introduction to couchdb

Open Source Business Systems www.opusvl.com

Introduction to CouchDB

Jon Allen (JJ) – [email protected]

YAPC::Europe 2010



What is CouchDB?

•  Document Oriented Database



What is CouchDB?

•  Document Oriented Database – No schema

•  Stores "documents" (i.e. data structures)

– No SQL •  Uses "MapReduce" queries written in JavaScript



What is CouchDB?

•  Document Oriented Database – No schema

•  Stores "documents" (i.e. data structures)

– No SQL •  Uses "MapReduce" queries written in JavaScript

•  Written in Erlang •  REST API •  Replication, scalability



Why use CouchDB?

•  Store complex data structures (JSON) •  Store variable data structures (no schema) •  De-normalised / self contained •  Add attachments to documents •  Scalable and fault tolerant

•  Better fit for certain problem domains – Messaging – CMS



Using CouchDB from Perl

•  HTTP REST API •  No DBI, DBD, DBIx::Class etc

•  CPAN modules – CouchDB::Client –  AnyEvent::CouchDB



Creating a database


use CouchDB::Client; use Try::Tiny;

my $client = CouchDB::Client->new( uri => 'http://localhost:5984' );

my $db = $client->newDB('jj_test'); # lower case!

try { $db->create; } catch { die "Could not create DB"; };


Inserting a document


my $client = CouchDB::Client->new(); my $db = $client->newDB('jj_test');

my $doc = $db->newDoc('docid', undef, { type => 'message', text => 'Hello, World', to => ['JJ', 'Barbie', 'Brian'], });

try { $doc->create; } catch { die "Could not create document"; };


Inserting a document


my $client = CouchDB::Client->new(); my $db = $client->newDB('jj_test');

my $doc = $db->newDoc('docid', undef, { type => 'message', text => 'Hello, World', to => ['JJ', 'Barbie', 'Brian'], });

try { $doc->create; } catch { die "Could not create document"; };

Document ID, must be unique

Revision ID


CouchDB GUI



Querying documents


Map function

Key, Value

Key, Value

Key, Value

Key, Value

All Documents

Query by Key Key, Value

Key, Value


Views

•  A view is a JavaScript function –  Like a stored procedure in SQL

•  Map function is executed on every document in the database

•  Emits key/value pairs – Can emit 0, 1, or more KV pairs for each document

in the database –  Keys and values can be complex data structures

•  KV pairs are then ordered and indexed by key



Queries

•  Queries are run against views –  i.e. searches the "key" of the list of KV pairs

•  Query types: –  Exact: key = x –  Range: key is between x and y – Multiple: key is in list (x,y,z)

•  Reduce functions –  e.g. count, sum, group



Designing a view

•  Show messages for a specific user


// This is JavaScript

function(doc) { if (doc.type && doc.to && doc.text) { if (doc.type == 'message') { for (var dest in doc.to) { emit(doc.to[dest], doc.text); } } } }


Creating a view

•  Can either use the GUI or the API

•  View stored in database as a "Design Document" –  A design document can contain multiple views

•  Example – Design document "_design/myview" –  View name "messages_to"



Querying a view


use Data::Dumper;

my $client = CouchDB::Client->new(); my $db = $client->newDB('jj_test'); my $view = $db->newDesignDoc('_design/myview') ->retrieve;

my $result = try { $view->queryView('messages_to', (key => 'JJ')); } catch { die "Could not query view"; };

say Dumper($result);


Query results


varos:talk_scripts jj$ perl5.10.1 query_view.pl

$VAR1 = { 'total_rows' => 3, 'rows' => [ { 'value' => 'Hello, World', 'id' => 'docid', 'key' => 'JJ' } ], 'offset' => 2 };


Conclusion

•  Different mindset to SQL database •  Quite low-level, but very powerful •  Additional features

–  Replication – Document revisions –  Reduce functions – Changes API

•  More information: http://books.couchdb.org/relax



KTHKSBYE!


Thank you for listening!

Any questions?

http://www.opusvl.com

http://perl.jonallen.info/talks

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