Hey, Here are a few thoughts on the research paper from Amazon on infinite-scalability that Niclas referenced some time ago. I found it very interesting, and have been thinking about how the ideas presented in the paper could be implemented in Qi4j.
First of all, terminology. What is called "entity" in the paper would be an "aggregate entity" for us (an indivisible and fully locally "located" thing), and what they call "object" is an entity or value object in Qi4j. It seems like a good idea to start with the basic assumptions. First of all, assume that we are building an almost-infinitely scalable system. This means a lot of entities in the system, partitioned over a set of computers. Assume that locally communication almost always works and services almost always complete requests (disk i/o is reliable etc.). Assume also that network communication almost never works, and there is no way to know if a call over the network will make it. From these assumptions the paper concludes that synchronous requests can be made within an Aggregate without much error handling, and that for one Aggregate to talk to another it must use asynchronous messaging. To coordinate things each aggregate keeps track of what conversations it is currently having with other Aggregates so as to manage resends and response actions. In Qi4j the Activities concept could be implemented as a mixin on the root Aggregate, which would have a ManyAssociation<Qualifier<Entity,Conversation>> collection to keep track of who it is currently talking to (see, Qualifiers are neat!). To send a message from one Aggregate to another we could use the Messaging ideas already outlined, meaning, to send a message the application code would create an EntityComposite which contains the to-header and body (minimally), and the EntityStore would then trigger a send of it to the recipient. In this case the to-header would simply be set to another Aggregate id in the same system. The sending mechanism would send the Message Entity to the server where the receiving Aggregate resides (which could be the same machine, or on the other side of the planet), and there it would be consumed by the Aggregate itself rather than some message processing code, as outlined in the paper. Responses would be created and handled in the same way. There's a whole lot to be said of how to implement this, for example using RESTful ideas, but this is at least a rough outline of how the ideas presented in the paper could map to our "world". I think it would be very useful, and would provide a toolbox for people who want to relatively easily create almost-infinitely scalable systems. What I like about the model is that there is relatively little error handling going on in the application code, since all operations are assumed to almost always work (since both local calls and message creation is local). Thoughs and comments on this? /Rickard _______________________________________________ qi4j-dev mailing list [email protected] http://lists.ops4j.org/mailman/listinfo/qi4j-dev
