=======
Indices
=======

Status
======

:Date: 2007-07-14

This document describes the indexing facilities within breezy.


.. contents::


Motivation
==========

To provide a clean concept of index that can be reused by different
components within the codebase rather than being rewritten every time
by different components.


Terminology
===========

An **index** is a dictionary mapping opaque keys to opaque values.
Different index types may allow some of the value data to be interpreted
by the index. For example the ``GraphIndex`` index stores a graph between
keys as part of the index.


Overview
========

Breezy is moving to a write-once model for repository storage in order to
achieve lock-free repositories eventually. In order to support this, we are
making our new index classes **immutable**. That is, one creates a new
index in a single operation, and after that it is read only. To combine
two indices a ``Combined*`` index may be used, or an **index merge** may
be performed by reading the entire value of two (or more) indices and
writing them into a new index.

General Index API
=================

We may end up with multiple different Index types (e.g. GraphIndex,
Index, WhackyIndex). Even though these may require different method
signatures to operate would strive to keep the signatures and return
values as similar as possible. e.g.::

    GraphIndexBuilder - add_node(key, value, references)
    IndexBuilder - add_node(key, value)
    WhackyIndexBuilder - add_node(key, value, whackiness)

as opposed to something quite different like::

    node = IncrementalBuilder.get_node()
    node.key = 'foo'
    node.value = 'bar'

Services
--------

An initial implementation of indexing can probably get away with a small
number of primitives. Assuming we have write once index files:

Build index
~~~~~~~~~~~

This should be done by creating an ``IndexBuilder`` and then calling
``insert(key, value)`` many times. (Indices that support sorting,
topological sorting etc, will want specialised insert methods).

When the keys have all been added, a ``finish`` method should be called,
which will return a file stream to read the index data from.

Retrieve entries from the index
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This should allow random access to the index using readv, so we probably
want to open the index on a ``Transport``, then use ``iter_entries(keys)``,
which can return an iterator that yields ``(key, value)`` pairs in
whatever order makes sense for the index.

Merging of indices
~~~~~~~~~~~~~~~~~~

Merging of N indices requires a concordance of the keys of the index. So
we should offer a ``iter_all_entries`` call that has the same return type
as the ``iter_entries`` call.

Index implementations
=====================

GraphIndex
----------

``GraphIndex`` supports graph based lookups. While currently unoptimised
for reading, the index is quite space efficient at storing the revision
graph index for Breezy. The ``GraphIndexBuilder`` may be used to create one
of these indices by calling ``add_node`` until all nodes are added, then
``finish`` to obtain a file stream containing the index data. Multiple
indices may be queried using the ``CombinedGraphIndex`` class.




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