- Flickr uses PHP for its page and application logic, along with technologies like Smarty, PEAR, Perl, and Java. The site has over 60,000 lines of PHP code and templates. - The logical architecture includes page logic, application logic, photo storage, APIs, templates, third-party apps. The physical architecture includes static servers, database servers, and node servers. - PHP is used for page and application logic. Smarty is used for templating. MySQL is used for storage. Java is used in the node service. Flickr scales horizontally by adding more hardware over time.