Noah Slater wrote:
On 16 Aug 2010, at 21:38, Miles Fidelman wrote:
There have been some of these built on top of HTTP, but the purist in me really
dislikes violating layering.
I'm a little out of my depth here, but is that really the case? The things
you're talking about sound like patterns to me, not layer-specific details.
What makes that pattern any less applicable at a higher level in the network
stack?
well, the whole point of protocol layering is to isolate different
functions in different layers - as soon as the same function is
available in multiple layers, things just get very confusing and brittle
a simple example that I think will illustrate the difference (note -
it's easier to read stacks from the bottom up)
a RESTful stack that doesn't violate layering:
RSS - specific encoding of data
HTTP - client-server protocol - deals primarily with addressing data
items and encodings
TLS - adds security to a connection
TCP - reliable connections
IP - unreliable datagram transport across multiple networks (glues
individual networks into an internet)
802.x - unreliable data over a single local network
USB (or whatever) - hardware level device connection
vs.
WS-* protocols: try to do everything, including things that are already
available at lower layers (addressing, security, connections, ...)
SOAP:
HTTP:
TLS
TCP
IP
802.x
USB
essentially, a lot of what the WS-* protocols do is reinvent things that
have been worked out and incorporated into lower layer protocols - the
result is that:
1. the WS-* protocols tend to do a poor, and complicated, job of
functions that are available in lower layers of the network stack, and
simply ignore those capabilities, and/or,
2. an awful lot of SOAP-based applications generate transactions that
contain huge numbers of empty XML headers
The impact, in practice: We recently wrote an application that
translated OpenSearch formatted queries into SOAP formatted queries, for
execution by an EBRIM Registry/Repository, and then translated the
responses back into OpenSearch format (actually RSS). When you looked
inside the transactions, you'd see:
a 1-line OpenSearch query turns into 1 line of SQL, surrounded by about
100 lines of empty XML headers
the response starts as 100+ lines of XML, mostly empty headers, that can
be compressed into about 10 lines of XML in RSS format
Layering ends up being about managing complexity. Doing things at the
right layer simplifies things, doing it at the wrong layer makes things
intractable.
Miles
--
In theory, there is no difference between theory and practice.
In<fnord> practice, there is. .... Yogi Berra
