Who am I ? What am I doing here?• Twitter: @mcwoods

• Email: chris.woods@mind-flip.com

• Mind-Flip Blog:www.mind-flip.com/theBlog

• Linked In: http://www.linkedin.com/in/mindflip

• When I think I’m being profound, or funny: www.WithBigHair.com

Birth of an Idea…

ApplicationDesign

Application Design

Data Structure Design

• Resulting Structures– User Object– Feed Object– Story Object– Comment Object

WWR: VocZie’s Object linking

User ObjectFeed Object

Comment ObjectStory Object

1

1

1

1

0..1

1

*

1

VocZie’s REST API

Examples:

• /feeds/<feed_id>• /feeds/<feed_id>/stories• /feeds/<feed_id>/story/<story_id>/comments• /feeds/<feed_id>/story/<story_id>

Selecting a Technology

MongoDB Structures

• Collections• Objects and their identity• The two ways of relating objects– Via pointers (aka _id)– Via embedding

Transaction Problem?UpdateStoriesFromFeed( url )

if ( url is being updated ) then wait for notificationelse

lock out everyone else from updating urlfetch and parse stories from feed’s urlfor each story returned

if ( story doesn’t already exist in DB)add storyend if

end forremove lock preventing anyone else from updating url

end ifEnd function

Implementing a lock manually

• update if current– Aka Compare and Swap – Atomic MongoDB actions allow for manual

creation of locks– http://www.mongodb.org/display/DOCS/Atomic+

Operations#AtomicOperations-TheABANuance

Stumbling along

Moment of Terror

500 – Internal server error.

There is a problem with the resource you are looking for, and it cannot be displayed.

More Terror

root@1310-1064-2283:~# mongod

mongod --help for help and startup optionsMon Jul 23 14:38:35 [initandlisten] MongoDB starting : pid=26105 port=27017dbpath=/data/db/ 64-bit

** WARNING: You are running in OpenVZ. This is known to be broken!!!

The Technical Lesson

• VMs change over time…

More More Terror….

• Have I lost all my data ?• Can I back up now?

cat Terror | less

• The no backup – backup…• Check out /var/lib/mongodb

Conclusions

• Good– No SQL DB is great for avoiding the DAL translation

pain– Aid development time– MongoDB Core Documentation is good

• Bad– Small print – VM compatibility– MongoDB Driver Documentation isn’t as good as

the core

Questions ?• Twitter: @mcwoods• Email: woods.mc@gmail.com• Mind-Flip Blog:

www.mind-flip.com/theBlog• Linked In: http://

www.linkedin.com/in/mindflip • When I think I’m being profound, or funny:

www.WithBigHair.com

Application Design

• User Functionality First– What data does the UI need?

• What structure would best present that data?