Today I shall be tackling Service Oriented Architecture. I think that that particular buzz phrase has annoyed me a lot in the past. CTOs and middle management talk about how the read in a magazine something about SoA or SaaS (Software As A Service) as being the next big thing and that we should switch all of our stuff to that! Meanwhile there isn't a very good explanation of how or why to do such a thing. I had written it off mostly as I never saw how independent services could be pulled together into an application. Sure it sounded neat for some other project that wasn't mine. But no way did that model fit in with the project I'm doing.
Recently however I've been tasked with rewriting the section of code that generates reports in our application. The entire application is just a series of Java classes nested together and packaged up as one WAR file that is deployed to a Tomcat server. To scale, we deploy the same war file to another server (ideally everything is stateless and share-nothing so there's no sessions to worry about). The whole thing is behind a load balancer that uses a round-robin strategy to send requests to various machines. Seems like a pretty good way to scale horizontally I thought.
However the horizontal scaling of this system is fairly coarse grain. If we find that getting hit with a lot of requests to generate reports is slowing down a server, our option is to duplicate the entire application on another server to distribute the load. However that is distributing not only report generation but also every other aspect of the application. So now we have an additional front end, service, and database layer running just to speed up one area. It seems like a bit of a waste of resources.
So instead of scaling a monolithic application, how about we break the whole thing up into 'services'. Instead of the Report system just being a set of APIs and implementations in the applications source code, we instead make an entire new application that just generates reports as its whole goal. It has a basic interface and API that takes in a request with certain criteria of what type of report you want and in what format and returns that report. It can be a completely stand-alone application that just sits and waits, listening on a port for a request to come in. When one does it processes the request, generates a report, then sends the information back over that port to the client and waits for the next request. Naturally we make it multi-threaded so that we can handle multiple requests at a time, putting a limit on that number and queuing any overflow.
The benefit of this is manyfold. For starters you gain fine-grain horizontal scalability. If you find that one service is too slow or receives a lot of requests you can just deploy that service on additional machines. You only deploy that service however rather than the whole application. The service can be done via RPC, direct sockets, web services, or whatever else you like to listen for your requests. A controller near the front end would just call each individual service to gather the end data up to pass back to the user. Put it behind some sort of load balancer with failover and you have fine-grain horizontal scalability.
Second, you gain faster deployment. Since each service is self-contained they can be deployed independent of the other services. Launching a new version does not have to replace the entire system, just the services that you're upgrading. In addition, since each service can be running (and should be running) on multiple machines, you can upgrade them piecemeal and perform bucket tests. For instance, if you have a service that is running on 10 machines you can upgrade only 2 of them and monitor user interaction with those services. This way you can have 20% of your users actually utilize your new code, the rest will hit the old code. When you see that everything is going fine you can upgrade the other 8 servers to have the new version of the service.
Because each part of the program is just a small separate program itself, problems become easier to manage and bugs become easier to fix. You can write tests for just the service you're working on and make sure that it meets its contract obligation. It helps manage the problem with cyclical dependencies and tightly coupled code. It also reinforces the object-oriented strategy of message passing between components.
Further more, try to avoid language specific messages such as Java Serialization for communicating between your services. Utilize a language agnostic format such as JSON or Google's Protocol Buffers. I would avoid XML because it's fairly verbose and slower to transmit over a network and parse than those others. The advantage of using a language agnostic format is that each service can be written in a different language depending on the needs of the service. If you have a service that really needs to churn through a lot of data and you need every ounce of speed out of it you can write it in C or even assembly. For those areas where it would be good to do something concurrently you might use Scala, Erlang, or Clojure. The main idea is each part of your program can be written in a language that is best for solving that services problem.