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<h3 class="section">3.11 Tuning POST and PUT requests</h3>
<p><a name="index-POST-request-172"></a><a name="index-PUT-request-173"></a><a name="index-expectContinue-174"></a>
The main assumption behind the design of the HTTP protocol is that
requests are idempotent: since a request can be repeated by a client,
a server is allowed to drop a connection at any time. This fact, more
than anything else, explains the amazing scalability of the protocol.
<p>This assumption breaks down in the case of POST requests. Indeed, a
POST request usually causes some action to be performed (a page to be
printed, a significant amount of money to be transferred from your
bank account, or, in Florida, a vote to be registered), and such a
request should not be repeated.
<p>The only solution to this problem is to reserve HTTP to idempotent
activities, and use reliable protocols for action-effecting ones.
Notwithstanding that, HTTP/1.1 makes a weak attempt at making POST
requests slightly more reliable and efficient than they are in
HTTP/1.0.
<p>When speaking to an HTTP/1.1 server, an HTTP client is allowed to
request that the server check <em>a priori</em> whether it intends to
honour a POST request. This is done by sending <dfn>an expectation</dfn>,
a specific header with the request, ‘<samp><span class="samp">Expect: 100-continue</span></samp>’, and
waiting for either an error message or a ‘<samp><span class="samp">100 Continue</span></samp>’ reply
from the server. If the latter arrives, the client is welcome to send
the rest of the POST request<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.
<p>Polipo's behaviour w.r.t. client expectations is controlled by the
variable <code>expectContinue</code>. If this variable is false, Polipo
will never send an expectation to the server; if a client sends an
expectation, Polipo will fail the expectation straight away, causing
the client (if correctly implemented) to retry with no expectation.
If <code>expectContinue</code> is <code>maybe</code> (the default), Polipo will
behave in a standards-compliant manner: it will forward expectations
to the server when allowed to do so, and fail client expectations
otherwise. Finally, if <code>expectContinue</code> is <code>true</code>, Polipo
will always send expectations when it is reasonable to do so; this
violates the relevant standards and will break some websites, but
might decrease network traffic under some circumstances.
<div class="footnote">
<hr>
<h4>Fußnoten</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> This, of course, is only part of
the story. Additionally, the server is not required to reply with
‘<samp><span class="samp">100 Continue</span></samp>’, hence the client must implement a timeout.
Furthermore, according to the obsolete RFC2068, the server is
allowed to spontaneously send ‘<samp><span class="samp">100 Continue</span></samp>’, so the client must
be prepared to ignore such a reply at any time.</p>
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