One of the most interesting presentations in Architecture & Design world was the eBay Architecture presentation by Randy Shoup and Dan Pritchett. The presentation was only one hour long, so Randy and Dan didn't cover all the topics in the slide. Here are some of the insight I took from this presentation.

Architecture evolution  -  eBay actually went through several architecture revolutions. Their initial architecture cannot even begin to scale to their current loads. It was, however, a very good fit for their initial quality attributes - specifically, the emphasis on time to market and costs.  This shows the importance of balancing quality attributes. Sure an architectural change is painful but if they'd future proofed too much I doubt they would ever get something working.

V2 demonstrated that traditional 3-tier architecture would only scale so far. It was nice to see how it evolved though. Also with the move from version 2.4 to 2.5 and later to 3 we see eBay learning about  CAP - the hard way. In its final (current) incarnation eBay's data architecture prefers partitioning and availability over consistency. This doesn't mean they forgo consistency altogether - just that they trade the comfort zone of ACID transactions with the BASE approach. Where BASE - stands for Basically Available, Scalable/Soft state & Eventually Consistent. .
eBay partitions thier data in two levels one is a SOA like division by business areas (users, items etc.) and the second level is an horizental partitioning based on access paths.This BASE approach to data was dubbed by Dathan Pattishall (from Flicker and Friendster) as sharding (via HighScalability). This approach means things like high partitioning, no distributed transactions (also see below), denormalization etc. (you might also want to read the item I wrote on denormalizaiton in InfoQ yesterday).
The more major implication here is that when it comes to internet scale, the database looses its importnace - or as Bill de Hora nicely puts it:

The use of RDBMSes as data backbones have to be rethought under these volumes; as a result system designs and programming toolchains will be altered. When the likes of Adam Bosworth, Mike Stonebraker, Pat Helland and Werner Vogels are saying as much, it behooves us to listen.

As I said the data architecture of eBay is SOAish -  partitioned their components and data along business lines, and they apply many of SOA principles. They don't however unite data and components to create a service and  they don't (seem) to have the same contract boundaries that SOA promotes (Randy told me that they are currently contemplating SOA).

Returning to the  eBay do not use transactions. "no transactions" which seems very controversial  - but if we just consider some of the points I made on transactions between services in previous posts - it is the only logical way to ensure scaling. By the way, as can be expected  they do use transactions - when they are local e.g. if the users table is spread over a couple of table both will be updated together).

The application layers also follow the segmentation by business areas. eBay cacse metadata/immutable data as much as possible. keep the application stateless (i.e. state comes from client/db) e.g. they don't use sessions. The DAL virtualized the horizontal partitioning mentioned above for the rest of the code.

It was also interesting to  that eBay developed its own messaging infrastructure - though Randy and Dan did not provide alot of details on that

Development process - It seems that eBay is using some hybrid of feature driven development with waterfall (i.e. the development is feature by feature - but the development of a feature is waterfallish). The do have a constant delivery rate which they synchronize using the concept of a train. if you have a features that is it will be added to the train which is scheduled to arrive around the time your feature will be ready. Several features are delivered as a package which gives a predictable (weekly). I guess it also gives them some nice metaphors to use such as a feature that doesn't make it - misses the train or the train leaves on time etc.

The slides of the presentation can be downloaded from Dan Pritchett's site (They not from the same event but they are pretty much the same slides. Also you can read Elliotte Rusty Harold's account of the presentation.