Execution and Exceptions

That’s pretty nifty how we started up a Netty server in a few lines of code, just like with Jetty, right? But while Planify is really handy for experimenting in the console, it makes a lot of assumptions that are unlikely to hold for a live server.

A Less Simple Plan

Let’s see what the plan looks like with those defaults made explicit.

import unfiltered.netty._

trait MyPlan extends cycle.Plan with
cycle.ThreadPool with ServerErrorResponse

object Hello extends MyPlan {
  def intent = {
    case _ => ResponseString("hello world")

The traits define methods needed by cycle.Plan. But what for?

Deferred Execution

Netty handlers are invoked on an I/O worker thread. In some scenarios (including this one, actually) it would be just fine to prepare a response on this thread. If the work is CPU-bound (does not perform blocking operations), you can mix in the cycle.SynchronousExecution trait instead, for no-overhead execution.

But a typical web application does need to access a database or something, and the typical way to do this is with a blocking call. That’s why Planify uses the cycle.ThreadPool trait, which defers application of the intent partial function to a cached thread pool.

Deference has its Memory Limits

Unfortunately, that’s not the end of the story with deferred execution. Applications also need to worry about running out of memory. If you defer request handling jobs to an executor queue as fast as Netty can accept requests, any heap limit can be exceeded with a severe enough usage spike.

To avoid that kind of failure, you should equip your plans with an executor that consumes a limited amount of memory and blocks on incoming requests when that limit is reached. If you’ve defined a simple local base plan (like the MyPlan above), you can customize it later (ideally, before your server throws an out of memory exception) with a memory-aware thread pool executor.

trait MyPlan extends cycle.Plan with
cycle.DeferralExecutor with cycle.DeferredIntent with
ServerErrorResponse {
  def underlying = MyExecutor.underlying
object MyExecutor {
  import org.jboss.netty.handler.execution._
  lazy val underlying = new MemoryAwareThreadPoolExecutor(
    16, 65536, 1048576)

Expecting Exceptions

The ServerErrorResponse trait also implements behavior that your application will likely need to customize, sooner or later. Instead of mixing in that trait, you can implement onException directly in your base plan. For a starting point see the source for the provided exception handler, which logs the stack trace to stdout and serves a very terse error response. Normally an application will hook into its own logger and serve a custom error page or redirect.

The source code for this page can be found here.