A server-side JavaScript runtime based on coroutines
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Latest commit 739bd3b Sep 25, 2010 @pgriess Add benchmarking tools.
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Corona is an experimental server-side JavaScript runtime that provides concurrency support using coroutines. It builds on the work of the NodeJS team, particularly in its integration with the V8 JavaScript engine.

Runtime Model

Like NodeJS, Corona executes in a single process, with JavaScript executing in a single thread. No API calls exposed to JavaScript will cause the process itself to block.

However, in contrast to NodeJS, the Corona runtime can support multiple independent control flows in JavaScript. These control flows are implemented under the covers as coroutines. Individual coroutines can invoke API calls that block the coroutine (but not the process itself). This allows usage the linear programming style familiar to most programmers without sacraficing performance.

A programming model with multiple flows of control, regardless of the underlying implementation requires some mechanism for mitigating race conditions. Most thread-like systems implement synchronization primitives such a mutexes, semaphores and the like. It is an explicit design goal of Corona to provide a programming model that does not require use of these constructs: threads are hard.

That said, there must be some model for allowing control flows to interact with and reason about global state in the presence of blocking API calls. Corona's attempt to address this is the following set of runtime features

  • Support for futures: When invoking an function call that needs to block to compute its return value, rather than blocking the process immediately, the function can return a Future object immediately, without blocking. This Future represents a holder for the future return value of the function. Only when this object is interrogated does the process block. This allows developers to implement atomic codepaths that span multiple nominally blocking functions.
  • Toggling blocking/non-blocking mode: A set of primitives will be provided to ensure that the runtime does not block. These can be used to implement critical sections. An exception will be thrown when any system call is invoked that would otherwise block the currently executing control flow.

Corona vs. NodeJS

Corona's value proposition in comparison to NodeJS is one of ease-of-development. While JavaScript is a language uniquely capable in its support for an asynchronous programing model (chiefly due to closures and anonymous functions), there is some room for improvement over the purely asynchronous environment provided by NodeJS:

  • Purely asynchronous programming requires developers to maintain the application state machine in two places: the stack, and state explicitly maintained and passed around to callbacks.
  • In practice, purely asynchronous programming can necessitate deeply nested callback structures. Such a control flow is more concisely represented in a linear sequence of blocking calls.
  • Correlating callbacks to the codepath that created them is often difficult. The canonical example for this is throwing an exception inside an event handler -- who "owns" the event handler and where did it come from?