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HTTP Pipelining explained

This contents is from the curl/lib/README.pipelining file.

HTTP Pipelining with libcurl
Since pipelining implies that one or more requests are sent to a server before
the previous response(s) have been received, we only support it for multi
interface use.
When using the multi interface, you create one easy handle for each transfer.
Bascially any number of handles can be created, added and used with the multi
interface - simultaneously. It is an interface designed to allow many
simultaneous transfers while still using a single thread. Pipelining does not
change any of these details.
We've added a new option to curl_multi_setopt() called CURLMOPT_PIPELINING
that enables "attempted pipelining" and then all easy handles used on that
handle will attempt to use an existing pipeline.
- A pipeline is only created if a previous connection exists to the same IP
  address that the new request is being made to use.
- Pipelines are only supported for HTTP(S) as no other currently supported
  protocol has features resemembling this, but we still name this feature
  plain 'pipelining' to possibly one day support it for other protocols as
- HTTP Pipelining is for GET and HEAD requests only.
- When a pipeline is in use, we must take precautions so that when used easy
  handles (i.e those who still wait for a response) are removed from the multi
  handle, we must deal with the outstanding response nicely.
- Explicitly asking for pipelining handle X and handle Y won't be supported.
  It isn't easy for an app to do this association. The lib should probably
  still resolve the second one properly to make sure that they actually _can_
  be considered for pipelining. Also, asking for explicit pipelining on handle
  X may be tricky when handle X get a closed connection.