constness of it all

As a designer of libraries and code that others use, I have come to an impasse with Java. I am convinced that it was originally designed for ultimate portability and application development and not for efficiency and library design. One sticking point that has bothered me since I got JDK 1.0 back in 1996 was the absence of const, commonplace in C/C++. const keyword means something is not allowed to be changed (barring any void* hacks).

So if someone writes:

const char *p;

You know that this is a pointer to a char array that cannot be changed. But const is not always this easy to understand, lets look at this example:

const char *const p;


This means that the pointer contents cannot be changed and the pointer value (memory location of the char data) cannot be changed. Further you can really complicate things with:

const char *const get(const char *const) const;

Here a class member function accepts a pointer to a char array that cannot be changed and pointer that cannot be reassigned, it returns the same and on top of that the class data cannot change. Little much but const can be a beautiful thing in the hands of an interface designer.

Let's take a simple example of a class that is a global cache that you cannot change:

class GlobalCache
{
public:
const ObjectData& get(const KeyName&) const;
};


Simple enough you give it a reference to a key object and it looks up and returns a constant ObjectData reference without changing its own contents. The fact that reference is used to pass data around is efficient because no memory is allocated/copied in the process. The method is const (the last const in that declaration) so nothing is written to the object during that call and the most important const of them all is the first one. It means that the data you get you cannot change in any way. Why is it important? You don't need to synchronize access to that object, in a multi-threaded environment you can have as many threads as you want reading that object without a need to synchronize it in any way.

There is no way to do this in java. The closest thing is to return a copy of the ObjectData (and ObjectData can be huge):

class GlobalCache
{
public:
public ObjectData get(const KeyName key) {
return new ObjectData(someinternalObjectaData;
}
};

Java has to make a copy of this object to guarantee consistency. If you return the actual object then anyone can change it which can have very unexpected results if the system is multi-threaded (object consistency is lost). Before you mention final keyword, remember that it means the reference cannot change and makes no guarantees about the object itself.

Not having a const means you need make a lot more complicated of a design to guarantee that the code behaves as intended (because you can never expect the users to do what is expected, they will always do what is not). This is one of the reasons java code tends to bloat in the class count area and it is never fun to have several classes doing what really one class should have done.

I am hoping in one of the upcoming java versions implements a const-like behavior (afterall 5.0 has template-like behavior)... More and more java is starting to look like C++. Funny how that happens.

AOS (AsynchObjectServer) is undergoing tons of changes, I decided to rewrite the processing queues to modularize the server a bit more. The first 2 stages are: HTTP reader and socket selector; once a listener socket accepts a connection (either HTTP or HTTPS, both treated identically having a common base class AFile_Socket_SSL is a child of AFile_Socket). SSL sockets are slightly different, but when data is available we can use the AFile_Socket_SSL::read and the cyphers are handled automatically, so the socket selector queue just needs to determine that there is data waiting and the HTTP reader queue will then process the socket. Once HTTP reader determines the HTTP request header is complete, it creates a AOSContext object based on AOSRequest and pushes it into the first "application logic" stage.

The AOSInputProcessor-based queue which then handles the input; primarily it splits the query string, appends FORM data if needed or handle a custom request document.

Once finished it pushes the context into AOSMOduleExecutor-based queue which executes applicable modules for the given request command and once that is finished it will push context to the next queue.

AOSOutputGenerator-based queue then performs the output generator, by default the XML document is returns and nothing is needed to generate but it can do XSLT or template substitution/concatination or whatever output is needed.

Splitting AOSRequest (socket request) and AOSContext (application context) queues allows the server to have multiple entry points into application execution, since AOSContext relies on AFile interface it allows anything that behaves like a file to work as a recepient of the execution logic. AFile interface is implemented for string buffer, physical file, and stdc++ IO stream.

This is a rough overview of the queue chains (I omitted the error queue). Each queue is really a set of queues with N threads working them asynchronously. Overall throughput of this setup (which is very much like SEDA but I didn't know about their papers until I read them few weeks ago) is quite high and in my initial load tests was about 3-4 times faster than Apache mina-0.9.4. I am going to rewrite memory allocation optimizations and improve concurrency then retest. I am hoping to have a working server again in a few weeks.

Initium

So this is a continuation of the progress report on Asynchronous Object Server I have been writing for the last 9 months or so. Rest of the progress notes are at http://www.achacha.org/aos/, blog format is probably easier to maintain and read.