The Year of Living Asynchronously

Happy New Year! Asynchronicity is busting out all over the web and my prediction is that 2010 will be the year of “events”:

  • Of course Twitter has brought The concept of publish/subscribe messaging to the masses and we enjoyed their journey of discovery to the heights of scalability in 2009.
  • XMPP has been embraced by the real-time web crowd, most publicly in Google Wave but also in other “back-web” contexts such as Gnip.
  • Web sockets is an experimental feature of HTML 5 which enables push messages directly to web pages.
  • New frameworks for event-driven programming are emerging such as EventMachine, Twisted, Node.js.
  • In 2009 every major software vendor had a CEP product.

In the meantime, SOA has become so damn synchronous. But it doesn’t have to be!

One of the fundamental tennets of SOA is that reducing coupling between systems makes them more scalable, reliable and agile (easier to change). SOA goes a long way to reducing coupling by providing a contract-based, platform independent mechanism for service providers and consumers to cooperate. However I still think we can improve on current SOA practices in further reducing coupling.

Coupling still intrudes into many aspects of how SOA is practiced today:

  • HTTP transports tie us to a regimen of synchronous request-reply with timeouts which creates tight couplings between provider and consumer. Even though one-way MEPs were an original feature of SOAP, message-oriented transports remain the forgotten orphan of web-services standards.
  • Many SOA services are conceived, implemented and maintained as point-to-point entities…providers and consumers forced into lock-step due to inadequate versioning and lifecycle management.
  • Process orchestration layers often form a bridge between service providers and consumers, which on the face of it provides some level of indirection. But in many cases orchestration provides limited value and may actually serve to increase the overall system coupling.

In many cases we can achieve the benefits of service orientation to much greater effect by exercising a little scepticism toward some of these shibboleths of the web services world and embracing a more asynchronous, event-oriented way of building processes. So this year, embrace your asynchronous side and do something to reduce your system coupling: build some pub/sub services, learn about Event Processing or Event-Driven Architecture, try one of the technologies I pointed to above.

Just as developers should embrace multiple languages to broaden their skills, so should architects embrace and be fluent in multiple architectural styles.

Push versus Pull

From OSCON via O’Reilly Radar here’s a good case study of an architectural decision driven by the system requirements rather than the usual religious considerations that pollute the bloggosphere.

FriendFeed needed update info from Flikr but a REST-based “pull” approach is highly inefficient in this case. Instead the solution architects opted for a “push” approach using xmpp as the message transport. This is a really good presentation because it goes into the architectural choices and implications of “push” versus “pull”.

I characterize this as “pull vs push” rather than “REST vs xmpp” (or “REST vs *” or “why REST is crap”) because fundamentally it comes down to the best choice of how to synchronize changes between systems. You make this choice based on the usage characteristics of the different systems, the likely traffic volumes this will result in and the consequential resource impacts. Having made the choice between push or pull you then choose the appropriate message transport.

The web doesn’t do a lot of “push” and consequently there is not a lot of discussion about push and REST. Dare Obasanjo characterises it nicely:

Polling is a good idea for RSS/Atom for a few reasons

  • there are a thousands to hundreds of thousands clients that might be interested in a resource so the server keeping track of subscriptions is prohibitively expensive
  • a lot of these end points aren’t persistently connected (i.e. your desktop RSS reader isn’t always running)
  • RSS/Atom publishing is as simple as plopping a file in the right directory and letting IIS or Apache work its magic

The situation between FriendFeed and Flickr is almost the exact opposite. Instead of thousands of clients interested in document, we have one subscriber interested in thousands of documents. Both end points are always on or are at least expected to be. The cost of developing a publish-subscribe model is one that both sides can afford.

Inside the firewall, the situation is often more akin to that between FriendFeed and Flikr. This is why messaging is more common inside the firewall than outside – not because of any universal superiority between REST versus messaging, but because the system requirements are different and often favour a push approach rather than pull.

While your over at Dare’s excellent Blog, be sure to also check out his discussion of push versus pull in the context of scaling Twitter and MS Exchange.  These are important considerations for designers of federated systems such as federated databases or federated messaging systems. The example of FriendFeed to Flikr could be considered as the first incremental step toward a federation.