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From Alex Karasulu <aok...@bellsouth.net>
Subject Re: [asn.1] BER codec design
Date Sat, 24 Jan 2004 04:37:33 GMT
Some additions inline:

> 
> From: Alex Karasulu <aok123@bellsouth.net>
> Date: 2004/01/23 Fri PM 11:30:13 EST
> To: <directory-dev@incubator.apache.org>
> Subject: [asn.1] BER codec design
> 
> Hi all,
> 
> It seems to me like Wes' breaking down the BER encoding/decoding into two
> steps with an intermediate structure.  This intermediate structure is the
> (BER specific) TLV tree Wes mentioned before.  
> 
> 1). The first is to go from the binary encoding to a binary encoded TLV 
>     tree and vice versa.  The TLV tree is just a structure reflecting the
>     containment tree of the ASN.1 datatype.  Each leaf in the tree is a
>     simple (non-composite) ASN.1 type with label and value.  I'm very
>     interested in your ideas for representing this TLV tree BTW.
> 
> 2). The second takes you from a TLV tree to a populated instance of a
>     data structure stub (an Object of the stub class) and vice versa.
> 
> Question is do we design this process into the server with non-blocking
> staged processing in mind or do you generate stuff conforming to what 
> works best for this BER codec?  I think we design for the BER codec
> but keep in mind we perform chunk wise operations.  So when encoding
> we ask for the first chunk and keep going back for more until we're
> told all has been transferred for that encoding.  Likewise for decoding
> we give chunks one chunk at a time until we're told we have enough for
> a single decoding session.
> 
> Also I think we might have to define multiple sub-codecs composing the
> stages of a larger ASN.1 BER codec like so:
> 
>           codec A           codec B
>               
> --------- decoder --------- decoder ---------
> -       - ------> -       - ------> -       -
> -Encoded-         -TLVTree-         -Decoded-
> -       - <------ -       - <------ -       -
> --------- encoder --------- encoder ---------
> 
> Let's just look at codec A.  Here we of course need to know what our
> tree classes are to build the right args but for now presume we are
> working with Objects.  
> 
> interface EncoderA
> {
>     SessionKey encode( ByteBuffer a_buf, Object a_inTree ) 
>         throws EncodingException ;
> 
>     boolean encode( ByteBuffer a_buf, SessionKey a_key ) ;
> }
> 
> The encoder encodes binary data into an NIO byte buffer.  Both
> methods reflect this.  However notice that the first returns a
> SessionKey and actually takes the TLVTree as an Object type.
> The first method returns null if it can fit all of the encoded
> image of the TLVTree into the buffer.  If it cannot it saves its
> state in the encoding process initializing a session for the
> encoding and returns a key to access that state once again to
> continue the process.  So for the most part small response PDUs 
> should only require one call to the first encode overloaded 
> method without incurring any session and key obj creation 
> overheads.
> 
> If however the encoded image cannot fit into the buffer then
> the session objects are created and a non-null key is returned.
> The buffer btw should be completely full.  Calls to the second
> method get the next chunk of encoded data which is again deposited 
> into the provided ByteBuffer.  This method unlike the first 
> requires the session key to continue the process.

Note that the boolean return value in the second encode() is used
to tell use if another encode operation is required to complete the
encode session.

> I'll come back to the rest later but this is how we get an encoder
> to return chunk wise data that is conducive to the SEDA architecture.
> I think we could use this interface as the interface for the entire
> BER encoder and not just for this phase of the encoding within codec 
> A.
> 
> I'll come back to this again later.

And another thing please give me some feedback.  I'm I overcomplicating
this or is this an expected cost in complexity to have non-blocking 
stuff working?

Alex



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