<title>Bug-Tracking In Fossil</title>
<h2>Introduction</h2>

A bug-report in fossil is called a "ticket".  Tickets are tracked
separately from code check-ins.

Some other distributed bug-tracking systems store tickets as files within
the source tree and thereby leverage the syncing and merging capabilities
of the versioning system to sync and merge tickets.  This approach is
rejected in fossil for three reasons:

  1.  Check-ins in fossil are immutable.  So if
      tickets were part of the check-in, then there would be no way to add
      new tickets to a check-in as new bugs are discovered.

  2.  Any project of reasonable size and complexity will generate thousands
      and thousands of tickets, and we do not want all those ticket files
      cluttering the source tree.

  3.  We want tickets to be managed from the web interface and to have a
      permission system that is distinct from check-in permissions.
      In other words, we do not want to restrict the creation and editing
      of tickets to developers with check-in privileges and an installed
      copy of the fossil executable.  Casual passers-by on the internet should
      be permitted to create tickets.

Recall that a fossil repository consists of an
unordered collection of <i>artifacts</i>.  (See the
<a href="fileformat.wiki">file format document</a> for details.)
Some artifacts have a special format, and among those are
<a href="fileformat.wiki#tktchng">Ticket Change Artifacts</a>.
One or more ticket change artifacts are associated with each
ticket.  A ticket is created by a ticket change artifact.
Each subsequent modification of the ticket is a separate artifact.

The "push", "pull", and "sync" algorithms share ticket change artifacts
between repositories in the same way as every other artifact.  In fact,
the sync algorithm has no knowledge of the meaning of the artifacts it
is syncing.  As far as the sync algorithm is concerned, all artifacts are
alike.  After the sync has occurs, the individual repositories must
make sense of the meaning of the various artifacts for themselves.

<h2>Interpretation Of Ticket Change Artifacts</h2>

<i>Note: The following is implementation detail which can be safely ignored by
casual users of fossil.</i>

Every ticket change artifact contains (among other things)

  *  a timestamp,
  *  a ticket ID, and
  *  one or more name/value pairs.

The current state of a ticket is found by replaying all ticket
change artifacts with the same ticket ID in timestamp order.
For a given ticket, all values are initially NULL.  As each
ticket change artifact is encountered, values are either replaced
or appended, according to a flag on the name/value pair.  The current
values for the fields of a ticket are the values that remain at the
end of the replay process.

To create a new ticket, one inserts a ticket change
artifact with a new ID.  The ticket ID
is a random 40-character lower-case hexadecimal number.  The "tktnew"
page in the fossil web interface creates new ticket IDs
using a good source of randomness to insure uniqueness.
The name/value pairs on the initial ticket change
artifact are the initial values for the fields in the ticket.

Amending a ticket means simply creating a new artifact with the
same ticket ID and with name/value pairs for those fields which
are changing.  Fields of the ticket which are not being modified
should not appear as name/value pairs in the new artifact.

This approach to storing ticket state means that independently entered
changes are automatically merged together when artifacts are shared
between repositories.  Tickets do not branch.  This approach also makes
it trivial to track the historic progression of changes to a ticket.

In order for this scheme to work, the system clocks on machines that
add new ticket changes artifacts have to be set close to reality.
It is OK for a ticket change artifact timestamp to be off by a few
minutes or even a few hours.  But if a timestamp on a ticket change
artifact is off by months or years, it can seriously confuse the
replay algorithm for determining the current ticket state.  There
are techniques available to cause rogue artifacts to be ignored
by fossil.  So if a ticket change artifact with a bad timestamp
does get into a repository, it can be removed by an administrator.
But the best approach is to take steps to insure that timestamps
are approximately correct in the first place.

<h2>Local Configuration</h2>

The ticket change artifacts are part of the global state for a project.
The global state is that which is shared between repositories during
a sync.  Each repository also has local state which is not (normally)
shared.  The name/value pairs for a ticket are part of global state,
but the interpretation and display of that information is local state.
Hence, each repository is free to set up its own ticket display and
input formats and reporting rules according to its own particular needs.

Each repository defines its own TICKET table in its database.  There is
one row in the TICKET table for each unique ticket ID.  The
names of columns in the TICKET table correspond to the names in
the name/value pairs of ticket change artifacts.  When running the replay
algorithm, if a name/value pair is seen which has no corresponding column
in the TICKET table, then that name/value pair is ignored.  Columns can
be added or removed from the TICKET table at any time.  Whenever the
TICKET table is modified, the replay algorithm automatically reruns
to repopulate the table using the new column names.  Note that the TICKET
table schema and content is part of the local state of a repository
and is not shared with other repositories during a sync, push, or pull.

Each repository also defines scripts used to generate web pages for
creating new tickets, viewing existing tickets, and modifying an
existing ticket.  These scripts consist of HTML with an embedded
scripts written in a Tcl-like language called "[./th1.md|TH1]".  Every
new fossil repository is created with default scripts. Administrators
wishing to customize their ticket entry, viewing, and editing screens
should modify the default scripts to suit their needs.  These screen
generator scripts are part of the local state of a repository and are
not shared with other repositories during a sync, push, or pull.

<i>To be continued...</i>
