public-inbox-extindex-format(5)

1PUBLIC-INBOX-EXTINDEX-FORMATp(u5b)lic-inbox user manPuUaBlLIC-INBOX-EXTINDEX-FORMAT(5)
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NAME

6       public-inbox-extindex-format - external index format description
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DESCRIPTION

9       The extindex is an index-only evolution of the per-inbox SQLite and
10       Xapian indices used by public-inbox-v2-format(5) and
11       public-inbox-v1-format(5).  It exists to facilitate searches across
12       multiple inboxes as well as to reduce index space when messages are
13       cross-posted to several existing inboxes.
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15       It transparently indexes messages across any combination of v1 and v2
16       inboxes and data about inboxes themselves.
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DIRECTORY LAYOUT

19       While inspired by v2, there is no git blob storage nor "msgmap.sqlite3"
20       DB.
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22       Instead, there is an "ALL.git" (all caps) git repo which treats every
23       indexed v1 inbox or v2 epoch as a git alternate.
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25       As with v2 inboxes, it uses "over.sqlite3" and Xapian "shards" for WWW
26       and IMAP use.  Several exclusive new tables are added to deal with
27       "XREF3 DEDUPLICATION" and metadata.
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29       Unlike v1 and v2 inboxes, it is NOT designed to map to a NNTP
30       newsgroup.  Thus it lacks "msgmap.sqlite3" to enforce the unique
31       Message-ID requirement of NNTP.
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33   INDEX OVERVIEW AND DEFINITIONS
34         $SCHEMA_VERSION - DB schema version (for Xapian)
35         $SHARD - Integer starting with 0 based on parallelism
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37         foo/                              # "foo" is the name of the index
38         - ei.lock                         # lock file to protect global state
39         - ALL.git                         # empty, alternates for inboxes
40         - ei$SCHEMA_VERSION/$SHARD        # per-shard Xapian DB
41         - ei$SCHEMA_VERSION/over.sqlite3  # overview DB for WWW, IMAP
42         - ei$SCHEMA_VERSION/misc          # misc Xapian DB
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44       File and directory names are intentionally different from analogous v2
45       names to ensure extindex and v2 inboxes can easily be distinguished
46       from each other.
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48   XREF3 DEDUPLICATION
49       Due to cross-posted messages being the norm in the large Linux kernel
50       development community and Xapian indices being the primary consumer of
51       storage, it makes sense to deduplicate indexing as much as possible.
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53       The internal storage format is based on the NNTP "Xref" tuple, but with
54       the addition of a third element: the git blob OID.  Thus the triple is
55       expressed in string form as:
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57               $NEWSGROUP_NAME:$ARTICLE_NUM:$OID
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59       If no "newsgroup" is configured for an inbox, the "inboxdir" of the
60       inbox is used.
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62       This data is stored in the "xref3" table of over.sqlite3.
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64   misc XAPIAN DB
65       In addition to the numeric Xapian shards for indexing messages, there
66       is a new, in-development Xapian index for storing data about inboxes
67       themselves and other non-message data.  This index allows us to speed
68       up operations involving hundreds or thousands of inboxes.
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BENEFITS

71       In addition to providing cross-inbox search capabilities, it can also
72       replace per-inbox Xapian shards (but not per-inbox over.sqlite3).  This
73       allows reduction in disk space, open file handles, and associated
74       memory use.
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CAVEATS

77       Relocating v1 and v2 inboxes on the filesystem will require extindex to
78       be garbage-collected and/or reindexed.
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80       Configuring and maintaining stable "newsgroup" names before any
81       messages are indexed from every inbox can avoid expensive reindexing
82       and rely exclusively on GC.
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LOCKING

85       flock(2) locking exclusively locks the empty ei.lock file for all non-
86       atomic operations.
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THANKS

89       Thanks to the Linux Foundation for sponsoring the development and
90       testing.
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COPYRIGHT

93       Copyright 2020-2021 all contributors <mailto:meta@public-inbox.org>
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95       License: AGPL-3.0+ <http://www.gnu.org/licenses/agpl-3.0.txt>
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