Developing request handlers

This document explains how to implement and deploy a custom request handler.

In most cases, implementing your own request handlers is unnecessary, as both searchers and processors can access the request data directly. However, there are a few cases where custom request handlers are useful:

  1. You need to implement a custom REST API.
  2. Your application needs to control which parameters are used to route requests to a particular search or processing chain.

Implementing a request handler

Upon receiving a request, the request handler must consume its content, process it, and then return a response. The most convenient way to implement a request handler is by subclassing the ThreadedHttpRequestHandler.

This utility base class uses a synchronous API and a multithreaded execution model. It also implements a lot of functionality that is needed by most request handlers:

  • queries are automatically written to the access log
  • an HTTP date header is added to the response (if your own code adds a date header, it will not be overwritten, though)
  • logging of exceptions and queries that time out
  • automatic shutdown when an Error is thrown

Example request handler implementations

The Vespa sample apps on GitHub contains a few example request handler implementations:

Handler Description
DemoHandler A handler that modifies a request before dispatching it to the ProcessingHandler. This handler is also used in the HTTP API tutorial. Note that since this depends on ProcessingHandler you must add <processing/> to your <container> tag to use it. If you want to issue Queries instead, have com.yahoo.search.searchchain.ExecutionFactory injected instead and use it to create executions and call search/fill on them.

Deploying a request handler

To deploy a request handler in an application, use the handler element in services.xml: 

<container id="default" version="1.0">
    <handler id="my.package.MyRequestHandler" bundle="the name in <artifactId> in your pom.xml">
        <binding>http://*/*</binding>
    </handler>

A request handler may be bound to zero or more URI patterns by adding a binding element for each pattern.