Introduction to CouchDB and CouchApps. Example of a Rails + CouchApp hybrid built called i.canhazthread.com

1. #geekcampsg 2009
CouchApps - Applications built on CouchDB
Arun Thampi
@iamclovin
http://mclov.in
Saturday, August 22, 2009

2. 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
Saturday, August 22, 2009

3. 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)
Saturday, August 22, 2009

4. Profit
Saturday, August 22, 2009

5. 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”]
}
Saturday, August 22, 2009

6. 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 ?
Saturday, August 22, 2009

7. Saturday, August 22, 2009

8. 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
Saturday, August 22, 2009

9. Example Map Reduce
Map Function
function(doc) {
if(doc.type == “customer”) {
emit(doc.first_name, 1);
}
}
Saturday, August 22, 2009

10. Example Map Reduce (contd)
Reduce Function
function(keys, values, re) {
return sum(values);
}
Saturday, August 22, 2009

11. Example Map Reduce (contd)
{
..
{
“key”: “Seth”,
“value”: 1
},
reduce([“Seth”, “Fogell”], [1, 1])
{
“key”: “Fogell”,
“value”: 1
}
..
}
Saturday, August 22, 2009

12. CouchApps
• HTTP / HTML / Javascript is the language of the web
• Coincidentally, that’s what CouchDB speaks too
• Why not have CouchDB apps serve apps?
Saturday, August 22, 2009

13. 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
Saturday, August 22, 2009

14. CouchApps
• Available at http://github.com/
couchapp/couchapp
• couchapp generate icanhazthread
• couchapp push icanhazthread http://
127.0.0.1:5984/icanhazthread
Saturday, August 22, 2009

15. 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
Saturday, August 22, 2009

16. 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
Saturday, August 22, 2009

17. icanhazthread.com
nginx
CouchDB API [RSS, Realtime]
Rails Mongrels CouchDB
Saturday, August 22, 2009

18. 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”
}
Saturday, August 22, 2009

19. 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
Saturday, August 22, 2009

20. 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);
}
}
Saturday, August 22, 2009

21. 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
Saturday, August 22, 2009

22. Future Directions
• Make icanhazthread real-time (Utilize the _changes API in CouchDB)
• RSS Feeds (Dead simple with the CouchApp list)
• etc etc
Saturday, August 22, 2009

23. 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
Saturday, August 22, 2009

24. Thank you
Q &A
Saturday, August 22, 2009