Scaling Web Applications and Tagging - Digg, Del.icio.us, StackOverflow

Question

How do websites like Digg, Del.icio.us, and StackOverflow implement tagging?

I know this other question has an accepted answer of a many-to-many relation with a cros

Answer 1

Here is the oft-quoted article which breaks down tagging schemas by real performance metrics: http://tagging.pui.ch/post/37027746608/tagsystems-performance-tests

The author notes that the founder of delicious explains using an RDBMS for tagging simply does not scale to many millions of items under load. An alternative like Lucene may fit better in such a case.

Answer 2

I am sure that the additional JOIN queries would be too expensive in a very large system.

The tags are either stored non-normalised in the main table, or there may be a separate tag table which has a row for each tagged item.