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72 lines
3.3 KiB
Plaintext
72 lines
3.3 KiB
Plaintext
Doing HTTP Pipelining with libcurl
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==================================
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Background
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Since pipelining implies that one or more requests are sent to a server before
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the previous response(s) have been received, it cannot be implemented easily
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into libcurl's easy interface due to its synchronous nature. We therefore only
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aim on adding it for multi interface use.
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Considerations
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When using the multi interface, you create one easy handle for each transfer.
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Bascially any number of handles can be created, added and used with the multi
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interface - simultaneously. It is an interface designed to allow many
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simultaneous transfers while still using a single thread.
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Pipelining however, will force us to allow apps to somehow "connect" two (or
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more) easy handles that are added to a multi handle. The first one sends a
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request and receives a response, just as normal, while the second (and
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subsequent) ones need to be attached to the first handle so that it can send
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its request on the same connection and then sit and wait until its response
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comes.
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API
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We add a new option to curl_multi_setopt() called CURLMOPT_PIPELINING that
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enables "attempted pipelining" and then all easy handles used on that handle
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will attempt to use an existing pipeline.
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Decisions Already Made
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- A pipeline is only created if a previous connection exists to the same IP
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address that the new request is being made to use.
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- Pipelines are only supported for HTTP(S) as no other currently supported
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protocol has features resemembling this, but we still name this feature
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plain 'pipelining' to possibly one day support it for other protocols as
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well.
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- HTTP Pipelining is for GET and HEAD requests only.
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- When a pipeline is in use, we must take precautions so that when used easy
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handles (i.e those who still wait for a response) are removed from the multi
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handle, we must deal with the outstanding response nicely.
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- Explicitly asking for pipelining handle X and handle Y won't be supported.
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It isn't easy for an app to do this association. The lib should probably
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still resolve the second one properly to make sure that they actually _can_
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be considered for pipelining. Also, asking for explicit pipelining on handle
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X may be tricky when handle X get a closed connection.
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To Ponder About
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- We need options to control max pipeline length, and probably how to behave
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if we reach that limit. As was discussed on the list, it can probably be
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made very complicated, so perhaps we can think of a way to pass all
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variables involved to a callback and let the application decide how to act
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in specific situations. Either way, these fancy options are only interesting
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to work on when everything is working and we have working apps to test with.
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- Currently (before pipelining) we do not have any code or concept that lets
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multiple handles share the same physical connection. We need to carefully
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make sure that each easy handle knows exactly what they can do and when, on
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the shared connection.
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- We need to keep a linked list of each handle that is part of a single pipe
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so that if it breaks, we know which handles that need to resend their
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requests. The pipe linked-lists could very well be "held" in the multi
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handle struct so that they won't "belong" to a particular easy handle that
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happens to be part of the pipeline during a certain period.
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