Techniques to avoid serializing the input or output when stacking middleware ¶

Title:: Avoiding Serialization When Stacking Middleware
Author:: Ian Bicking <ianb@colorstudy.com>
Discussions-To:: Python Web-SIG <web-sig@python.org>
Status:: Proposed
Created:: 06-03-2007

Abstract ¶

This proposal gives a strategy for avoiding unnecessary serialization and deserialization of request and response bodies. It does so by attaching attributes to wsgi.input and the app_iter, as well as a new environment key x-wsgiorg.want_parsed_response.

Rationale ¶

Output-transforming middleware often has to parse the upstream content, transform it, then serialize it back to a string for output. The original output may have already been in the parsed form that the middleware wanted. Or there may be more middleware that does similar transformations on the same kind of objects.

The same things apply to the parsing of wsgi.input, specifically parsing form data. A similar strategy is presented to avoid unnecessarily reparsing that data.

Specification ¶

WSGI applications (or middleware) can return an app_iter that not only serializes the output, but also has extra attributes. An attribute is given here, app_iter.x_wsgiorg_parsed_response which is a function/method that takes one argument, the “type” of object that you want to receive. It may return that type of object, or None (meaning it cannot produce that type of object). Consumers should fall back on normal parsing of the response if the method does not exist, or returns None.

Similarly the wsgi.input object may have the same method, with the same meaning.

WSGI applications that want to lazily serialize their output have a problem: they probably cannot calculate Content-Length without doing the actual serialization. Browsers typically want to know about Content-Length, but WSGI middleware seldom cares, since it just can get the content from app_iter regardless of its length. WSGI middleware that will transform the output can set environ['x-wsgiorg.want_parsed_response'] = True to give this hint to the application. Applications are thus encouraged to only lazily serialize their output when that key is present and true. (There is no equivalent concept for wsgi.input.)

The object returned by x_wsgiorg_parsed_response() may be modified in-place by the WSGI middleware using that object. Producers should make a copy if they do not want consumers modifying the object.

Example ¶

Two examples are provided: one for output, and one for input.

The output transformation parses the page with lxml.etree.HTML (from the lxml library) and replaces all <i> tags with <em> tags. First we show the middleware:

import lxml.etree

class EmTagMiddleware(object):
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        parent_wants_parsed = environ.get('x-wsgiorg.want_parsed_response')
        environ['x-wsgiorg.want_parsed_response'] = True
        written_output = []
        captured_headers = []
        def repl_start_response(status, headers, exc_info=None):
            if exc_info:
                raise exc_info[0], exc_info[1], exc_info[2]
            captured_headers[:] = [status, headers]
            return written_output.append
        app_iter = self.app(environ, repl_start_response)
        parsed = None
        if captured_headers and not written_output:
            method = getattr(app_iter, 'x_wsgiorg_parsed_response', None)
            if method:
                parsed = method(lxml.etree._Element)
        if parsed is None:
            # Have to manually parse, because:
            #  a) start_response was called lazily
            #  b) the start_response writer was used
            #  c) app_iter.x_wsgiorg_parsed_response didn't exist
            #  d) that method returned None
            try:
                for item in app_iter:
                    written_output.append(item)
            finally:
                if hasattr(app_iter, 'close'):
                    app_iter.close()
            parsed = self.parse_body(''.join(written_output))
        status, headers = captured_headers
        new_body = self.transform_body(parsed)
        for i in range(len(headers)):
            if headers[i][0].lower() == 'content-length':
                del headers[i]
                break
        if parent_wants_parsed:
            new_app_iter = self.make_app_iter(new_body)
        else:
            serialized_body = serialize(new_body)
            headers.append(('Content-Length', str(len(serialized_body))))
            new_app_iter = [serialized_body]
        return new_app_iter

    def parse_body(self, body):
        return lxml.etree.HTML(body)

    def transform_body(self, root):
        for el in root.xpath('//i'):
            el.tag = 'em'
        return root

    def make_app_iter(self, body):
        return LazyLXML(body)

def serialize(element):
    return lxml.etree.tostring(element)

class LazyLXML(object):
    def __init__(self, body):
        self.body = body
        self.have_yielded = False
    def __iter__(self):
        return self
    def next(self):
        if self.have_yielded:
            raise StopIteration
        self.have_yielded = True
        return serialize(self.body)
    def x_wsgiorg_parsed_response(self, type):
        if type is lxml.etree._Element:
            return self.body
        return None

Here’s a simpler example for parsing normal form inputs in wsgi.input:

import cgi
import urllib
from cStringIO import StringIO

def parse_form(environ):
    content_type = environ.get('CONTENT_TYPE', '')
    assert content_type in ['application/x-www-form-urlencoded', 'multipart/form-data']
    wsgi_input = environ['wsgi.input']
    method = getattr(wsgi_input, 'x_wsgiorg_parsed_response', None)
    if method:
        parsed = method(cgi.FieldStorage)
        if parsed is not None:
            return parsed
    form = cgi.FieldStorage(fp=wsgi_input, environ=environ, keep_blank_values=True)
    environ['wsgi.input'] = FakeFormInput(form)
    return form

class FakeFormInput(object):
    def __init__(self, form):
        self.form = form
        self.serialized = None
    def x_wsgiorg_parsed_response(self, type):
        if type is cgi.FieldStorage:
            return self.form
        return None
    def read(self):
        if self.serialized is None:
            self._serialize()
        return self.serialized.read()
    def readline(self, *args):
        if self.serialized is None:
            self._serialize()
        return self.serialized.readline(*args)
    def readlines(self, *args):
        if self.serialized is None:
            self._serialize()
        return self.serialized.readlines(*args)
    def __iter__(self):
        if self.serialized is None:
            self._serialize()
        return iter(self.serialized)
    def _serialize(self):
        # XXX: Doesn't deal with file uploads, and multipart/form-data generally
        data = urllib.urlencode(self.form.list, True)
        self.serialized = StringIO(data)

Problems ¶

Obviously the code is not simple, but this is the nature of WSGI output-transforming middleware. Ideally a framework of some sort would be used to construct this kind of middleware.

Something that replaces wsgi.input (like the example) may change the CONTENT_LENGTH of the request; normalization alone may change the length, even if the data is the same (e.g., there are multiple ways to urlencode a string). However, there’s no way without actually serializing to determine the proper length. Ideally requests like this should allow simply reading to the end of the object, without needing a CONTENT_LENGTH restriction (this is not true for socket objects). Ideally something like CONTENT_LENGTH="-1" would indicate this situation (simply a missing CONTENT_LENGTH generally means 0). Another option is to set it to 1 and simply return the entire serialized response all at once. cgi.FieldStorage actually protects against this. Or set it to a very very large value, and allow reading past the end (returning ""). This is likely to work with most consumers. I’m not sure what effect -1 will have on different code.

Other Possibilities ¶

You could simply parse everything ever time.
You could pass data through callbacks in the environment (but this can break non-aware middleware).
You can make custom methods and keys for each case.
You can use something other than WSGI.

I think this specification offers advantages over all these options.

Open Issues ¶

Should “type” be the class object? A string describing the type? Things like lxml.etree._Element are a little unclean, since the actual class isn’t a public object (only the factory function lxml.etree.Element()). Also, there are occasionally times when multiple classes implement the same interface.

The boolean x-wsgiorg.want_parsed_response doesn’t really give any idea of what kind of object you want. This is actually something of a problem, because sometimes it’s impossible to give that kind of object. For instance, if you want to transform images you might want the PIL object for the image. But if the response is HTML there’s no way to give this type. Similarly if you are transforming HTML then images don’t mean anything to you, and you probably do want them to come out as normal. And potentially both a image transformer and an HTML transformer are in the stack. Should that key actually hold a list of types that are of interest?

x_wsgiorg_parsed_response() isn’t a very good name for the method on wsgi.input, as it’s not a response.

Techniques to avoid serializing the input or output when stacking middleware ¶

Abstract ¶

Rationale ¶

Specification ¶

Example ¶

Problems ¶

Other Possibilities ¶

Open Issues ¶

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