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lib: Document the metadata stored within the Xapian database.

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We are currently storing "version", "last_thread_id", and
"thread_id_*" values so document how each of these are used.
This commit is contained in:
Carl Worth 2010-04-12 15:15:14 -07:00
parent af49741228
commit 328626d0fd
1 changed files with 45 additions and 1 deletions
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46
lib/database.cc
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@ -44,7 +44,8 @@ typedef struct {
/* Here's the current schema for our database (for NOTMUCH_DATABASE_VERSION):
*
* We currently have two different types of documents: mail and directory.
* We currently have two different types of documents (mail and
* directory) and also some metadata.
*
* Mail document
* -------------
@ -118,6 +119,49 @@ typedef struct {
*
* The data portion of a directory document contains the path of the
* directory (relative to the database path).
*
* Database metadata
* -----------------
* Xapian allows us to store arbitrary name-value pairs as
* "metadata". We currently use the following metadata names with the
* given meanings:
*
* version The database schema version, (which is distinct
* from both the notmuch package version (see
* notmuch --version) and the libnotmuch library
* version. The version is stored as an base-10
* ASCII integer. The initial database version
* was 1, (though a schema existed before that
* were no "version" database value existed at
* all). Succesive versions are allocated as
* changes are made to the database (such as by
* indexing new fields).
*
* last_thread_id The last thread ID generated. This is stored
* as a 16-byte hexadecimal ASCII representation
* of a 64-bit unsigned integer. The first ID
* generated is 1 and the value will be
* incremented for each thread ID.
*
* thread_id_* A pre-allocated thread ID for a particular
* message. This is actually an arbitarily large
* family of metadata name. Any particular name
* is formed by concatenating "thread_id_" with a
* message ID. The value stored is a thread ID.
*
* These thread ID metadata values are stored
* whenever a message references a parent message
* that does not yet exist in the database. A
* thread ID will be allocated and stored, and if
* the message is later added, the stored thread
* ID will be used (and the metadata value will
* be cleared).
*
* Even before a message is added, it's
* pre-allocated thread ID is useful so that all
* descendant messages that reference this common
* parent can be recognized as belonging to the
* same thread.
*/
/* With these prefix values we follow the conventions published here: