On Jun 5, 2011, at 3:35 PM, Martin v. Löwis wrote:
> And that's all fine. I still claim that you have to *understand*
> sockets in order to use it properly. By this, I mean stuff like
> "what is a TCP connection? how is it established?", "how is UDP
> different from TCP?", "when data arrives, what layers of software
> does it go through?", "what is a port number?", etc.
Yes, these are all excellent concepts to be familiar with. But the word
"socket" (and the socket HOWTO) refers to a specific way to interface with
those concepts, the Berkeley socket API:
<http://en.wikipedia.org/wiki/Berkeley_sockets>. Which you don't have to know
anything about if you're going to use Twisted. You should know about IPC in
general, and TCP/UDP specifically if you're going to use Twisted, but sockets
are completely optional.
Also, I feel that I should point out that the sockets HOWTO does not cover even
a single one of these concepts in any useful depth. If you think that these
are what it should be explaining, it needs some heavy editing. Here's what it
has to say about each one:
> what is a TCP connection?
The only place that the characters "TCP" appear in the entire document is in
the phrase "... which is completely different from TCP_NODELAY ...". Nowhere
is a TCP connection explained at a conceptual level, except to say that it's
something a web browser does.
> how is UDP different from TCP?
The phrase "UDP" never appears in the HOWTO. DGRAM sockets get a brief mention
as "anything else" in the sentence: "... you’ll get better behavior and
performance from a STREAM socket than anything else ...". (To be fair, I do
endorse teaching that "the difference between TCP and UDP is that you should
not use UDP" to anyone not sufficiently advanced to read the relevant reference
documentation themselves.)
> when data arrives, what layers of software does it go through?
There's no discussion of this that I can find at all.
> what is a port number?
Aside from a few comments in the code examples, the only discussion of port
numbers is "low number ports are usually reserved for “well known” services
(HTTP, SNMP etc)."
It would be very good to have a "Python networking overview" somewhere that
explained this stuff at a very high level, and described how data might get
into or out of your program, with links to things like the socket HOWTO that
describe more specific techniques. This would be useful because most commonly,
I think that data will get into Python network programs via WSGI, not direct
sockets or anything like Twisted.
To be clear, having read it now: I do _not_ agree with Antoine that this
document should be deleted. I dimly recall that it helped me understand some
things in the very early days of Twisted. While it's far from perfect, it
might help someone in a similar situation understand those things as well
today. I just found it interesting that the main concepts one would associate
with such a HOWTO are nowhere to be found :).
-glyph
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