% public-inbox developer manual

=head1 NAME

public-inbox-v1-format - git repository and tree description (aka "ssoma")

=head1 DESCRIPTION

WARNING: this does NOT describe the scalable v2 format used
by public-inbox.  Use of ssoma is not recommended for new
installations due to scalability problems.

ssoma uses a git repository to store each email as a git blob.
The tree filename of the blob is based on the SHA1 hexdigest of
the first Message-ID header.  A commit is made for each message
delivered.  The commit SHA-1 identifier is used by ssoma clients
to track synchronization state.

=head1 PATHNAMES IN TREES

A Message-ID may be extremely long and also contain slashes, so using
them as a path name is challenging.  Instead we use the SHA-1 hexdigest
of the Message-ID (excluding the leading "E<lt>" and trailing "E<gt>")
to generate a path name.  Leading and trailing white space in the
Message-ID header is ignored for hashing.

A message with Message-ID of: E<lt>20131106023245.GA20224@dcvr.yhbt.netE<gt>

Would be stored as: f2/8c6cfd2b0a65f994c3e1be266105413b3d3f63

Thus it is easy to look up the contents of a message matching a given
a Message-ID.

=head1 MESSAGE-ID CONFLICTS

public-inbox v1 repositories currently do not resolve conflicting
Message-IDs or messages with multiple Message-IDs.

=head1 HEADERS

The Message-ID header is required.
"Bytes", "Lines" and "Content-Length" headers are stripped and not
allowed, they can interfere with further processing.
When using ssoma with public-inbox-mda, the "Status" mbox header
is also stripped as that header makes no sense in a public archive.

=head1 LOCKING

L<flock(2)> locking exclusively locks the empty $GIT_DIR/ssoma.lock file
for all non-atomic operations.

=head1 EXAMPLE INPUT FLOW (SERVER-SIDE MDA)

1. Message is delivered to a mail transport agent (MTA)

1a. (optional) reject/discard spam, this should run before ssoma-mda

1b. (optional) reject/strip unwanted attachments

ssoma-mda handles all steps once invoked.

2. Mail transport agent invokes ssoma-mda

3. reads message via stdin, extracting Message-ID

4. acquires exclusive flock lock on $GIT_DIR/ssoma.lock

5. creates or updates the blob of associated 2/38 SHA-1 path

6. updates the index and commits

7. releases $GIT_DIR/ssoma.lock

ssoma-mda can also be used as an L<inotify(7)> trigger to monitor maildirs,
and the ability to monitor IMAP mailboxes using IDLE will be available
in the future.

=head1 GIT REPOSITORIES (SERVERS)

ssoma uses bare git repositories on both servers and clients.

Using the L<git-init(1)> command with --bare is the recommend method
of creating a git repository on a server:

	git init --bare /path/to/wherever/you/want.git

There are no standardized paths for servers, administrators make
all the choices regarding git repository locations.

Special files in $GIT_DIR on the server:

=over

=item $GIT_DIR/ssoma.lock

An empty file for L<flock(2)> locking.
This is necessary to ensure the index and commits are updated
consistently and multiple processes running MDA do not step on
each other.

=item $GIT_DIR/public-inbox/msgmap.sqlite3

SQLite3 database maintaining a stable mapping of Message-IDs to NNTP
article numbers.  Used by L<public-inbox-nntpd(1)> and created
and updated by L<public-inbox-index(1)>.

Users of the L<PublicInbox::WWW> interface will find it
useful for attempting recovery from copy-paste truncations of
URLs containing long Message-IDs.

Automatically updated by L<public-inbox-mda(1)>,
L<public-inbox-learn(1)> and L<public-inbox-watch(1)>.

Losing or damaging this file will cause synchronization problems for
NNTP clients.  This file is expected to be stable and require no
updates to its schema.

Requires L<DBD::SQLite>.

=item $GIT_DIR/public-inbox/xapian$N/

Xapian database for search indices in the PSGI web UI.

$N is the value of PublicInbox::Search::SCHEMA_VERSION, and
installations may have parallel versions on disk during upgrades
or to roll-back upgrades.

This is created and updated by L<public-inbox-index(1)>.

Automatically updated by L<public-inbox-mda(1)>,
L<public-inbox-learn(1)> and L<public-inbox-watch(1)>.

This directory can always be regenerated with L<public-inbox-index(1)>.
If lost or damaged, there is no need to back it up unless the
CPU/memory cost of regenerating it outweighs the storage/transfer cost.

Since SCHEMA_VERSION 15 and the development of the v2 format,
the "overview" DB also exists in the xapian directory for v1
repositories.  See L<public-inbox-v2-format(5)/OVERVIEW DB>

Our use of the L</OVERVIEW DB> requires Xapian document IDs to
remain stable.  Using L<public-inbox-compact(1)> and
L<public-inbox-xcpdb(1)> wrappers are recommended over tools
provided by Xapian.

This directory is large, often two to three times the size of
the objects stored in a packed git repository.

=item $GIT_DIR/ssoma.index

This file is no longer used or created by public-inbox, but it is
updated if it exists to remain compatible with ssoma installations.

A git index file used for MDA updates.  The normal git index (in
$GIT_DIR/index) is not used at all as there is typically no working
tree.

=back

Each client $GIT_DIR may have multiple mbox/maildir/command targets.
It is possible for a client to extract the mail stored in the git
repository to multiple mboxes for compatibility with a variety of
different tools.

=head1 CAVEATS

It is NOT recommended to check out the working directory of a git.
there may be many files.

It is impossible to completely expunge messages, even spam, as git
retains full history.  Projects may (with adequate notice) cycle to new
repositories/branches with history cleaned up via L<git-filter-repo(1)>
or L<git-filter-branch(1)>.
This is up to the administrators.

=head1 COPYRIGHT

Copyright 2013-2021 all contributors L<mailto:meta@public-inbox.org>

License: AGPL-3.0+ L<http://www.gnu.org/licenses/agpl-3.0.txt>

=head1 SEE ALSO

L<gitrepository-layout(5)>, L<ssoma(1)>