>push</b> and the <b>update</b> command is decoupled from <b>pull</b>.
That means you have to do a few extra steps in order to accomplish the
<b>push</b> and <b>pull</b> tasks manually.

<ol>
<li>
Establish a local repository using either the <b>new</b> command
to start a new project, or the <b>clone</b> command to make a clone
of a repository for an existing project.   The default setting for
a new repository is with autosync on, so you will need to turn it off
using the <b>setting autosync off</b> command with a <b>-R</b> option
to specify the repository.
</li>

<li>
Establish one or more source trees by changing your working directory
to where you want the root of the source tree to be, then issuing
the <b>open</b> command with the name of the repository file as its
argument.
</li>

<li>
The <b>open</b> command in the previous step populates your local source
tree with a copy of the latest check-in.  Usually this is what you want.
In the rare cases where it is not, use the <b>update</b> command to
switch to a different check-in.
Use the <b>timeline</b> or <b>leaves</b> commands
to identify alternative check-ins to switch to.
</li>

<li>
Edit the code.  Add new files to the source tree using the <b>add</b>
command.  Omit files from future check-ins using the <b>rm</b> command.
(Even when you remove files from future check-ins, those files continue
to exist in historical check-ins.)  Test your changes.
</li>

<li>
Create a new check-in using the <b>commit</b> command.  You will be prompted
for a check-in comment and also for your GPG key if you have GPG installed.
The commit copies the edits you have made in your local source
tree into your local repository.
</li>

<li>
Use the <b>push</b> command to push your changes out to a server
where your co-workers can access them.
</li>

<li>
When co-workers make their own changes, use the <b>pull</b> command
to pull those changes into your local repository.  Note that <b>pull</b>
does not move the changes into your local source tree, only into your
local repository.
</li>

<li>
Once changes are in your local repository, use
the <b>update</b> command to merge them to your local source tree.
If you merge in some changes and find that the changes do not work out
or are not to your liking, you can back out the changes using the
<b>undo</b> command.
</li>

<li>
If two or more people ran "commit" against the same check-in, this will
result in a [./branching.wiki | fork] which you may want to resolve by
running <b>merge</b> followed by another <b>commit</b>.
</li>

<li>
Repeat all of the above until you have generated great software.
</li>
</ol>

<h2>5.0 Setting Up A Fossil Server</h2>

With other configuration management software, setting up a server is
a lot of work and normally takes time, patience, and a lot of system
knowledge.  Fossil is designed to avoid this frustration.  Setting up
a server with Fossil is ridiculously easy.  You have four options:</p>

<ol>
<li><p><b>Stand-alone server.</b>
Simply run the [/help?cmd=server|fossil server] or
[/help?cmd=ui|fossil ui] command from the command-line.

<li><p><b>CGI.</b>
Install a 2-line CGI script on a CGI-enabled web-server like Apache.

<li><p><b>SCGI.</b>
Start an SCGI server using the
[/help?cmd=server| fossil server --scgi] command for handling
SCGI requests from web-servers like Nginx.

<li><p><b>Inetd or Stunnel.</b>
Configure programs like inetd, xinetd, or stunnel to hand off HTTP requests
directly to the [/help?cmd=http|fossil http] command.
</ol>

See the [./server/ | How To Configure A Fossil Server] document
for details.

<h2>6.0 Review Of Key Concepts</h2>

<ul>
<li>The <b>fossil</b> program is a self-contained stand-alone executable.
    Just put it somewhere on your PATH to install it.</li>
<li>Use the <b>clone</b> or <b>new</b> commands to create a new repository.</li>
<li>Use the <b>open</b> command to create a new source tree.</li>
<li>Use the <b>add</b> and <b>rm</b> or <b>delete</b> commands to add and
    remove files from the local source tree.</li>
<li>Use the <b>commit</b> command to create a new check-in.</li>
<li>Use the <b>update</b> command to merge in changes from others.</li>
<li>The <b>push</b> and <b>pull</b> commands can be used to share changes
    manually, but these things happen automatically in the default
    autosync mode.</li>
</ul>
