Matthias Schorer talked about FIDUCIA IT AG and their service-oriented architecture. This financial services provider works with 780 banks in Europe, processing 35,000,000 transactions during the banking day. That works out to a little over 3.5 million transactions per hour.
Matthias described this as a service-oriented architecture, and it is. Be warned, however, that SOA does not imply or require web services. The services here exist in the middle tier. Instead of speaking XML, they mainly use serialized Java objects. As Matthias said, "if you control both ends of the communication, using XML is just crazy!"
They do use SOAP when communicating out to other companies.
They've done a couple of interesting things. They favor asynchronous communication, which makes sense when you architect for latency. Where many systems push data into the async messages, FIDUCIA does not. Instead, they put the bulk data into storage (usually files, sometimes structured data) and send control messages instructing the middle tier to process the records. This way, large files can be split up and processed in parallel by a number of the processing nodes. Obviously, this works when records are highly independent of each other.
Second, they have defined explicit rules and regulations about when to expect transactional integrity. There are enough restrictions that these are a minority of transactions. In all other cases, developers are required to design for the fact that ACID properties do not hold.
Third, they've build a multi-layered middle tier. Incoming requests first hit a pair of "Central Process Servers" which inspect the request. Requests are dispatched to individual "portals" based on their customer ID. Different portals will run different versions of the software, so FIDUCIA supports customers with different production versions of their software. Instead of attempting to combine versions on a single cluster, they just partition the portals (clusters.)
Each portal has its own load distribution mechanism, using work queues that the worker nodes listen to.
This multilevel structure lets them scale to over 1,000 nodes while keeping each cluster small and manageable.
The net result is that they can process up to 2,500 transactions per second, with no scaling limit in sight.