HTTP client API

[Anthony Vanelverdinghe]

Hi Tobias

As far as I know, the latest published Javadoc is at [1] (which was 
linked to in [2]). However, this Javadoc doesn't exactly match the 
current code either.

Below are some quick replies to some of your feedback.

1.)
 > What is the advantage of the Publisher<ByteBuffer> / 
Subscriber<ByteBuffer> API over plain old InputStream / OutputStream 
based APIs?
InputStream / OutputStream are blocking APIs, whereas Publisher / 
Subscriber are part of the Flow API, which corresponds to the Reactive 
Streams specification [3].

 > there is an implementation that discards all data [...] (and its 
generic type is called U rather than T)
The generic type is called U to avoid shadowing the type parameter in 
the enclosing HttpResponse.BodyProcessor<T>

 > one thing I found particularly challenging was the control flow 
progression [...] because it is push based and forces an application to 
relinquish control to the library rather than pulling data out of the 
library.
The application is still in control by means of its Flow.Subscription 
(see Flow.Subscriber::onSubscribe), isn't it?

 > 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?
As the Javadoc says, it's an Optional to be able to mark the end of the 
body. In case of an empty response body, the Consumer will therefore 
receive an Optional.empty() value.

2.) what happens when name is null, is specified by the following note 
in the package Javadoc: "Unless otherwise stated, null parameter values 
will cause methods of all classes in this package to throw 
NullPointerException."

3.) allowing custom redirection policies was brought up before [4][5], 
proposing that a redirection policy would simply be a 
BiFunction<HttpResponse, Integer, Optional<HttpRequest>>. Besides, 
analog to e.g. java.nio.file.StandardOpenOption, there would also be a 
StandardRedirectionPolicy enum [6].

4.) it doesn't, as is specified in HttpClient.Builder::version

5.) this was brought up in [7], and was addressed, I believe, in the 
latest API [1] by replacing CookieManager with java.net.CookieHandler. 
However, this change isn't in the current code (yet?).

6.) I'd assume the Executor is used to handle asynchronous requests 
(HttpClient::sendAsync in [1]). So by using a fixed thread pool, one can 
control the maximum number of concurrent requests.

Hope this helps,
Anthony

[1] http://cr.openjdk.java.net/~michaelm/httpclient/api.1/
[2] 
http://mail.openjdk.java.net/pipermail/net-dev/2016-September/010238.html
[3] http://www.reactive-streams.org/
[4] http://mail.openjdk.java.net/pipermail/net-dev/2016-February/009547.html
[5] http://mail.openjdk.java.net/pipermail/net-dev/2016-March/009608.html
[6] 
http://jep110-anthonyvdotbe.rhcloud.com/api/jep110/StandardRedirectionPolicy.html
[7] http://mail.openjdk.java.net/pipermail/net-dev/2016-August/010187.html


On 24/10/2016 21:33, Tobias Thierer wrote:
>
> Hi Michael and others -
>
>
> Thanks for publishing the latest HTTP client API docs 
> <http://cr.openjdk.java.net/%7Emichaelm/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/%7Emichaelm/httpclient/api/java/net/http/HttpRequest.BodyProcessor.html>and 
> HttpResponse.BodyProcessor 
> <http://cr.openjdk.java.net/%7Emichaelm/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 BodyProcessoraim to fulfill
>     beyond what the (simpler) abstraction of a Bodycould be?
>
>      o
>
>         Instead of describing the abstraction as a “processor” of
>         ByteBuffers / Java objects, wouldn’t it be simpler to say to
>         say that request / response bodiesare ByteBuffer / Java object
>         sources/ sinks? What is the advantage of the
>         Publisher<ByteBuffer> / Subscriber<ByteBuffer> API over plain
>         old InputStream / OutputStream based APIs?
>
>      o
>
>         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/%7Emichaelm/httpclient/api/java/net/http/HttpResponse.BodyProcessor.html#discard-U->(and
>         its generic type is called Urather 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?
>
>      o
>
>         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.
>
>      o
>
>         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?
>
>      o
>
>         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?
>
>      o
>
>         Naming inconsistency: HttpRequest.BodyProcessor.fromFile() vs.
>         HttpResponse.BodyProcessor.asFile(). How about calling all of
>         these of(), or alternatively renaming asFile() -> toFile() or
>         toPath()?
>
>      o
>
>         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
>     nameis 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)?
>
>      o
>
>         firstValueAsLong() looks particularly suspect: why would
>         anyone care particularly about long values? Especiallysince
>         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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