Erlang Everywhere

Brief History of CouchDB

Document Oriented Database
Started by Damien Katz: 2005
Originally XML Database written in C++
Rewritten in Erlang, moved to JSON
Became an Apache project 2008
Founded Couch.io 2010 -> CouchOne -> Couchbase

What is CouchDB?

JSON
HTTP
Map Reduce
Replication (Sync)
CouchApps
GeoCouch
Robust

JSON:

{
    "_id": "49c8b92961f5fbf9bc13fbcc00000484",
    "_rev": "2-bc797d9090695d916a89d6166d125581",
    "some": "json",
    "an": ["array", "of", "things"],
    "and": {"child": "objects"},
    "numbers": 42
}

HTTP API - Database

GET $HOST
{"couchdb":"Welcome","version":"1.2.0a-c856916-git"}

GET $HOST/demo
{"error":"not_found","reason":"no_db_file"}

PUT $HOST/demo
{"ok":true}

GET $HOST/demo
{"db_name":"demo","doc_count":0 ... }

HTTP API - Documents

PUT $HOST/demo/doc {"foo":"bar"}
{"ok":true,"id":"doc","rev":"1-4c6114c65e295552ab1019e2b046b10e"}

GET $HOST/demo/doc
{"_id":"doc","_rev":"4-42dd533bd9d3e47403c1914ad663ab8b","foo":"bar"}

POST $HOST/demo '{"foo":"bar"}'
{"ok":true,"id":"49c8b92961f5fbf9bc13fbcc0000128a", \
  "rev":"1-4c6114c65e295552ab1019e2b046b10e"}

HTTP API - Documents

PUT $HOST/demo/doc
  {"_rev":"1-4c6114c65e295552ab1019e2b046b10e", "new":"content"}
{"ok":true,"id":"doc","rev":"2-c8b28721ed96f3dc039a1ab3f85b0dba"}

DELETE $HOST/demo/doc?rev=2-c8b28721ed96f3dc039a1ab3f85b0dba
{"ok":true,"id":"doc","rev":"3-8d9fecfc76473aa0d412ca228468c8cc"}

PUT $HOST/demo/newdoc/test.html --data-binary @test.html
{"ok":true,"id":"newdoc","rev":"1-a2833f8c791e5ce692f2210ef3249d89"}

Map Reduce

Incrementally Built

Map

function(doc) {
  if (doc.type && doc.type === 'task' && doc.status === "complete") {
      emit([doc.index || 0], null);
  }
}

Reduce

_count

BTree

Uses B-Tree for both data and view storage
Minimise disk seeks / Concurrent access

Append Only

BTree is stored on disk as append only file
Updates / Deletions dont modify data
Only returns ok for writes when data is guaranteed to be
safe on disk (delayed_commits)
killdashnine party! Safe way to shutdown CouchDB is
```
kill -9
```

Changes Feed

Can read the database as a series of changes

$ curl $HOST/demo/_changes
{"results":[
{"seq":2,"id":"49c8b92961f5fbf9bc13fbcc0000128a","changes":
  [{"rev":"1-4c6114c65e295552ab1019e2b046b10e"}]},
{"seq":7,"id":"newdoc","changes":
  [{"rev":"1-a2833f8c791e5ce692f2210ef3249d89"}]},
{"seq":8,"id":"doc","changes":
  [{"rev":"4-42dd533bd9d3e47403c1914ad663ab8b"}]}
],
"last_seq":8}

Continous Changes can notify database activity

curl "$HOST/demo/_changes?since=8&feed=longpoll"

Replication (Sync)

Move data between databases

POST /_replicate {"source": "mydatabase", "target":"backup"}

Can specify specific documents / generic filter

function (doc, req) {
  if (doc._id.match("_design")) {
    return true;
  }
  return doc.type && doc.type === "task";
}

Can run continuously (changes are updated in real time)

GeoCouch

GET $HOST/places/_design/main/_spatial/points?bbox=0,0,180,90

R-Tree Index

GeoSpatial Queries

MapQuery

CouchApps

Mochiweb web server built in

Can serve files via attachments

Can build whole Web apps with just CouchDB

Example CouchApps

Couch in the wild

Canonical have built a file sync into Ubuntu itself, based on CouchDB it lets users access their files and program settings wherever they are.

Meebo, one of the most popular online chat applications, uses CouchDB to store its users logs for future access and analysis.

Couch in the wild

Dimagi uses CouchDB to provide healthcare data to remote clinics with no reliable internet.

Radical Dynamic built Group Complete, a mobile app for offline data collection on Android

Why Mobile

Phones are often offline, need local data and ability to sync
Phone can kill app at any time, needs robust storage
HTML5 and Geo capabilities

Sync Patterns

Couch has flexible / powerful sync

Sync Patterns - Follow / Privacy

Sync Patterns - Group Share

Android Overview

Modified glibc
Different shell (/system/bin/sh)
Lack of system libraries (blowfish)
Lots of fixing build files
Cant access beam files via filesystem

iOS Overview

No Dynamic linking
No Subprocesses (DNS and NIFs)
OS can kill the process at any time
Cant execute writeable memory

Erlang on Mobile

Mobile Networks will try to 'optimise' traffic
No way to reduce build reliably
Strip debug symbols
Surpisingly good power usage

Going Forward

Get patches into Erlang + Apache CouchDB
Reduce Build Size
Test thoroughly
Improve Startup Speed
General Release Imminent