[cpp-netlib-devel] Reusable filter chain design problem.

[<rhy...@gm...>]

Hi, all

I have posted this in asio user group.  And I think it is also important 
for cpp-netlib.

I think filter chain is an important facility for almost every some what 
complicated network application. Say, I want

1. do RC4 encryption/decryption,
2. zip/unzip the byte stream,
3. encode/decode a protocol specific class instance into/from plain byte 
stream.

Then we need 3 filters: an RC4 filter, a compression filter, and a 
protocol codec filter. We assemble these filters into a filter chain, 
and attach the filter chain to sessions.

There are several choices, say compile time static filter chain or 
runtime assembled dynamic filter chain? And filters often maintains 
session specific states, if we store the states in the filters, we 
should maintain one filter chain per session. If we store the states in 
the session object, and make all filters stateless, then all session can 
share a single filter chain. But the session object must meet the 
concept that all the filters requires. Say, the RC4 encoder/decoder pair 
are not stateless, thus cannot be stored in the rc4_filter instance, and 
rc4_filter may requires the session object having two public method:

     rc4_encoder& get_rc4_encoder();
     rc4_decoder& get_rc4_decoder();

What do you think? And I recommend a glance at the Apache MINA project, 
which have a rather flexible filter chain facility. Here's the site:

     http://mina.apache.org/


Cheers
Cheng