Ruby sittin' on the Couch

About me

• Alexander Lang
• Upstream Agile GmbH, Berlin
programmer, owner
• http://upstream-berlin.com

TDD
BDD
“agile”
Pair Programming

* all the f****** time

About me

• playing with CouchDB since 09/2008
• helped hack the CouchDB statistics module
(with @janl)
• wrote Couch Potato
• working on smaller production apps

Who is in this room?
• Chris Anderson
• George Palmer
• Paul Carey
• Johan Sørensen
• Cheah Chu Yeow/Carlos Villela

Who is in this room?
• Chris Anderson (couchrest + couchapp)
• George Palmer (couch_foo)
• Paul Carey (relaxdb)
• Johan Sørensen (couchbject)
• Cheah Chu Yeow/Carlos Villela (activecouch)

Agenda

• CouchDB introduction
• The CouchDB example wiki
• The frameworks
• Conclusion: where to go from here?
• Q &A

Apache Project
so it has to be good

Document oriented
Database

Store/read any JSON
document

Powerful map/reduce
views for querying*
* we’ll see what that is

Map/Reduce
the thing that made Google rich

No more SQL/Schema/
Migrations

can use existing
clients, libraries

integrated replication
(yes, multi master)

use existing load
balancer, proxies etc.
HTTP ftw

JSON
{
_id: “some UUID”,
_rev: “MVCC key”,
title: “page one”
body: “this is page one.”,
tags: [“ruby”, “couchdb”, “sor09”]
metadata: {created_at: “2009/03/28 06:34:00”,
author: “alex”}
}

HTTP API

• POST ‘/my_db’, {my_json}
• GET ‘/my_db/my_document_id’
• PUT ‘/my_db/my_document_id’,
{new_json}
• DELETE ‘/my_db/my_document_id’

Map/Reduce views

• views are documens
• provide a map (and optional reduce
function) written in JavaScript
• this creates an index over all documents
• query that index via GET

Map/Reduce views
{
title: “page one”,
tags: [“ﬁrst”, important”]
}

{
title: “page 2”,
tags: [“funny”]
}

Map/Reduce views
{ function(doc) {
emit(doc.title, doc.tags.length)
}
}

{
tags: [“funny”]
}

Map/Reduce views
{ function(doc) {
}
}
key value
{
“page one” 2
tags: [“funny”] “page 2” 1
}

Map/Reduce views
{ function(doc) {
}
}
key value
{
“page one” 2
}

function(keys, values) {
return sum(values);
}

Map/Reduce views
{ function(doc) {
}
}
key value
{
“page one” 2
}

return sum(values);
3
}

Query a View

/my_db/_design/wiki/_view/tags_count

Query a View

/my_db/_design/wiki/_view/tags_count?reduce=false

Query a View

/my_db/_design/wiki/_view/tags_count?limit=1

Query a View

/my_db/_design/wiki/_view/tags_count?limit=1
/my_db/_design/wiki/_view/tags_count?key=page one

Query a View
?key=‘page one’

Query a View
?key=‘page one’
?startkey=‘page 1’&endkey=’page 999’

Query a View
?key=‘page one’
?key=[‘composite’, ‘key’]

Query a View
?key=‘page one’
?key=[‘composite’, ‘key’]
?keys=[‘set’, ‘of’, ‘keys’]

That is CouchDB
(the basics)

• upload documents via POST/PUT
• read documents via GET
• create indexes by providing map/reduce
functions
• query views via GET, pass keys + other
options

Example Wiki
• ActiveRecord • RelaxDB
• CouchRest • Couch Potato
• ActiveCouch • CouchOject
• CouchFoo • CouchApp

• a wiki example app implemented in all
frameworks I could ﬁnd
• + in ActiveRecord for comparison

DISCLAIMER: implementations are insecure, have
bugs and aren’t meant for production at all

http://github.com/langalex/couchdb_example_wiki

Example Wiki

• a few simple features
• create a page
• add new pages by clicking on a
CamelCase link
• list of pages

Example Wiki

• and a few special cases
• keep history of each page, browse old
versions
• statistics: count occurrences of all words
in all pages

Example Wiki

• creating pages is easy
• versioning, statistics harder
• views are the source of CouchDB’s power

let me show you how it works
Example Wiki

routes.rb
map.resources :pages do |pages|
pages.resources :versions
end

map.resources :statistics
map.root :controller => quot;pagesquot;, :action => 'show'

page.rb
class Page < ActiveRecord::Base
acts_as_versioned

def to_param
title
end
end

pages/show.html.erb
<%= <%= simple_format linkify(@page.body) %>
simple_format linkify(@page.body) %>

def linkify(text)
text.gsub(/([A-Z][a-z]+([A-Z][a-z]+)+)/) do
link_to($1, page_path($1))
end
end

replace CamelCase words with links to #show

pages_controller.rb

def show
@page = Page.first unless params[:id]
@page ||= Page.find_by_title params[:id]
redirect_to new_page_path(:title => params[:id]) unless @page
end

redirect to #new if no page found

statistics
def self.word_counts
Page.all.map(&:body).join(quot; quot;).split(/s
+/).grep(/w+/i).inject(Hash.new(0)) do |res,
word|
res[word] += 1
res
end
end

schema.rb
create_table quot;page_versionsquot;, :force => true do |t|
t.integer quot;page_idquot;
t.integer quot;versionquot;
t.text quot;bodyquot;
t.datetime quot;created_atquot;
t.datetime quot;updated_atquot;
end

create_table quot;pagesquot;, :force => true do |t|
t.string quot;titlequot;
t.text quot;bodyquot;
t.datetime quot;created_atquot;
t.datetime quot;updated_atquot;
t.integer quot;versionquot;, :default => 1
end

AR Summary

• Page has_many PageVersions
• ugly schema with duplicated table
• acts_as_versioned does the magic for us
• statistics - ?!?

CouchDB Wiki
How does it work?

The Page
{
_id: “89765”,
_rev: “lb7tlb”,
type: “Page”,
body: “this is page one”
}

Page Versions
{
_id: “765”,
_rev: “lhjb97”
type: “PageVersion”,
body: “this is page one”,
version: 23,
page_id: “89765”
}

All in one namespace
{ {
_id: “ 89765”, {
_id: “ 9753”,
_id: “97865”,
_rev: “lb7tlb”, _rev: “lb7tlb”,
_rev: “lhjb97”
type: “Page”, type: “PageVersion”,
type: “Page”,
title: “page one”, title: “page one”,
body: “this is page one”,
body: “this is page one” version: 23,
body: “this is page one”
page_id: “89765”
} {
}
}
_id: “6437”,
_rev: “lhjb97” {
type: “PageVersion”, _id: “6367”,
{ { _rev: “lhjb97”
body: “this is page one”, type: “PageVersion”,
_id: “ 76538”, _id: “ 8975763”,
version: 23, title: “page one”,
page_id: “89765”
_rev: “lb7tlb”, } _rev: “lb7tlb”, body: “this is page one”,
version: 23,
type: “Page”, type: “Page”, page_id: “89765”
title: “page one”, title: “page one”, }
body: “this is page one” body: “this is page one”
} }

Finding pages
function(doc) { key value
if(doc.type == “Page”) {
emit(doc.title, doc);
“page 2” {...}
}
}
“page one” {...}

Finding Page Versions
function(doc) {
if(doc.type == “PageVersion”) {
emit([doc.page_id, doc.version], doc);
}
} key value
[“ladsb7gi”, 1] {...}
[“ladsb7gi”, 2] {...}
[“nloh79d”, 1] {...}

Finding Page Versions
key value
[“ladsb7gi”, 1] {...}
[“ladsb7gi”, 2] {...}
[“nloh79d”, 1] {...}

GET /mydb/_design/page_versions/_view/by_page?
startkey=[“ladsb7gi”, 1]&endkey=[“ ladsb7gi”,{}]

Counting Words - Map

{body: “page one”}
{body: “page 2”}

function(doc) {
if(doc.type == 'Page') {
var words = doc.body.split(/W/);
words.forEach(function(word) {
if (word.length > 0) emit(word, 1);
});}}


function(doc) {
if(doc.type == 'Page') {
});}}

key value
“page” 1
“page” 1
“one” 1
“2” 1

Counting Words - reduce
key value
“page” 1
“page” 1
“one” 1
“2” 1

key value
“page” 1
“page” 1
“one” 1
“2” 1

return sum(values);
}

key value
“page” 1
“page” 1
“one” 1
“2” 1
key value
“page” 2
return sum(values);
“one” 1
}

“2” 1

CouchDB summary

• no schema, arbitrary documents in one
namespace
• still use foreign keys to implement
associations
• create views instead of join tables
• views! views! views!

Show us the
frameworks already

What’s the framework’s job?

ActiveRecord

ActiveRecord

• map objects to relations and back
• do the whole SQL thing

ActiveRecord
schema attribute auto
tracking detection
serialized attributes pagination
groups
JOINS unique lazy loading
caching
• map objects to relations and back count
• do the whole SQL thing HABTM
create or update?
has_many :through conditions
eager association loading
connection DDL :dependent => :destroy

ActiveRecord
Page.all :include => :tags

SELECT pages.id ... JOIN tags ON ... WHERE ...
... [10 more lines] ...

ActiveRecord

big fat abstraction

SQL

ActiveCouchRelaxFooDBObject


<#134 Page title=”page one”>

{type: “Page”, title: “page one”}


POST /mydb/, {title: “page one”, type: “Page”}

GET /mydb/page-one


skinny
CouchDB abstraction


... are not *that* important

The frameworks

CouchPoato RelaxDB
CouchObject
ActiveCouch
CouchRest
CouchFoo

CouchRest

• 2 in 1
• foundation for most
other frameworks

Low level part

• relatively thin layer on top of RestClient
• store and retrieve JSON structures
• query views the Couch way

Class mapping part

• map Ruby classes to JSON documents
• CRUD
• declarative views with CouchDB
semantics

RelaxDB

• CRUD + very basic associations
• automatic view generation via view_by
• a bit of support for custom views via
RelaxDB::View
• CouchDB like view API

Couch Object

• last updated in 2007
• pretty low level - okay for learning
the details of Couch the DIY way
• no update, no properties, no
automatic view creation

Couch Potato
• CRUD, associations + JSON mapping
• built-in acts_as_versioned - 60 LOC but
doing it by hand only requires 7
• ViewQuery class for creating/querying
custom views
• AR like ﬁnders

CouchFoo

• takes ActiveRecord and makes it
work with CouchDB
• if you (have) to migrate an AR
app...
• doesn’t give you the power of
CouchDB

ActiveCouch
• “Object Relational Mapper for [..]
CouchDB”
• “Since, the Rubyists here at Wego are
already very familiar with ActiveRecord
semantics, care has been taken to ensure
that ActiveCouch resembled it in many
ways.”

ActiveCouch

• one database per model
• View class to upload #@!? views via Rake
task
• no support for custom views

Page CRUD
class Page < CouchRest::ExtendedDocument
update_callback :after, :create_version

property :title can’t infer attributes from table
property :body

view_by :title
auto-generate simple views
def create_version
PageVersion.new(:page_id => id,
:body => @body_was, :version => versions_count + 1).save!
end
create version on update
end

PageVersion

class PageVersion < CouchRest::ExtendedDocument
property :body
property :version
property :page_id

view_by [:page_id, :version]
end

Word Count
class WordCount < CouchRest::ExtendedDocument
view_by :all, :map => quot;function(doc) {
if(doc['couchrest-type'] == 'Page') {
});
}
}quot;,
:reduce => quot;function(keys, values) {
return sum(values);
}quot;
end

Querying Views
Page.by_title

Page.by_created_at(:limit => 1)

PageVersion.get params[:id]

Page.by_title(:key => title, :limit => 1).first

WordCount.all

CouchApp

• serve entire apps directly from CouchDB
• just JSON, HTML & JavaScript

What is CouchApp

• bunch of Ruby Python scripts to help with
development/deployment
• data, CouchDB views, validations, shows,
lists and assets in one database

AJAX apps
• serve HTML, CSS as assets

• do all the work in JavaScript

• in the browser: GET from couch and append
HTML, POST form data to CouchDB to
update documents

• in CouchDB: views to retrieve, compute data,
validations

“real” apps

• lists
• shows
• server HTML, XML etc. from CouchDB

DON’T

use ActiveCouch
unless you understand why it is
the way it is

or Couch Potato
I’ll break all of its bones and APIS

DON’T

or CouchFoo
unless you want more power
than AR can give you

Conclusion

forgetting about the ActiveRecord way
is more important than what framework

Conclusion

think in documents & views,
not in records and associations

Conclusion

CouchDB is not about ﬁnding your
records, it’s about clever map/reduce
to get exactly what you want

What you want is

a thin abstraction
CouchDB semantics, not ActiveRecord
start simple with CouchREST or RelaxDB

and relax...

time to relax

Resources
• The CouchDB book
http://books.couchdb.org/relax/

• jchris CouchApp talk
http://jchrisa.net/drl/CouchDB%20Talk%20at%20Vidoop/
VidoopCouchTalk.pdf

• janl CouchDB talk
http://www.slideshare.net/bbcwebdev/introduction-into-couchdb-
jan-lehnardt-presentation

• The example wiki
http://github.com/langalex/couchdb_example_wiki/

• http://couch.io
support, training, hosting, development

Questions?
• Email: alex@upstream-berlin.com
• Twitter: @langalex

time to relax

Ruby sittin' on the Couch

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