HTTP client API

[Tobias Thierer]

Hi Michael and others -

Thanks for publishing the latest HTTP client API docs
<http://cr.openjdk.java.net/~michaelm/httpclient/api/> (already slightly
outdated again), as well as for publishing the current draft code in the
sandbox repository!

Below is some concrete feedback, questions and brainstorming on how to

(a) increase the usefulness or

(b) decrease the semantic weight

of the API. Note that most of this is driven only by inspection of the API
and some brief exploration of the implementation code, not (yet) by a
substantial effort to write proof of concept client applications. I’d love
if I could help make this API as useful to applications as possible, so I’d
appreciate your feedback on how I can best do that and what the principles
were that guided your design choices.

1.) The HttpRequest.BodyProcessor
<http://cr.openjdk.java.net/~michaelm/httpclient/api/java/net/http/HttpRequest.BodyProcessor.html>
and HttpResponse.BodyProcessor
<http://cr.openjdk.java.net/~michaelm/httpclient/api/java/net/http/HttpResponse.BodyProcessor.html>
abstractions seem particularly hard to grasp / have a high semantics
weight.

   -

   What purpose does the abstraction of a BodyProcessor aim to fulfill
   beyond what the (simpler) abstraction of a Body could be?
   -

      Instead of describing the abstraction as a “processor” of ByteBuffers
      / Java objects, wouldn’t it be simpler to say to say that request /
      response bodies are ByteBuffer / Java object sources / sinks? What is
      the advantage of the Publisher<ByteBuffer> / Subscriber<ByteBuffer> API
      over plain old InputStream / OutputStream based APIs?
      -

      The term “processor” and the description of “converting incoming
      buffers of data to some user-defined object type T” is especially
      confusing (increases the semantic weight of the abstraction) given that
      there is an implementation that discards all data
      <http://cr.openjdk.java.net/~michaelm/httpclient/api/java/net/http/HttpResponse.BodyProcessor.html#discard-U->
      (and its generic type is called U rather than T). A BodyProcessor
      that has no input but generates the digits of Pi is also conceivable.
      Perhaps call these BodySource / BodySink, ByteBufferPublisher /
      ByteBufferSubscriber, or just Body?
      -

      The fact that you felt the need to introduce an abstraction
      HttpResponse.BodyHandler whose name is similar to but whose semantics are
      different from HttpResponse.BodyProcessor is another indication
that these
      concepts could be clarified and named better.
      -

      To explore how well the abstractions “fit”, I played with some draft
      code implementing the API on top of another one; one thing I found
      particularly challenging was the control flow progression:
      HttpClient.send(request, bodyHandler)
      -> bodyProcessor = bodyHandler.apply(); // called by the library
      -> bodyProcessor.onSubscribe() / onNext()
      because it is push based and forces an application to relinquish
      control to the library rather than pulling data out of the library.
      Perhaps the Response BodyHandler abstraction could be eliminated
      altogether? For example, wouldn’t it be sufficient to abort
downloading the
      body once an application thread has a chance to look at the Response
      object? Perhaps once a buffer is full, the download of the
further response
      body could be delayed until a client asks for it?
      -

      What’s the purpose of HttpRequest.bodyProcessor()’s return type being
      an Optional<BodyProcessor> (rather than BodyProcessor)? Why can’t this
      default to an empty body?
      -

      Naming inconsistency: HttpRequest.BodyProcessor.fromFile() vs.
      HttpResponse.BodyProcessor.asFile(). How about calling all of these
      of(), or alternatively renaming asFile() -> toFile() or toPath()?
      -

      asByteArrayConsumer(Consumer<Optional<byte[]>> consumer): Why is this
      an Optional? What logic decides whether an empty response body will be
      represented as a present byte[0] or an absent value?


2.) HttpHeaders: I love that there is a type for this abstraction! But:

   -

   Why is the type an interface rather than a concrete, final class? Since
   this is a pure value type, there doesn’t seem to be much point in allowing
   alternative implementations?
   -

   The documentation should probably specify what the methods do when name
   is not valid (according to RFC 7230 section 3.2?), or is null.
   -

   Do the methods other than map() really pull their weight (provide enough
   value relative to the semantic API weight that they introduce)?
   -

      firstValueAsLong() looks particularly suspect: why would anyone care
      particularly about long values? Especially since the current
      implementation seems to throw NumberFormatException rather than returning
      an empty Optional?


3.) Redirect

   -

   Stupid question: Should Redirect.SAME_PROTOCOL be called SAME_SCHEME
   (“scheme” is what the “http” part is called in a URL)? I’m not sure which
   one is better.
   -

   I haven’t made up my mind about whether the existing choices are the
   right ones / sufficient. Perhaps if this class used the typesafe enum
   pattern from Effective Java 1st edition rather than being an actual enum,
   the API would maintain the option in a future version to allow
   client-configured Redirect policies, allowing Redirect for URLs as long as
   they are within the same host/domain?


4.) HttpClient.Version:

   -

   Why does a HttpClient need to commit to using one HTTP version or the
   other? What if an application wants to use HTTP/2 for those servers that
   support it, but fall back to HTTP/1.1 for those that don’t?


5.) CookieManager

   -

   Is there a common interface we could add without making the API much
   more complex to allow us both RFC 2965 (outdated, implemented by
   CookieManager) and RFC 6265 (new, real world, actually used) cookies? Needs
   prototyping. I think it’s likely we’ll be able to do something
similar to OkHttp’s
   CookieJar
   <https://square.github.io/okhttp/3.x/okhttp/okhttp3/CookieJar.html>
   which can be adapted to RFC 2965 - not 100%, but close enough that most
   implementations of CookieManager could be reused by the new HTTP API, while
   still taking advantage of RFC 6265 cookies.


6.) HttpClient.Executor

   - The documentation isn’t very clear about what tasks run on this
   executor and how a client can control HTTP traffic through a custom
   Executor instance. What power does the current executor() API provide to
   clients? Perhaps it would be simpler to omit this API altogether until the
   correct API becomes clearer?


Thanks!

Tobias
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