GeekCamp SG 2009 - CouchApps with CouchDB

#geekcampsg 2009
CouchApps - Applications built on CouchDB

Arun Thampi
@iamclovin
http://mclov.in

Saturday, August 22, 2009

About myself

• Ruby Developer + Dabbler at Wego.com

• The kick-ass travel meta-search engine

• Work with great set of people

@winstonyw @reds @mongeslani
@chuyeow @jefﬂimar
@phungleson @ijuzwannatwit @garytheis


CouchDB FTW
• Schema-less Key-Value based • MVCC (No Locks)
document store
• Replication
• Erlang
• Scaling out of the box (HAProxy/
• Speaks HTTP / REST Nginx)

• Map Reduce • Full Text Search (Lucene/Sphinx)

• JSON • Attachments (binary ﬁles)

• Social Media • Web Interface (Futon)


Proﬁt


CouchDB - Built for the Web
{
“_id” : “abc”,
“_rev” : “abc123def”,
“text” : “Hello World”,
“screen_name” : “mclovin”,
“timestamp” : 123232,
“type” : “thread”,
“replies”: [ “reply 1”, “reply 2”, “reply 3”]
}


CouchDB - Built for the web
Inserting a document HTTP PUT

Retrieving a document HTTP GET

Deleting a document HTTP DELETE

Updating a document HTTP POST

Scaling Replication + load balancing

Querying ?


Map Reduce with Javascript Views
(No I wasn’t making that up)

Document 1 [Key 1, Value 1]

Document 2 Map Function [Key 2, Value 2] Reduce Function

[Key k,Value k]
Document n Value X


Example Map Reduce
Map Function

function(doc) {
if(doc.type == “customer”) {
emit(doc.first_name, 1);
}
}


Example Map Reduce (contd)
Reduce Function

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


Example Map Reduce (contd)
{
..
{
“key”: “Seth”,
“value”: 1
},
reduce([“Seth”, “Fogell”], [1, 1])
{
“key”: “Fogell”,
“value”: 1
}
..
}


CouchApps

• HTTP / HTML / Javascript is the language of the web

• Coincidentally, that’s what CouchDB speaks too

• Why not have CouchDB apps serve apps?


CouchApps - Why?

• Barriers to entry to develop powerful data-backed applications is
lowered

• Replication enables load-balancing + scaling

• No Impedance Mismatch - Data is Javascript, your views are in
Javascript, your HTML pages are rendered in Javascript


CouchApps
• Available at http://github.com/
couchapp/couchapp

• couchapp generate icanhazthread

• couchapp push icanhazthread http://
127.0.0.1:5984/icanhazthread


CouchApps - List & Show Functions
Schema-less
Model
• A view is queried and the results are JSON

passed to a list function

• List Function then renders the results
Map/Reduce
of your view in whatever format you Controller
View
please: HTML, JSON, XML, RSS, etc

• In a twisted way, its like a controller +
view concept in an MVC framework
View List/Show


Introducing i.canhazthread.com

• Inspired (in fact, cloned) from a.tinythread.com by Joshua Schacter

• Backend powered by a CouchApp (http://github.com/arunthampi/
icanhazthread)

• Uses Rails to do Twitter authentication + HTML rendering


icanhazthread.com

nginx

CouchDB API [RSS, Realtime]

Rails Mongrels CouchDB


Document Structure
Thread Reply

{ {
“_id” : “abc”, “_id” : “def”,
“_rev” : “abc123def”, “_rev” : “234abcdef”,
“text” : “Hello World”, “text” : “oh hai”,
“screen_name” : “mclovin”, “screen_name” : “snehamenon”,
“timestamp” : 123232, “timestamp” : 1232310,
“type” : “thread” “type” : “reply”,
} “thread_id” : “abc”
}


Interesting Map/Reduce problem
(at least for me)

• Interesting because replies are not
contained within a thread

• Also, because threads on the
homepage need to be sorted
according to timestamp (newest
ones ﬁrst)

• Views need to be commutative &
associative


Show me the code
Map Function (yawn)

function(doc) {
if(doc.type == ‘thread’) {
emit([doc.id, doc.time_created], doc);
} else if(doc.type == ‘reply’) {
emit([doc.thread_id, doc.time_created], doc);
}
}


Show me the code Thread Reply Result

Reduce Function
Sorting?

[K1, V1] [K2,V2] [K1, Summary for Thread 1]

num_replies: 2
[K1, RV1]
group_level =1 [K2, Summary for Thread 2]

[K1, RV2] num_replies: 0


Future Directions

• Make icanhazthread real-time (Utilize the _changes API in CouchDB)

• RSS Feeds (Dead simple with the CouchApp list)

• etc etc


Resources
• CouchDB Wiki: http://wiki.couchdb.org

• CouchDB Mailing lists (user + dev)

• Blogs: Chris Anderson, Harish Mallipeddi, Jan Lenhardt

• CouchDB Futon Test Suite (http://127.0.0.1:5984/_utils/
couch_tests.html)

• Other CouchApps: Toast, Chris Anderson’s blog


Thank you

Q &A


GeekCamp SG 2009 - CouchApps with CouchDB

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