Pragmatic Dictator

…..Three-tier architecture. Three tier architecture is really a logical partitioning of system functions into presentation, business logic and data-access. Of course, some frameworks have attempted to turn this into a physical reality which is fine but many people believe that such systems are or can be distributed which makes little sense – why?

Because as has been said elsewhere placing code far away from the data source makes little sense. If one is to place code away from the data source it’s going to be for one or more of the following reasons:

1. The computational weight demands separate scaling from the computational load inherent in managing the storage of the data.
2. The computational load in the data-storage layer can be better scaled elsewhere.
3. We can offset the additional latency introduced by network roundtrips.

Most three-tier architectures fail to satisfy any of the above criteria and thus aren’t good candidates for distribution.

…..Synchronous. This is because, in order to offset latency we must exploit asynchronous behaviour. Note that this does not imply the use of messaging rather it means adopting suitable asynchronous design patterns which can be implemented via RPC or messaging.

…..Completely consistent all of the time. Trying to enforce ACID properties across a distributed system is opening an enormous can of worms where one constantly attempts to defy the nature of the network. It’s not impossible to achieve but there is a tradeoff to be made. It’s often better to prefer eventual consistency in good-enough time i.e. something that approximates total consistency under most circumstances whilst degrading (hopefully gracefully) under load or in the presence of failure.

Many have attempted to implement distributed systems whilst falling into one or more of the traps above. Many have paid the price and many have consequently pronounced that distributed is inappropriate, impossible or insane. This is the motoring equivalent of strapping a massive turbo to an unmodified engine and complaining when the pistons explode through the bonnet and the oil is dumped all over the floor. In both worlds the remedy is the same, talk to an expert and be prepared to throw out a few beliefs.

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This entry was posted on Monday, July 9th, 2007 at 10:56 pm and is filed under Architecture, Distributed Systems. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.