[Haskell-cafe] http-enumerator: redirects, streaming and keep-alive

Michael Snoyman michael at snoyman.com
Wed Feb 2 14:57:30 CET 2011

Hi all,

There are a number of new feature requests for http-enumerator, and I
wanted to discuss some possible API changes with everyone:

* Allow keep-alive requests, which will reuse the same connection.
* Allow client code to determine whether it accepts a server's SSL certificate.
* Allow the request body to be an Enumerator so we can send large
request bodies without using lazy IO. This is already implemented via
streamingHttp, but it would make more sense to modify the Request
datatype to replace L.ByteString with (Int, Enumerator ByteString IO
* Fix redirect behavior. It seems that the right thing to do is change
a POST into a GET and remove the request body for a 303, and resend
the same request for all other 3xx codes. However, when using an
Enumerator for the request body, it is not guaranteed that we can
resend an Enumerator.

For addressing the last two, my plan is to:

* Change the Request datatype as mentioned, but provide a convenience
method for converting a lazy ByteString into a (Int, Enumerator
ByteString IO ()).
* Implement redirecting as I have described, and explain in the
documentation that automatic redirecting can only work with
Enumerators that can be run multiple times. AFAIK, this works with
most standard Enumerators, such as enumFile, and with the helper
function I will provide to convert lazy ByteStrings to Enumerators.
* Remove streamingHttp, as it will no longer be necessary.

As far as keep-alive goes, I still need to do a bit more research, but
my basic idea (with credit to Bryan O'Sullivan):

* http (and family) will all take an extra argument, Maybe Manager.
* Manager will be an abstract type that will keep an MVar (Map (Host,
Port, IsSecure) Socket).
* If http is provided with a Manager, then it uses the Socket
available in the Manager. If none is available, it creates a new
Socket and places it in the Manager.
* If http is *not* provided with a Manager, then it creates a new
socket and closes it before returning.
* There will be a newManager :: IO Manager, and a closeManager ::
Manager -> IO (), which closes all Sockets in the Manager and empties
out the inner Map.

Some open questions are:

* Who is responsible to recover if a server is no longer responding on
a Socket from the Manager: http or the client code? I would assume
* I haven't fully thought through how this will work with secure
connections: most likely in addition to storing the Socket in the
Manager I will need to store some certificate data.

And speaking of certificates, the main concern I have here is data
types: since http-enumerator can use either tls or OpenSSL, there
isn't a single certificate datatype I can use. If someone wants to
help out and write some code to convert an OpenSSL X509 datatype into
a Certificate from the certificate package (which tls uses), I will be
very grateful.

Assuming we had such a unified datatype, I would recommend changing
Request's secure record to be Certificate -> IO Bool, where returning
True means the certificate is accepted and False means it is rejected.
To get the current functionality of trusting any certificate, we would
use (const $ return True).

Any thoughts?


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