Waiting for File Descriptor Events¶
- Title:
Waiting for File Descriptor Events
- Author:
Christopher Stawarz <chris@pseudogreen.org>
- Discussions-To:
Python Web-SIG <web-sig@python.org>
- Status:
Proposed
- Created:
11-May-2008
Abstract¶
This specification defines a set of extensions that allow a WSGI application to suspend its execution until an event occurs on a specified file descriptor.
Rationale¶
The architecture of asynchronous (aka event driven) servers requires all I/O operations, including both interprocess and network communication, to be non-blocking. For a WSGI-compliant server, this requirement extends to all applications run on the server. However, the WSGI specification does not provide sufficient facilities for an application to ensure that its I/O is non-blocking. Specifically, it lacks a mechanism by which an application can suspend its execution until an arbitrary file descriptor (such as one belonging to a socket or pipe opened by the application) is ready for reading or writing. This specification defines a standard interface by which servers can provide such a mechanism to applications.
Specification¶
This specification introduces three new variables to the WSGI
environment: x-wsgiorg.fdevent.readable
,
x-wsgiorg.fdevent.writable
, and
x-wsgiorg.fdevent.timeout
.
The variables x-wsgiorg.fdevent.readable
and
x-wsgiorg.fdevent.writable
are callable objects that accept
two positional arguments, one required and one optional. In the
following description, these arguments are given the names fd
and
timeout
, but they are not required to have these names, and the
application must invoke the callables using positional arguments.
The first argument, fd
, is either an integer representing a file
descriptor or an object with a fileno
method that returns such an
integer. The set of acceptable file descriptors is defined to be
those accepted by select.select
. (Note that this set is platform
dependent: only sockets are allowed on Windows, whereas sockets,
pipes, and files are acceptable on Unix-like systems.) The second,
optional argument, timeout
, is either None
or a floating-point
value in seconds. If omitted, it defaults to None
.
When called, x-wsgiorg.fdevent.readable
and
x-wsgiorg.fdevent.writable
return the empty string (''
),
which must be yielded by the application iterable to the server.
(The result of calling x-wsgiorg.fdevent.readable
or
x-wsgiorg.fdevent.writable
and yielding a non-empty string,
or making multiple calls to x-wsgiorg.fdevent.readable
and/or x-wsgiorg.fdevent.writable
before yielding the empty
string, is undefined.) The server then suspends execution of the
application until one of the following conditions is met:
The specified file descriptor is ready for reading (if the application called
x-wsgiorg.fdevent.readable
) or writing (if the application calledx-wsgiorg.fdevent.writable
).timeout
seconds have elapsed without the desired file descriptor becoming readable (if the application calledx-wsgiorg.fdevent.readable
) or writable (if the application calledx-wsgiorg.fdevent.writable
), unless the value oftimeout
isNone
, in which case the wait will never timeout.The server detects an error or “exceptional” condition (such as out-of-band data) on the file descriptor.
Put another way, if the application calls
x-wsgiorg.fdevent.readable
and yields the empty string, it
will be suspended until select.select([fd],[],[fd],timeout)
would
return. If the application calls x-wsgiorg.fdevent.writable
and yields the empty string, it will be suspended until
select.select([],[fd],[fd],timeout)
would return.
The variable x-wsgiorg.fdevent.timeout
is an object whose
truth value can be changed by the server. (For example, it could be a
list
instance, whose truth value is false when empty, true
otherwise.) If timeout
seconds elapse without the desired file
descriptor event occurring, x-wsgiorg.fdevent.timeout
will
be true when the application resumes; otherwise, it will be false.
The truth value of x-wsgiorg.fdevent.timeout
when the
application is first started or after it yields each response-body
string is undefined.
The server may use any technique it desires to detect events on an application’s file descriptors. (Most likely, it will add them to the same event loop that it uses for accepting new client connections, receiving requests, and sending responses.)
Handling of the Input Stream¶
While technically outside the scope of this specification, the
application’s input stream (wsgi.input
) is another
source of potentially blocking I/O that deserves mention.
The methods provided by the input stream follow the semantics of the
corresponding methods of the file
class. In particular, each of
these methods can invoke the underlying I/O function (in this case,
recv
on the socket connected to the client) more than once,
without giving the application the opportunity to check whether each
invocation will block. Although authors of asynchronous servers may
be tempted to provide a non-standard input stream that supports
on-demand, non-blocking reads, such an input stream would be
incompatible with WSGI middleware.
In order to avoid these problems, it is strongly recommended that asynchronous servers pre-read the entire request body (to an in-memory buffer or temporary file) before invoking the application, either by default or as a configurable option. Doing so will ensure that the input stream is compatible with middleware and that reads from it will not block waiting for data from the client.
Examples¶
The following application acts as a proxy to python.org. It uses a pycurl.CurlMulti
instance
to perform the outgoing HTTP request in a non-blocking fashion. When
the CurlMulti.perform()
method detects that its next I/O
operation would block, it returns control to the application, which
then yields until the file descriptor of interest becomes readable or
writable as required. If the descriptor is not ready after one
second, the application sends a 504 Gateway Timeout
response to
the client and terminates:
def pyorg_proxy(environ, start_response):
result = StringIO()
c = pycurl.Curl()
c.setopt(pycurl.URL, 'http://python.org' + environ['PATH_INFO'])
c.setopt(pycurl.WRITEFUNCTION, result.write)
m = pycurl.CurlMulti()
m.add_handle(c)
while True:
while True:
ret, num_handles = m.perform()
if ret != pycurl.E_CALL_MULTI_PERFORM:
break
if not num_handles:
break
read, write, exc = m.fdset()
if read:
yield environ['x-wsgiorg.fdevent.readable'](read[0], 1.0)
else:
yield environ['x-wsgiorg.fdevent.writable'](write[0], 1.0)
if environ['x-wsgiorg.fdevent.timeout']:
msg = 'The request to python.org timed out.'
start_response('504 Gateway Timeout',
[('Content-Type', 'text/plain'),
('Content-Length', str(len(msg)))])
yield msg
return
start_response('200 OK', [('Content-Type', 'application/octet-stream'),
('Content-Length', str(result.len))])
yield result.getvalue()
The following adapter allows an application that uses the
x-wsgiorg.fdevent
extensions to run on a server that does not
support them, without any modification to the application’s code:
def with_fdevent(application):
def wrapper(environ, start_response):
select_args = [None]
def readable(fd, timeout=None):
assert (not select_args[0])
select_args[0] = ([fd], [], [fd], timeout)
return ''
def writable(fd, timeout=None):
assert (not select_args[0])
select_args[0] = ([], [fd], [fd], timeout)
return ''
environ['x-wsgiorg.fdevent.readable'] = readable
environ['x-wsgiorg.fdevent.writable'] = writable
timeout = False
class TimeoutWrapper(object):
def __nonzero__(self):
return timeout
environ['x-wsgiorg.fdevent.timeout'] = TimeoutWrapper()
for result in application(environ, start_response):
assert (not (result and select_args[0]))
if result or (not select_args[0]):
yield result
else:
ready = select.select(*select_args[0])
timeout = (ready == ([], [], []))
select_args[0] = None
return wrapper
Problems¶
The empty string yielded by an application after calling
x-wsgiorg.fdevent.readable
orx-wsgiorg.fdevent.writable
must pass through any intervening middleware and be detected by the server. Although WSGI explicitly requires middleware to relay such strings to the server (see Middleware Handling of Block Boundaries), some components may not, making them incompatible with this specification.
Other Possibilities¶
To prevent an application that does blocking I/O from blocking the entire server, an asynchronous server could run each instance of the application in a separate thread. However, since asynchronous servers achieve high levels of concurrency by expressly avoiding multithreading, this technique will almost always be unacceptable.
The greenlet package enables the use of cooperatively-scheduled micro-threads in Python programs, and a WSGI server could potentially use it to pause and resume applications around blocking I/O operations. However, such micro-threading is not part of the Python language or standard library, and some server authors may be unwilling or unable to make use of it.
Open Issues¶
Some third-party libraries (such as PycURL) provide non-blocking interfaces that may need to monitor multiple file descriptors for events simultaneously. Since this specification allows an application to wait on only one file descriptor at a time, application authors may find it difficult or impossible to use such libraries, or they may be limited to a subset of the libraries’ capabilities.
Although this specification could be extended to include an interface for waiting on multiple file descriptors, it is unclear whether it would be easy (or even possible) for all servers to implement it. Also, the appropriate behavior for a multi-descriptor wait is not obvious. (Should the application be resumed when a single descriptor is ready? All of them? Some minimum number?)