Home

judd edited this page Sep 12, 2010 · 36 revisions
Clone this wiki locally

UPDATE / REORGANISATION
====
The acts_as_xapian Google Group is now the main place to look for updates and news about the project.

  • Read about the latest news on the welcome screen of the Google Group! The project is now maintained by Lukas Rieder (alias Overbryd).
  • You can fork from here now: http://github.com/Overbryd/acts_as_xapian/tree/master
  • You’re welcome on the Google Group. Feel free to ask or discuss anything about the acts_as_xapian project.
  • If you have found a bug or you want to request a feature do use the Lighthouse Tracker
  • This wiki page will remain as the Readme with instructions as it was before.

From Francis Irving, the creator of acts_as_xapian:
I’ve stopped working on acts_as_xapian, in order to focus on other projects. Please ask any questions
to the community of people at the acts_as_xapian Google Group -
and this being github, you can get updates to acts_as_xapian by pulling from other people’s repositories.
Ask on the Google Group who to pull from! — Francis Irving

Do patch this file if there is documentation missing / wrong. It’s called
README.txt and is in git, using Textile formatting. The wiki page is just
copied from the README.txt file.

Contents
====

  • a. Introduction to acts_as_xapian
  • b. Installation
  • c. Comparison to acts_as_solr (as on 24 April 2008)
  • d. Documentation – indexing
  • e. Documentation – querying
  • f. Configuration
  • g. Support

a. Introduction to acts_as_xapian
=========

Xapian is a full text search engine library which has
Ruby bindings. acts_as_xapian adds support for it to Rails. It is an
alternative to acts_as_solr, acts_as_ferret, Ultrasphinx, acts_as_indexed,
acts_as_searchable or acts_as_tsearch.

acts_as_xapian is deployed in production on these websites.

The section “c. Comparison to acts_as_solr” below will give you an idea of
acts_as_xapian’s features.

acts_as_xapian was started by Francis Irving in May 2008 for search and email
alerts in WhatDoTheyKnow, and so was supported by mySociety
and initially paid for by the JRSST Charitable Trust

Installation
===

Retrieve the plugin directly from the git version control system by running
this command within your Rails app.

git clone git://github.com/frabcus/acts_as_xapian.git vendor/plugins/acts_as_xapian

Xapian 1.0.5 and associated Ruby bindings are also required.

Debian or Ubuntu – install the packages libxapian15 and libxapian-ruby1.8.

Mac OSX – follow the instructions for installing from source on
the Installing Xapian page – you need the
Xapian library and bindings (you don’t need Omega).

There is no Ruby Gem for Xapian, it would be great if you could make one!

c. Comparison to acts_as_solr (as on 24 April 2008)
=========

  • Offline indexing only mode – which is a minus if you want changes
    immediately reflected in the search index, and a plus if you were going to
    have to implement your own offline indexing anyway.
  • Collapsing – the equivalent of SQL’s “group by”. You can specify a field
    to collapse on, and only the most relevant result from each value of that
    field is returned. Along with a count of how many there are in total.
    acts_as_solr doesn’t have this.
  • No highlighting – Xapian can’t return you text highlighted with a search
    query. You can try and make do with TextHelper::highlight (combined with
    words_to_highlight below). I found the highlighting in acts_as_solr didn’t
    really understand the query anyway.
  • Date range searching – this exists in acts_as_solr, but I found it
    wasn’t documented well enough, and was hard to get working.
  • Spelling correction – “did you mean?” built in and just works.
  • Similar documents – acts_as_xapian has a simple command to find other models
    that are like a specified model.
  • Multiple models – acts_as_xapian searches multiple types of model if you
    like, returning them mixed up together by relevancy. This is like
    multi_solr_search, only it is the default mode of operation and is properly
    supported.
  • No daemons – However, if you have more than one web server, you’ll need to
    work out how to use Xapian’s remote backend.
  • One layer – full-powered Xapian is called directly from the Ruby, without
    Solr getting in the way whenever you want to use a new feature from Lucene.
  • No Java – an advantage if you’re more used to working in the rest of the
    open source world. acts_as_xapian, it’s pure Ruby and C++.
  • Xapian’s awesome email list – the kids over at
    xapian-discuss
    are super helpful. Useful if you need to extend and improve acts_as_xapian. The
    Ruby bindings are mature and well maintained as part of Xapian.

d. Documentation – indexing
=======

Xapian is an offline indexing search library – only one process can have the
Xapian database open for writing at once, and others that try meanwhile are
unceremoniously kicked out. For this reason, acts_as_xapian does not support
immediate writing to the database when your models change.

Instead, there is a ActsAsXapianJob model which stores which models need
updating or deleting in the search index. A rake task ‘xapian:update_index’
then performs the updates since last change. You can run it on a cron job, or
similar.

Here’s how to add indexing to your Rails app:

1. Put acts_as_xapian in your models that need search indexing. e.g.

acts_as_xapian :texts => [ :name, :short_name ], :values => [ [ :created_at, 0, “created_at”, :date ] ], :terms => [ [ :variety, ‘V’, “variety” ] ]

Options must include:

  • :texts, an array of fields for indexing with full text search.
    e.g. :texts => [ :title, :body ]
  • :values, things which have a range of values for sorting, or for collapsing.
    Specify an array quadruple of [ field, identifier, prefix, type ] where
    • identifier is an arbitary numeric identifier for use in the Xapian database
    • prefix is the part to use in search queries that goes before the :
    • type can be any of :string, :number or :date

e.g. :values => [ [ :created_at, 0, “created_at”, :date ],
[ :size, 1, “size”, :string ] ]

  • :terms, things which come with a prefix (before a :) in search queries.
    Specify an array triple of [ field, char, prefix ] where
    • char is an arbitary single upper case char used in the Xapian database, just
      pick any single uppercase character, but use a different one for each prefix.
    • prefix is the part to use in search queries that goes before the :
      For example, if you were making Google and indexing to be able to later do a
      query like “site:www.whatdotheyknow.com”, then the prefix would be “site”.

e.g. :terms => [ [ :variety, ‘V’, “variety” ] ]

A ‘field’ is a symbol referring to either an attribute or a function which
returns the text, date or number to index. Both ‘identifier’ and ‘char’ must be
the same for the same prefix in different models.

Options may include:

  • :eager_load, added as an :include clause when looking up search results in
    database
  • :if, either an attribute or a function which if returns false means the
    object isn’t indexed

2. Generate a database migration to create the ActsAsXapianJob model:

script/generate acts_as_xapian rake db:migrate

3. Call ‘rake xapian:rebuild_index models=“ModelName1 ModelName2”’ to build the index
the first time (you must specify all your indexed models). It’s put in a
development/test/production dir in acts_as_xapian/xapiandbs. See f. Configuration
below if you want to change this.

4. Then from a cron job or a daemon, or by hand regularly!, call ‘rake xapian:update_index’

e. Documentation – querying
=======

Testing indexing
-——————-

If you just want to test indexing is working, you’ll find this rake task
useful (it has more options, see tasks/xapian.rake)

rake xapian:query models=“PublicBody User” query=“moo”

You can also specify options such as offset, limit, sort_by_prefix and collapse_by_prefix

rake xapian:query models=“PublicBody InfoRequestEvent” query=“foobar” limit=30 offset=20

The example above query both models for ‘foobar’ starting at the 20th entry and limit your result to 30 (instead of the default 10)

Performing a query
-———————-

To perform a query from code call ActsAsXapian::Search.new. This takes in turn:

  • model_classes – list of models to search, e.g. [PublicBody, InfoRequestEvent]
  • query_string – Google like syntax, see below

And then a hash of options:

  • :offset – Offset of first result (default 0)
  • :limit – Number of results per page
  • :sort_by_prefix – Optionally, prefix of value to sort by, otherwise sort by relevance
  • :sort_by_ascending – Default true (documents with higher values better/earlier), set to false for descending sort
  • :collapse_by_prefix – Optionally, prefix of value to collapse by (i.e. only return most relevant result from group)

Google like query syntax is as described in
Xapian::QueryParser Syntax
Queries can include prefix:value parts, according to what you indexed in the
acts_as_xapian part above. You can also say things like model:InfoRequestEvent
to constrain by model in more complex ways than the :model parameter, or
modelid:InfoRequestEvent-100 to only find one specific object.

Returns an ActsAsXapian::Search object. Useful methods are:

  • description – a techy one, to check how the query has been parsed
  • matches_estimated – a guesstimate at the total number of hits
  • spelling_correction – the corrected query string if there is a correction, otherwise nil
  • words_to_highlight – list of words for you to highlight, perhaps with TextHelper::highlight
  • results – an array of hashes each containing:
    • :model – your Rails model, this is what you most want!
    • :weight – relevancy measure
    • :percent – the weight as a %, 0 meaning the item did not match the query at all
    • :collapse_count – number of results with the same prefix, if you specified collapse_by_prefix

Finding similar models
-—————————-

To find models that are similar to a given set of models call ActsAsXapian::Similar.new. This takes:

  • model_classes – list of model classes to return models from within
  • models – list of models that you want to find related ones to

Returns an ActsAsXapian::Similar object. Has all methods from ActsAsXapian::Search above, except
for words_to_highlight. In addition has:

  • important_terms – the terms extracted from the input models, that were used to search for output
    You need the results methods to get the similar models.

f. Configuration
====

If you want to customise the configuration of acts_as_xapian, it will look for a file called ‘xapian.yml’
under RAILS_ROOT/config. As is familiar from the format of the database.yml file, separate :development,
:test and :production sections are expected.

The following options are available:

  • base_db_path – specifies the directory, relative to RAILS_ROOT, in which acts_as_xapian stores its
    search index databases. Default is the directory xapiandbs within the acts_as_xapian directory.

g. Support
==

Please ask any questions on the
acts_as_xapian Google Group

The official home page and repository for acts_as_xapian are the
acts_as_xapian github page

For more details about anything, see source code in lib/acts_as_xapian.rb

Merging source instructions “Using git for collaboration” here:
http://www.kernel.org/pub/software/scm/git/docs/gittutorial.html