Component Reference

A component is any Java class whose lifetime is controlled by the container, see the Developer Guide for an introduction. Components are specified and configured in services.xml and can have other components, and config (represented by generated "Config" classes) injected at construction time, and in turn be injected into other components.

Whenever a component or a resource your component depends on is changed by a redeployment, your component is reconstructed. Once all changed components are reconstructed, new requests are atomically switched to use the new set and the old ones are destructed.

If you have multiple constructors in your component, annotate the one to use for injection by @com.yahoo.component.annotation.Inject.

Identifiable components must implement com.yahoo.component.Component, and components that need to destruct resources at removal must subclass com.yahoo.component.AbstractComponent and implement deconstruct().

See the example for common questions about component uniqueness / lifetime.

Component Types

Vespa defined various component types (superclasses) for common tasks:

Component configurations

This illustrates a typical component configuration set up by the Vespa container:

The network layer associates a Request with a response handler and routes it to the correct type of request handler (typically based on URI binding patterns).

If an application needs lightweight request-response processing using decomposition by a series of chained logical units, the processing framework is the correct family of components to use. The request will be routed from ProcessingHandler through one or more chains of Processor instances. The exact format of the output is customizable using a Renderer.

If doing queries, SearchHandler will create a Query object, route that to the pertinent chain of Searcher instances, and associate the returned Result with the correct Renderer instance for optional customization of the output format.

The DocumentProcessingHandler is usually invoked from messagebus, and used for feeding documents into an index or storage. The incoming data is used to build a Document object, and this is then feed through a chain of DocumentProcessor instances.

If building an application with custom HTTP APIs, for instance arbitrary REST APIs, the easiest way is building a custom RequestHandler. This gets the Request, which is basically a set of key-value pairs, and returns a stream of arbitrary data back to the network.

Injectable Components

These components are available from Vespa for injection into applications in various contexts:

Component Versioning

Components as well as many other artifacts in the container can be versioned. This document explains the format and semantics of these versions and how they are referred.

Format

Versions are on the form:

version ::=    major [ "." minor [ "." micro [ "." qualifier ]]]

Where major, minor, and micro are integers and qualifier is any string.

A version is appended to an id separated by a colon. In cases where a file is created for each component version, the colon is replaced by a dash in the file name.

Ordering

Versions are ordered first by major, then minor, then micro and then by doing a lexical ordering on the qualifier. This means that a:1 < a:1.0 < a:1.0.0 < a:1.1 < a:1.1.0 < a:2

Referencing a versioned Component

Whenever component is referenced by id (in code or configuration), a fully or partially specified version may be included in the reference by using the form id:versionSpecification. Such references are resolved using the following rules:

• An id without any version specification resolves to the highest version not having a qualifier.
• A partially or full version specification resolves to the highest version not having a qualifier which matches the specification.
• Versions with qualifiers are matched only by exact match.

Example: Given a component with id a having these versions: [1.1, 1.2, 1.2, 1.3.test, 2.0]

• The reference a will resolve to a:2.0
• The reference a:1 will resolve to a:1.2
• The only way to resolve to the "test" qualified version is by using the exact reference a:1.3.test
• These references will not resolve: a:1.3, a:3, 1.2.3

Merging specifications for chained Components

In some cases, there is a need for merging multiple references into one. An example is inheritance of chains of version references, where multiple inherited chains may reference the same component.

Two version references are said to be compatible if one is a prefix of the other. In this case the most specific version is used. If they are not compatible they are conflicting. Example:

<search>
    <searcher id="Searcher:2.3" class="com.yahoo.search.example.Searcher" bundle="the name in <artifactId> in your pom.xml" />
    <searcher id="Searcher:2.4" class="com.yahoo.search.example.Searcher" bundle="the name in <artifactId> in your pom.xml" />
    <chain id="parenta">
        <searcher id="Searcher:2"> bundle="the name in <artifactId> in your pom.xml" </searcher>
    </chain>
    <chain id="parentb">
        <searcher id="Searcher:2.3"> bundle="the name in <artifactId> in your pom.xml" </searcher>
    </chain>
    <chain id="parentc">
        <searcher id="Searcher:2.4"> bundle="the name in <artifactId> in your pom.xml" </searcher>
    </chain>

    <!-- This chain will get Searcher:2.3 -->
    <chain id="childa" inherits="parenta parentb" />

    <!-- Error, as Searcher:2.3 and Searcher:2.4 are conflicting -->
    <chain id="childb" inherits="parentb parentc" />
</search>