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From "Alan D. Cabrera" <l...@toolazydogs.com>
Subject Re: [MINA 3.0] Initial thoughts on FilterChain
Date Tue, 15 Dec 2009 15:43:06 GMT

On Dec 15, 2009, at 4:30 AM, Emmanuel LŽcharny wrote:

> Hi,
> there is another approach if we switch to a SM : actions don't need  
> to know about the next action, as it's computed by the SM. We can  
> end with methods like :
> messageReceived() {
> blah
> }

Yes, this is along the lines of what I was thinking.

> and when the method returns, the SM decide which filter to call  
> next. This end up with something like :
> while ( not Done ) {
> nextFilter = computeNext(session);
> nextFilter.messageReceived(session);
> }

This is not much better than what we had before.  The chains should be  

while (!session.closed())
     List<IoFilter> filters = session.getChain(session.getState());
     for (IoFilter filter : filters) filter.messageReceived(session);

> The only constraints being that we don't have code like :
> messageReceived(session) {
> blah();
> call next filter; // Not necessary anymore...
> post_blah(); /// Wrong !!!
> }


> Now, why did we used Filters at the origin ? It's important to know  
> that when Alex thought about what should be MINA 6 years ago, and  
> before Trustin joined the project, the idea was to implement a SEDA  
> based framework.
> What does it imply in real world ? Many things. First, transition  
> between one filter and another should allow the use of a queue, so  
> interactions are asynchronous. Another aspect is that we may have  
> more than one thread running on one session (some decoding can occur  
> while a new message is being received). Another consequence is that  
> we may have unordered messages : if two threads are being processed  
> for the same session, one can be faster to decode than the second  
> one, and the second one can perfectly well hit the Handler before  
> the first one. We have some mchanism to protect the user from such a  
> problem.
> Ig we have to keep this SEDA approach, then we must be careful and  
> be sure that we can process each filter separately. looking at the  
> loop I exposed above, we will have a problem because the loop is  
> executed sequencially by one single thread, so we can't anymore  
> implement any SEDA mechanism.
> If the filter is responsible for the call of the next filter, then  
> it's a totally different story.
> We have to think about this before drafting some implementation, and  
> decide if we want to stick to SEDA.

Agreed.  This is something we should support.  I think that the  
original complexity came from mixing concerns.

What if we had channels that could contain things like queues, state  
machines, etc.?  Channels would be bidirectional, i.e. messages would  
move up and down in a single channel.  We would then compose channels  
in fixed DAGs.

> Now, some comment in line about Alan's last mail
> Alan D. Cabrera a écrit :
>> <Snip/>
>>>>> Not sure this is possible in another way than with those  
>>>>> computed nextFilter() inside the filters.
>>>> I agree but it's my contention that it's a bad practice that  
>>>> supports an ill thought out protocol.
>>> The biggest advantage is that it eases the implementor work most  
>>> of the cases.
>> IMO, it's sloppy and error prone and obfuscates code.  If no one  
>> else agrees then I'm happy to drop my point.
> You are probably right. If you look at the existing filters, there  
> is no reason we should not be able to avoid such code.
>>> Now, it does not preclude that we should not allow someone to  
>>> implement his protocol using a complete state machine. May be we  
>>> should provide both mechanisms :
>>> - one which is driven by the code (ie, the code 'pull' the next  
>>> step),
>>> - one which is driven by the state machine (your way).
>> I would argue against this.  Mina is afflicted w/ bloat.
> I can't agree more :)
>> One the goals should be to get rid of as many useless "helpful"  
>> classes and methods as we can.
> +1
>>  Either we all agree that adding filters in an ad hoc manner is a  
>> best practice for network protocol state machines and we loose the  
>> state machine or we agree that it's an anti-pattern that should be  
>> avoided.  If the community thinks that ad hoc filters are a best  
>> practice I'm happy to drop my point.
> I would like to keep the SM approach, but as explained above (SEDA  
> thing), I think we should be driven by the code.
>> <snip/>
>> I totally agree with this approach to deciding on the API and am  
>> happy to help out w/ some protocols, e.g. HTTP and SSL.
>> I am curious, what project feels that it needs to do an "implicit"  
>> state machine?  I would love to take a peek at the code.
> What do you mean by "implicit" state machine ?

In the "ad hoc" state transition approach the state transitions are  
implicit in the code rather than explicit and fixed in a set of data  
structures.  To understand the implicit state machine one must  
carefully read all the code to understand what's going on.


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