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
}
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);
}
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.
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 ?
