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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
/**
* <p>Provides classes and interfaces for implementing a {@link com.yahoo.jdisc.handler.RequestHandler
* RequestHandler}.</p>
*
* <h2>RequestHandler</h2>
* <p>All {@link com.yahoo.jdisc.Request Requests} in a jDISC application are processed by RequestHandlers. These are
* components created by the {@link com.yahoo.jdisc.application.Application Application}, and bound to one or more URI
* patterns through the {@link com.yahoo.jdisc.application.ContainerBuilder ContainerBuilder} API. Upon receiving a
* Request, a RequestHandler must return a {@link com.yahoo.jdisc.handler.ContentChannel ContentChannel} into which the
* caller can asynchronously write the Request's payload. The ContentChannel is an asynchronous API for ByteBuffer
* hand-over, with support for asynchronous completion-notification (through the {@link
* com.yahoo.jdisc.handler.CompletionHandler CompletionHandler} interface). Once the Request has been processed (which
* may or may not involve dispatching one or more child-Requests), the RequestHandler must prepare a {@link
* com.yahoo.jdisc.Response Response} object and asynchronously pass that to the corresponding {@link
* com.yahoo.jdisc.handler.ResponseHandler ResponseHandler}. One of the most vital parts of the RequestHandler definition
* is that it must provide exactly one Response for every Request. This guarantee simplifies the usage pattern of
* RequestHandlers, and allows other components to skip a lot of bookkeeping. If a RequestHandler decides to create and
* dispatch a child-Request, it is done through the same {@link com.yahoo.jdisc.application.BindingSet BindingSet}
* mechanics that was used to resolve the current RequestHandler. Because all {@link
* com.yahoo.jdisc.service.ServerProvider ServerProviders} use "localhost" for Request URI hostname, most RequestHandlers
* are also bound to "localhost". Those that are not typically provide a specific service for one or more remote hosts
* (these are {@link com.yahoo.jdisc.service.ClientProvider ClientProviders}).</p>
*
<pre>
@Inject
MyApplication(ContainerActivator activator, CurrentContainer container) {
ContainerBuilder builder = activator.newContainerBuilder();
builder.serverBindings().bind("http://localhost/*", new MyRequestHandler());
activator.activateContainer(builder);
}
</pre>
*
* <p>Because the entirety of the RequestHandler stack (RequestHandler, ResponseHandler, ContentChannel and
* CompletionHandler) is asynchronous, an active {@link com.yahoo.jdisc.Container Container} can handle as many
* concurrent Requests as the sum capacity of all installed ServerProviders. Furthermore, the APIs have been designed in
* such a way that the ContentChannel returned back to the initial call to a RequestHandler can be the very same
* ContentChannel as is returned by the final destination of a Request. This means that, unless explicitly implemented
* otherwise, a jDISC application that is intended to forward large streams of data can do so without having to make any
* copies of that data as it is passing through.</p>
*
* <h2>ResponseHandler</h2>
* <p>The complement of the Request is the Response. A Response is a numeric status code and a set of header fields.
* Just as Requests are processed by RequestHandlers, Responses are processed by ResponseHandlers. The ResponseHandler
* interface is fully asynchronous, and uses the ContentChannel class to encapsulate the asynchronous passing of
* Response content. Where the RequestHandler is part of the Container and it's BindingSets, the ResponseHandler is part
* of the Request context. With every call to a RequestHandler you must also provide a ResponseHandler. Because the
* Request itself is not part of the ResponseHandler API, there is no built-in feature to tell a ResponseHandler which
* Request the Response corresponds to. Instead, one should create per-Request light-weight ResponseHandler objects that
* encapsulate the necessary context for Response processing. This was a deliberate design choice based on observed
* usage patterns of a different but similar architecture (the messaging layer of the Vespa platform).</p>
*
* <p>A Request may or may not have an assigned timeout. Both a ServerProvider and a RequestHandler may choose to assign
* a timeout to a Request, but only the first to assign it has an effect. The timeout is the maximum allowed time for a
* RequestHandler to wait before calling the ResponseHandler. There is no monitoring of the associated ContentChannels
* of either Request or Response, so once a Response has been dispatched a ContentChannel can stay open indefinetly.
* Timeouts are managed by a jDISC core component, but a RequestHandler may ask a Request at any time whether or not it
* has timed out. This allows RequestHandlers to terminate CPU-intensive processing of Requests whose Response will be
* discarded anyway. Once timeout occurs, the timeout manager calls the appropriate {@link
* com.yahoo.jdisc.handler.RequestHandler#handleTimeout(Request, ResponseHandler)} method. All future calls to that
* ResponseHandler is blocked, as to uphold the guarantee that a Request should have exactly one Response.</p>
*
* @see com.yahoo.jdisc
* @see com.yahoo.jdisc.application
* @see com.yahoo.jdisc.service
*/
@com.yahoo.api.annotations.PublicApi
package com.yahoo.jdisc.handler;
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