This week, I attended a very interesting presentation by Dan Pritchett and Randy Shoup, both senior technologists at eBay, on eBay’s architecture. Some of it was as I would have expected, other things were, shall we say, counter-intuitive. Here is a random collection of notes, with some special exclamation marks:

• 212 million registered users, 1 billion photos
• 1 billion page views a day, 105 million listings, 2 petabytes of data, 3 billion API calls a month
• something like a factor of 35 in page views, e-mails sent, bandwidth from June 1999 to Q3/2006.
• 99.94% availability, measured as "all parts of site functional to everybody" vs. at least one part of a site not functional to some users somewhere
• 15,000 application servers, all J2EE. About 100 groups of functionality aka “apps”. Notion of a “pool”: “all the machines that deal with selling”… Well over 200 databases.
• Everything is planned with the question “what if load increases by 10x”. Scaling only horizontal, not vertical: many parallel boxes.
• leverages MSXML framework for presentation layer (even in Java)
• Oracle databases, WebSphere Java (still 1.3.1)
• split databases by primary access path, modulo on a key
• every database has at least 3 on-line databases. Distributed over 8 data centers
• some database copies run 15 min behind, 4 hours behind
• no stored procedures. some very simple triggers.
• move cpu-intensive work moved out of the database layer to applications applications layer: referential integrity, joins, sorting done in the application layer! Reasoning: app servers are cheap, databases are the bottleneck.
• no client-side transactions. no distributed transactions
• J2EE: use servlets, JDBC, connection pools (with rewrite). Not much else.
• no state information in application tier. transient state maintained in cookie or scratch database
• app servers do not talk to each other — strict layering of architecture
• Search, in 2002: 9 hours to update the index running on largest Sun box available — not keeping up
• Average item on site changes its search data 5 times before it is sold (e.g. price), so real-time search results are extremely important.
• “Voyager”: real-time feeder infrastructure built by eBay.. Uses reliable multicast from primary database to search nodes, in-memory search index, horizontal segmentation, N slices, load-balances over M instances, cache queries

There were way more questions by the packed audience of architects and other techies than there was time. Absolutely worth everybody’s time.

Dan put the slides on his blog: eBaySDForum2006-11-29.pdf.