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Add high-level explanation of transports and protocols early on.

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Guido van Rossum 2013-03-18 15:43:00 -07:00
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pep-3156.txt
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@ -76,6 +76,47 @@ interface; there is an API to submit a function to an executor (see
PEP 3148) which returns a Future that is compatible with the event
loop.
A Note About Transports and Protocols
-------------------------------------
For those not familiar with Twisted, a quick explanation of the
difference between transports and protocols is in order. At the
highest level, the transport is concerned with *how* bytes are
transmitted, while the protocol determines *which* bytes to transmit
(and when).
A transport represents a pair of streams (one in each direction) that
each transmit a sequence of bytes. The most common transport is
probably the TCP connection. Another common transport is SSL. But
there are some other things that can be viewed as transports, for
example an SSH session or a pair of UNIX pipes. Typically there
aren't many different transport implementations, and most of them
come with the event loop implementation.
A transport has two "sides": one side talks to the network (or the
subprocess, or whatever low-level interface it wraps), and the other
side talks to the protocol. The former uses whatever API is necessary
to implement the transport; but the interface between transport and
protocol is standardized by this PEP.
A protocol represents some kind of "application-level" protocol such
as HTTP or SMTP. Its primary interface is with the transport. While
some popular protocols will probably have a standard implementation,
often applications implement custom protocols. It also makes sense to
have libraries of useful 3rd party protocol implementations that can
be downloaded and installed from pypi.python.org.
There is also a somewhat more general notion of transport and
protocol, where the transport wraps some other communication
abstraction. Example include an interface for sending and receiving
datagrams, or a subprocess manager. The separation of concerns is the
same as for stream transports and protocols, but the specific
interface between transport and protocol can be different in each
case.
Details of the interfaces between transports and protocols are given
later.
Non-goals
=========