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So here I am reading one of my favorite software pattern books (Pattern-Oriented Software Architecture - Patterns for Concurrent and Networked Objects), specifically the sections on Proactor/Reactor asynchronous IO patterns. I can see how by using selectable channels I can implement a Reactor style asynchronous IO mechanism quite easy (and have done so). But, I cannot see how I would implement a proper Proactor mechanism with non-blocking writes. That is taking advantage of OS managed non-blocking write functions.
Functionality supported by OS specific calls like GetQueuedCompletionStatus under win32.
I did see that Java 7 brings some updates to NIO with asynchronous completion handlers (which seems to be in the right direction). That being said... Given the lack of unified cross-platform support for OS managed async operations (specifically async write) I am assuming that this is a quassy-implementation that is not utilizing native OS support.
So my questions are, is proactor based IO handling possible in Java in such a way that it is advantageous to use for specific scenarios; and, if Java NIO does support proactor based IO handling (either in Java 6 or Java 7) is OS managed asynchronous IO support (i.e. completion callbacks from the OS) being utilized? Furthermore, if the implementation is purely in-VM are the performance benefits so little that using proactive event handling offers nothing more than a different (possibly simpler) way of constructing concurrent network handling software.
For anyone interested in proactive event handling here is a good article that outlines pros / cons and a comparison to both traditional thread-per-connection and reactive IO models.
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Unlike Java IO, Java NIO is a non-blocking IO. This means that if a thread is invoking a read() or write() operation, that thread is not blocked until there is some data to read or the data is fully written rather the thread go on something else.
The asynchronous channel APIs were introduced into the existing java. nio. channels package, simply put – by prefixing the class names with the word Asynchronous. Some of the core classes include: AsynchronousSocketChannel, AsynchronousServerSocketChannel and AsynchronousFileChannel.
So what happens when you call a non-blocking API? Very well, it returns instantly and will not block the thread. This means the thread can immediately continue executing the code that comes after calling the API. When data has returned from IO, the caller will be notified that the data is ready.
Java IO's various streams are blocking. It means when the thread invoke a write() or read(), then the thread is blocked until there is some data available for read, or the data is fully written. Non blocking I/O. Non blocking IO does not wait for the data to be read or write before returning.
There are lots of factors involved in this one. I will try to summarize my findings as best as possible (aware of the fact that there is contention regarding the usefulness of reactor and proactor IO handling implementations).
Is proactor based IO handling possible in Java in such a way that it is advantageous to use for specific scenarios.
Java 1.4 introduced non-blocking IO which is NOT the same as asynchronous IO. Java SE 7 introduces asynchronous IO with JSR203 making "true" proactor style IO handling implementations possible.
See AsyncrhonousSocketChannel, AsynchronousServerSocketChannel
and, if Java NIO does support proactor based IO handling (either in Java 6 or Java 7) is OS managed asynchronous IO support (i.e. completion callbacks from the OS) being utilized?
Reading through the JSR 203 specs, completion handlers using new asynchronous channels are definitely supported and it is reported that native OS features are being utilized but I have not ascertained to what extent yet. I may follow up on this after an analysis of the Java 7 source (unless someone beats me to it).
Furthermore, if the implementation is purely in-VM are the performance benefits so little that using proactive event handling offers nothing more than a different (possibly simpler) way of constructing concurrent network handling software.
I have not been able to find any performance comparisons regarding new Asynchronous IO features in Java 7. I'm sure they will become available in the near future.
As always, when presented with more than one way to tackle a problem the questions of which approach is better is almost always answered with "depends". Proactive event handling (using asynchronous completion handlers) is included with Java 7 and cannot simply exist without purpose. For certain applications, it will make sense to use such IO handling. Historically a common example given where proactor has good applicability is in a HTTP server where many short requests are issued frequently. For a deeper explanation give this a read (provided only to highlight the advantages of proactor so try to overlook the fact that example code is C++).
IMO it seems obvious that in many circumstances reactor/proactor complicate what would otherwise be a very simple design using a more traditional approach and in other more complex systems they offer a high degree of simplification and flexibility.
. . .
On a side note I highly recommend reading through the following presentation about NIO which offers performance comparison between NIO and the "traditional" approach. Though I would also advise caution regarding the results presented as the NIO implementation in the benchmark was based on the pre Java 1.4 NBIO NIO library and not the NIO implementation shipped in 1.4.
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