Question

I am working on a web application which needs to get data from some local and some non local resources and then display it. As it could take arbitrary amount of time to get the data from these resources I am thinking of using the actors concept so that each actor is responsible for getting data from the respective resource. The request thread will wait for each actor to finish its task and then use ajax to update only the portion of the web page that is dependent on that data. This way user will start seeing the data as soon as it is received rather than wait for all of them to finish and then get a first look at the data.

I am planning to look into scala/lift framework for this. I have read some articles on the web for scala/lift and want to explore if this is the correct way to approach this problem and also if scala/lift is good platform of choice. I have worked in Java and C# previously. Any opinions, comments, suggestions are welcome.

Thanks, gary

Answer 1

Take a look a message queue technology like Java s JMS. Message queues allow you to handle long running background tasks asynchronously and reliably. This is the technique sites like Flickr and YouTube use to do media transcoding asynchronously. You can use a Java EE server, or a JMS technology like Apache s ActiveMQ, and then layer your Scala/Lift code on top of it.

Richard Monson-Haefel s book on JMS covers it well.

For more general help with web site scaling and construction, take a look at Todd Hoff s excellent blog, highscalability.com/. There are some good pointers to using message queues to offload long-running tasks this way.

BTW, Twitter uses Scala for something much like what you re considering. Here s an interview with some of their developers; they describe one way they use Scala:

Robey Pointer: A lot of our architecture is based on letting Rails do what it does best, which is the AJAX, the web front ends, the website—what the user sees. Anything we can offload out of the request/response cycle, we do. So we queue those tasks into a messaging system and have back-end daemons handle them.

Answer 2

If the non-local resources originate at some other service or system, Event Driven Architecture might work for you. Instead of pulling from the non-local resources you could set up this web-application as a subscriber to the events published by these services. Upon receiving a message regarding part of its functionality it would cache locally the data it s interested in. This should let you escape the issue of asynchronous update of parts of the page (all data would be accessible locally).

Udi Dahan blogs about this approach a lot and is also an author of a .NET message bus (NServiceBus) that can be used in such scenarios. See for example http://msdn.microsoft.com/en-us/architecture/aa699424.aspx

Answer 3

Actors would be a way to go. You re essentially setting up a light weight version of JMS. And Lift does the comet stuff very well. In addition to the Scala actors, and the Lift Actors, you also have akka actors. When Scala Swarm becomes production ready you ll be ready for that too.

Answer 4

If the delayed information is distinct from the information that needs to display immediately, with Lift you can use a LazyLoad snippet that has the longer-running computation (calling the web service) as part of its logic. Lift will take care of inserting it into the page when its ready.

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