Wednesday, April 28, 2010

A Platform of Services

Back in the 70's and 80's, when multi-threaded processes and languages were still very much on the research agenda, distributed systems were developed based on a services architecture, with services as the logical unit of deployment, replication and fault containment. If you wanted to service multiple clients concurrently then you'd typically fire off one server instance per client. Of course your servers could share information between themselves if necessary to give the impression of a single instance, using sockets, shared memory, disk etc.

Now although those architectures were almost always forced on us by limitations in the operating systems, languages and hardware at the time, it also made a lot of sense to have the server as the unit of deployment, particularly from the perspectives of security and fault tolerance. So even when operating systems and languages started to support multi-threading, and we started to look at standardising distributed systems with ANSA, DCE and CORBA, the service remained a core part of the architecture. For example, CORBA has the concept of Core Services, such as persistence, transactions, security and although the architecture allows them to be colocated in the same process as each other or the application for performance reasons, many implementations continued to support them as individual services/processes.

Yes there are trade-offs to be made between, say, performance and fault tolerance. Tolerating the crash of a thread within a multi-threaded process is often far more difficult than tolerating the crash of an individual process. In fact a rogue thread could bring down other threads or prevent them from making forward progress, preventing the process (server) from continuing to act on behalf of multiple clients. However, invoking services as local instances (e.g., objects) in the same process is a lot quicker than if you have to resort to message passing, whether or not based on RPC.

However, over the past decade or so as threading became a standard part of programming languages and processor performance increased more rapidly than network speeds, many distributed systems implementations moved to colocating services as the default, with the distributed aspect really only applying to the interactions between the business client and the business service. In some cases this was the only way in which the implementation worked, i.e., putting core infrastructure services in separate processes and optionally on different machines was simply no longer an option.

Of course the trade-offs I mentioned kicked in and were made for you (enforced on you) by the designers, often resulting in monolithic implementations. With the coining of the SOA term and the rediscovery of services and loose coupling, many started to see services as beneficial to an architect despite any initial issues with performance. As I've said before, SOA implementations based on CORBA have existed for many years, although of course you need more than just services to have SOA.

Some distributed systems implementations that embraced SOA started to move back to a service-oriented architecture internally too. Others were headed in that direction anyway. Still others stayed where they were in their colocated, monolithic worlds. And then came Cloud. I'm hoping that as an industry we can leverage most of what we've been implementing over the past few decades, but what architecture is most conducive to being able to take advantage of the benefits Cloud may bring? I don't think we're quite there yet to be able to answer that question in its entirety, but I do believe that it will be based on an architecture that utilises services. So if we're discussing frameworks for developing and deploying applications to the cloud we need to be thinking that those core capabilities needed by the application (transactions, security, naming etc.) will be remote, i.e., services, and they may even be located on completely different cloud infrastructures.

Now this may be obvious to some, particularly given discussions around PaaS and SaaS, but I'm not so sure everyone agrees, given what I've seen, heard and read over the past months. What I'm particularly after is a services architecture that CORBA espoused but which many people overlooked or didn't realise was possible, particularly if they spoke with the large CORBA vendors at the time: an architecture where the services could be from heterogeneous vendors as well as being based on different implementation languages. This is something that will be critical for the cloud, as vendors and providers come and go, and applications need to choose the right service implementation dynamically. The monolithic approach won't work here, particularly if those services may need to reside on completely different cloud infrastructures (cf CORBA ORB). I'm hoping we don't need to spend a few years trying to shoehorn monoliths in to this only to have that Eureka moment!

The lack of standards in the area will likely impact interoperability and portability in the short term, but once standards do evolve those issues should be alleviated somewhat. The increasing use of REST should help immediately too though.

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