lei design notes

Daemon architecture

The use of a persistent daemon works around slow startup time of
Perl.  This is especially important for built-in support for
shell completion.  It attempts to support inotify and EVFILT_VNODE
background monitoring of Maildir keyword changes.

If lei were reimplemented in a language with faster startup
time, the daemon architecture would likely remain since it also
lets us easily decouple the local storage from slow IMAP/NNTP
backends and allow us to serialize writes to git-fast-import,
SQLite, and Xapian across multiple processes.

The coupling of IMAP and NNTP network latency to local storage
is a current weakness of public-inbox-watch.  Therefore, -watch
will likely adopt the daemon architecture of lei in the future.

Read/write vs read-only storage

public-inboxes are intended to be written and read by different
Unix users.  Commonly, a single Unix user or group will write to
a public-inbox, but the inbox will be served by a user with
read-only permissions (e.g. "www-data" or "nobody").

lei/store is intended to be read and written by a single user,
thus we can rely on the Write-Ahead-Log journal of SQLite to
improve performance: <https://sqlite.org/wal.html>