Capped collections in MongoDB allow you to limit the size and number of documents in a collection. This is useful for applications that involve logging, news feeds, or caching where collections need to be bounded. While documents cannot be deleted or updated, objects are returned in insertion order and the collection size is prevented from growing out of control. Geospatial indexing allows finding documents close to a location point, making it useful for location-based services. Real world examples of using capped collections include activity feeds, event logging, caching, and data pages.

1. Geo & capped collections
with MongoDB:
Use cases & performance
in the real world

2. /usr/bin/whoami
Russell Smith
Consultant for UKD1 Limited
Specialising in helping companies going through rapid growth
Help with code, architecture, infrastructure, devops, sysops,
capacity planning, etc
<3 gearman, mongodb, neo4j, mysql, kohana, riaksearch, php,
debian

3. What are capped collections?
Special option for collections which allow you to;
Limit the size of a collection
Limit the number of documents in a collection

4. Why is this useful?
Logging
News feeds
Caching

5. Restrictions
Can’t delete objects (you can drop the collection though)
Can’t update objects (well...not 100% true - you can only if the size
doesn’t change)

6. Neat things
Objects are returned in the insertion order
Don’t have to worry about things growing out of control
No more cronjobs to delete old data

7. Real world example
SongFor.com:
Activity feeds for the front of the site are using capped collections
StrongHold Kingdoms (MMO):
Logging of events from syslog / internal events are stored in a
capped collection (Graylog2 & a custom dashboard system)
Lioness: cache + log of data, used to cut lag & reduce API access
requests to Facebook
Findlunch.in: Caching of page data

8. How to setup...
db.createCollection("mycoll", {capped:true, size:100000})
db.createCollection("mycoll", {capped:true, size:100000, max:100});
(limit to 100 documents - this is slower as it adds overhead)

9. What is Geospatial indexing?
Allows easily finding things close to a point - great for location based
services
Insert a document as normal, but with some co-ordinates...
ensureIndex({location_field: ‘2d’});

10. Other magic things
You can search $within a $box, or around $center point
(circle)...which is handy
Results are sorted by distance from points
geoNear (command) will return distance, average distance, etc

11. Versions & support
> v1.3.3 for initial version, which is pretty old
>= 1.7.0 added spherical support (more in a minute)
< 1.7.2 doesn’t support geo in sharded collections. It still has issues
see: http://jira.mongodb.org/browse/SERVER-926

12. and the point is?
Really really easy to find documents around a specific point
You can get distances from that point as well
Lookups are fast and accurate
This is a pain to do in MySQL

13. Example
FindLunch.in lets people find lunch close to where they are.
Geocode lookup of ~2 million postcodes from the UK
All London tube stations & most boroughs are also listed
Average query time Xms over 20k of test documents

14. Questions?
Any questions?